Report on the experimentally determined explosion … · SAFEKINEX-Report – Explosion Limits,...

SAFEKINEX-Report – Explosion Limits, Explosion Pressures and Rates of Pressure Rise - Part 1 page 1 (149)

Programme “Energy, Environment and Sustainable Development” Project SAFEKINEX: SAFe and Efficient hydrocarbon oxidation processes by KINetics and Explosion eXpertise Contract No. EVG1-CT-2002-00072

Report on the experimentally determined explosion limits, explosion pressures and rates of

explosion pressure rise - Part 1: methane, hydrogen and propylene

Contractual deliverable No. 8

Coordinating participant: Federal Institute for Materials Research and Testing (BAM) Contact: Kai Holtappels ([email protected])


Contents: 0 Introduction .............................................................................................................................. 4

1 Terms and Definitions ............................................................................................................. 5

2 Experimental............................................................................................................................. 6

2.1 Standard Operating Procedure SOP/EL................................................................................. 6

2.2 Work Plan ...............................................................................................................................7

2.3 Set-up ..................................................................................................................................... 7 2.3.1 Explosion vessels ......................................................................................................... 7

2.3.1.1 TUD.......................................................................................................................... 7 2.3.1.2 BAM ......................................................................................................................... 8 2.3.1.3 WUT......................................................................................................................... 9 2.3.1.4 INERIS ................................................................................................................... 10 2.3.1.5 BASF...................................................................................................................... 12

2.3.2 Data acquisition systems............................................................................................ 14 2.3.2.1 TUD........................................................................................................................ 14 2.3.2.2 BAM ....................................................................................................................... 14 2.3.2.3 WUT....................................................................................................................... 14 2.3.2.4 INERIS ................................................................................................................... 15 2.3.2.5 BASF...................................................................................................................... 18

2.3.3 Ignition sources .......................................................................................................... 20 2.3.3.1 Exploding wire igniter (TUD, BAM, WUT, INERIS, BASF)..................................... 20 2.3.3.2 Spark ignition (WUT, INERIS)................................................................................ 20 2.3.3.3 Hot spot igniter (INERIS)........................................................................................ 21 2.3.3.4 Pyrotechnical match (INERIS) ............................................................................... 24

2.4 Raw Data Analysis ............................................................................................................... 24 2.4.1 Explosion pressure pex................................................................................................ 24 2.4.2 Rate of explosion pressure rise (dp/dt)ex .................................................................... 25

3 Results .................................................................................................................................... 26

3.1 Hydrogen .............................................................................................................................. 26 3.1.1 Volume dependence................................................................................................... 26

3.1.1.1 Explosion limits ...................................................................................................... 27 3.1.1.2 Explosion pressure ratios....................................................................................... 28 3.1.1.3 KG-values ............................................................................................................... 31

3.1.2 Pressure dependence ................................................................................................ 34 3.1.2.1 Explosion limits ...................................................................................................... 34 3.1.2.2 Explosion pressure ratios....................................................................................... 38 3.1.2.3 KG-values ............................................................................................................... 41 3.1.2.4 Normalized KG-values ............................................................................................ 43

3.1.3 Temperature dependence .......................................................................................... 46 3.1.3.1 Explosion limits ...................................................................................................... 46 3.1.3.2 Explosion pressure ratios....................................................................................... 49 3.1.3.3 KG-values ............................................................................................................... 52

3.1.4 Flame propagation: dependence on hydrogen concentration .................................... 54 3.1.5 Discussion .................................................................................................................. 57


3.2 Hydrogen/oxygen mixtures................................................................................................... 62


3.3 Methane................................................................................................................................ 653.3.1 Volume dependence................................................................................................... 65


3.3.2 Pressure dependence ................................................................................................ 73 3.3.2.1 Explosion limits ...................................................................................................... 73 3.3.2.2 Explosion pressure ratios....................................................................................... 76 3.3.2.3 KG-values ............................................................................................................... 79 3.3.2.4 Normalized KG-values ............................................................................................ 82

3.3.3 Temperature dependence .......................................................................................... 86 3.3.3.1 Explosion limits ...................................................................................................... 86 3.3.3.2 Explosion pressure ratios....................................................................................... 88 3.3.3.3 KG-values ............................................................................................................... 90

3.3.4 Flame propagation: dependence on methane concentration ..................................... 92 3.3.5 Flame propagation: dependence on initial pressure and location of the

ignition source ............................................................................................................ 96 3.3.6 Discussion ................................................................................................................ 101

3.3.5.1 Explosion limits .................................................................................................... 101 3.3.5.2 Explosion pressure ratios..................................................................................... 102 3.3.5.3 KG-values ............................................................................................................. 104 3.3.5.4 Normalized KG-values .......................................................................................... 105

3.4 Propylene ........................................................................................................................... 106 3.4.1 Explosion limits......................................................................................................... 106 3.4.2 Explosion pressure ratios ......................................................................................... 111 3.4.3 Normalized KG-values............................................................................................... 115 3.4.4 Combustion times..................................................................................................... 118 3.4.5 Discussion ................................................................................................................ 121

3.4.5.1 Explosion limits .................................................................................................... 121 3.4.5.2 Explosion pressure ratios pex/pinitial ....................................................................... 122 3.4.5.3 KG-values ............................................................................................................. 126 3.4.5.4 Combustion times ................................................................................................ 127

3.4.6 Thermal explosion and detonation ........................................................................... 127 3.4.6.1 Experimental observations................................................................................... 127 3.4.6.2 Suggested explanation for the experimental observations .................................. 137 3.4.6.3 Pressure load on the vessel................................................................................. 139 3.4.6.4 Origin of pressure oscillations observed in case of detonations .......................... 140 3.4.6.5 Detonative range at pinitial = 30 bara and Tinitial = 25 °C................................... 143 3.4.6.6 Variation of the detonative range with vessel volume .......................................... 143 3.4.6.7 Predetonation distance of propene/O2-mixtures in long tubes ............................ 144

4 Conclusions.......................................................................................................................... 146

4.1 Hydrogen ............................................................................................................................ 146

4.2 Methane.............................................................................................................................. 146

4.3 Propene .............................................................................................................................. 146

5 Perspectives ......................................................................................................................... 147

6 References............................................................................................................................ 148


0 Introduction Explosion indices as described in section 1 (Terms and definitions) are not the type of independent physicochemical material characteristics such as melting temperature or density of a substance, pure or mixed. As with most other safety characteristics they are influenced by the test apparatus and determination procedure. Therefore the evaluation and standardisation of determination methods for safety characteristics is particularly important for chemical safety engineering and other fields of application, e.g. classification of dangerous substances and goods. With the creation of European directives in the field of explosion prevention, which apply uniformly to all member states, it became necessary to develop new European standards for the determination of explosion characteristics. The leadership for this project is with the European Technical Committee 305 “Potentially explosive atmospheres – Explosion prevention and protection” (CEN/TC305). The working group 1 is responsible for the development of new standards for the determination of safety characteristics of gases and vapours. In this field the new standard EN 1839 “Determination of explosion limits of gases and vapours” fixes the experimental basic principles of explosion indices determination, also valid for other indices than explosion limits. Two different procedures are proposed in this standard, a tube method (method T) and a bomb method (method B). Both options are valid for atmospheric conditions, only. An important task of the SAFEKINEX project is the development and validation of standard methods for the determination of explosion indices at elevated conditions. Furthermore it is of great interest to enlarge the knowledge of pressure and temperature respectively volume dependencies on the explosion indices by carrying out systematic examinations of different industrial important fuel gases. Thereby it is to take into consideration that low temperature oxidation phenomena may occur especially at non-atmospheric conditions. The present report deals with the validation of SAFEKINEX Standard Operating Procedure SOP/EL [1] and the corresponding experimental determined explosion indices explosion limits, explosion pressures and rates of explosion pressure rise for the three fuel gases hydrogen, methane and propylene. Starting the experiments it became obvious that it was reasonable to combine the experiments of both work packages WP 2.3 and WP 2.4 in order to reduce the experimental effort. More time efficient procedures concerning both data evaluation and experimental work plan were introduced, e.g. leading to reduction of modification frequency according to test series with different initial conditions. This also results in a different reporting structure as it was defined in the project plan. Thus, instead of four deliverables, differentiated in pressure and temperature dependencies of the explosion indices (originally planned), the synergy-optimized procedure formally results in just two deliverables, differentiated in the substances.


1 Terms and Definitions Used definitions in this report as already defined in deliverable report no.2 are: Explosion range Range of the concentration of a flammable substance in air or oxygen/inert

gas, within an explosion can occur.

EL Explosion Limits. Limits of the explosion range.

LEL Lower explosion limit. Highest concentration (in mol-%) of a flammable gas in gaseous mixture (fuel lean ), in which a flame just fails to propagate after ignition.

UEL Upper explosion limit. Lowest concentration (in mol-%) of a flammable gas in gaseous mixture (fuel rich), in which a flame just fails to propagate after ignition.

LOC Limiting oxygen concentration. Highest oxygen concentration in a ternary mixture of type combustible/oxygen/inert gas, in which a flame just fails to propagate for any ratio between combustible and inert gas.

pex Maximum pressure occurring in a closed vessel during an explosion.

pdet Chapman-Jouguet pressure of a detonation

(dp/dt)ex Highest rate of pressure rise of an explosion in a closed vessel.

pmax Highest value of pex obtained by variation of fuel concentration.

(dp/dt)max Highest value of (dp/dt)ex obtained by variation of fuel concentration.

pi Initial pressure (before ignition), also used: pinitial

pex/pi Explosion pressure ratio (possible criterion of an ignition), also used: pex/pinitial

pheat_initial Pressure of the unreacted gas mixture at the moment when thermal explosion occurs.

pdet_initial Pressure of the unreacted gas mixture at the moment when the deflagration to detonation transition occurs.

pheat_initial/pi Precompression factor thermal explosion. Ratio between pheat_initial and pi.

pdet_initial/pi Precompression factor detonation. Ratio between pdet_initial and pi. If the precompression factor is larger than 1, the Chapman-Jouguet pressure of the detonation rises accordingly. Typically, in volumes with characteristic dimensions being only slightly larger than the predetonation distance, the pressure in the unreacted gas at the moment of the occurrence of the DDT may be much larger than pinitial due to the expansion of the reaction gases generated by the initial deflagration.

Ti Initial temperature (before ignition), also used: Tinitial

Tex/Ti Temperature rising factor (possible criterion of an ignition), also used: Tex/Tinitial


KG Deflagration index given by (dp/dt)ex normalized to a 1 m3 vessel. Characteristic

value calculated according to the cubic law from the rate of pressure rise (KG = (dp/dt)ex * V1/3).

KGmax Maximum deflagration index given by (dp/dt)max normalized to a 1 m3 vessel (KGmax = (dp/dt)max * V1/3)

KG/pi KG-value normalized to an initial pressure of 1 bara.

tcombustion Combustion time. Time difference between the moment of ignition and the time when the pressure in the vessel reaches pex

Lpredet Predetonation distance (also referred to as “run up distance”).

D diameter

H height

2 Experimental 2.1 Standard Operating Procedure SOP/EL On the basis of dev.-rep. no. 2 "Report on the experimental factors influencing explosion indices determination" a Standard Operating Procedure SOP/EL for the determination of explosion limits, explosion pressures and rates of explosion pressure rise has been proposed. Main task of this SOP is to adjust the experimental important factors in order to achieve reliable, comparable and reproducible results from all of the different institutes involved in the project. The main characteristics of this SOP/EL are: During the experiments it may be necessary to modify the SOP/EL in some points as it was done for the sampling frequency. In most cases a frequency of 1 kHz is enough in order to determine explosion characteristics especially for gases with a low burning velocity. Other modifications of the SOP/EL will be reported in following deliverable reports, if necessary. It will be done latest in dev.-rep. no. 13, which deals with the determination of non-standardized explosion indices or experiments for industrial end-users. Ignition vessel closed spherical or cylindrical steel vessel (bomb) Volume pi ≤ 5 bara V > 5.0 dm3

pi > 5 bara V > 2.6 dm3

Initial temperature 20 °C up to 400 °C; TCK or PT100 , ø = 1 mm to 2 mm Initial pressure up to 100 bar(a) Mixture status quiescent when ignited Ignition source high voltage spark or fusing (exploding) wire

EIgnition = 10 J to 20 J [recommended] Explosion criterion pressure rise of pex/pi > 1.05 (5 % of initial pressure) Sampling frequency > 1 kHz Step size for EL 10% of sample concentration below 2 mol-%,

0.2 mol-% above 2 mol-% sample concentration Repetition of tests for EL 2 Explosion limit last non ignition point Step size (pex and (dp/dt)ex) in dependence of the explosion range, min. 8 concentrations Repetition pex and (dp/dt)ex none (single experiments)


2.2 Work Plan In order to test the validity of the SOP/EL, experiments at fixed initial conditions of 1 bara and 10 bara at 20 °C and 200 °C were defined for hydrogen and methane. These two fuel gases were chosen due to their extremely different laminar burning velocities which enables analysis of experimental important factors. Furthermore these validation tests were suitable for the determination of the volume dependencies of explosion limits, explosion pressures and rates of pressure rise. For this purpose, vessel volumes ranging from 2.8 10-3 m3 up to 2 m3 were used. Concerning the test program, defined in the Detailed Project Plan (DPP), each participant started to carry out experiments for a single fuel gas at any of the different initial conditions (TUD - methane; BAM - hydrogen; BASF - propene). Such division of work enabled investigations to be performed on a high level of efficiency and thus over a wide range of initial conditions. This includes also fuel gases of different substance classes. The initial conditions are combinations of pressure steps up to 30 bara and temperature steps up to 250 °C. 2.3 Set-up 2.3.1 Explosion vessels 2.3.1.1 TUD The TUD developed 20-dm3 explosion test apparatus exists of three major parts: an explosion sphere, a storage vessel and a fast acting valve. The cross section of the apparatus is presented in Fig. 1.

T3 T2 T 1

T6

T5

T4

T3 T2 T 1T3 T2 T 1

T6

T5

T4

P1 P2T3 T2 T 1T3 T2 T 1

T6

T5

T4

T3 T2 T 1T3 T2 T 1

T6

T5

T4

P1 P2

Fig. 1: 20-dm3 sphere of TUD (D = 340 mm) with an additional storage canister (2-dm3) and a

fast acting valve. The official pressure rating is 150 bara at 250 °C. All components are drawn to scale. On the right side the arrangement of the pressure transducers and the thermocouples are shown. 1 Explosion sphere 2 Fast acting valve 3 Storage vessel (0.6-dm3, 2-dm3 or 6.5-dm3)


Both the explosion sphere and the storage vessel have double walls to allow circulation of a thermo-fluid heating or cooling of the vessels. The thermo-fluid circulation system is constructed such that it enables simultaneous heating to the same or different temperatures. The explosion sphere and the storage vessel are connected by a duct. The fast acting valve opens and closes the borehole of the duct within an adjustable time. The movement of the fast acting valve to the closed or opened position is done by compressed air and is controlled by two electromagnetic valves. The volume of the storage vessel can be changed by attaching one of three storage vessels of 0.6-dm3, 2.0-dm3 of 6.5-dm3. The explosion sphere is equipped with two independent pressure transducers that record pressure development during the course of an experiment. For temperature measurement six thermocouples are installed inside the sphere at different locations in order to determine the flame propagation during an experiment. 2.3.1.2 BAM At BAM three different vessels with volumes of 3-dm3, 6-dm3 (both cylinders, German steel code: 1.4122; composition in units of %-w/w: C=0.4, Si=1, Mn=1.5, P=0.04, S=0.015, Cr=16.5, Mo=1, Ni≤1, Fe=78.5) and 14-dm3 (sphere, German steel code: 1.4571; composition in units of %-w/w: C≤0.08, Si=1, Mn=2, P=0.045, S=0.015, Cr=17.5, Mo=2.5, Ni=12, Fe=64.9) are used for the determination of explosion indices. They are shown schematically in Fig. 2.

Fig. 2: 3-dm3 (H = 190 mm, D = 150 mm, H/D = 1.27, official pressure rating: 550 bara at 400 °C)

and 6-dm3 (H = 210 mm, D = 207 mm, H/D = 1.01 , official pressure rating: 450 bara at 400 °C) cylindrical explosion vessels and 14-dm3 (D = 297 mm, official pressure rating: 60 bara at 20 °C) spherical explosion vessel. Blue marked pressure transducer Red marked thermocouple Yellow marked ignition source

Also shown are the locations of the pressure transducers (blue), the thermocouples (red) and the ignition electrodes (yellow). Which vessel is used is dependent on the initial conditions of the experiments and the expected explosion pressures. Therefore the 14-dm3 sphere can only be used at ambient temperature and initial pressures of max. 5 bara. The cylinders can be used for all initial conditions defined in the detailed project plan. The explosion vessels are located in heatable aluminium cylinders. Due to the good heat conduction of the aluminium, a homogeneous temperature distribution is guaranteed. Using this aluminium cylinder the temperature difference between the top and the bottom of the vessel is less than 2 K. For heating the set-up heating jackets are used, which enable temperatures up to 500 °C to be reached. The test mixtures are prepared separately in mixture vessels equipped with stirrers according to the partial pressures of the components. Therefore no mixing devices are necessary in the explosion vessels.


From further experiments [1] it is known that for initial pressures higher than 5 bara a vessel volume of min. 2.8-dm3 is required in order to get reasonable results. Thereupon it was decided to carry out the explosion experiments at initial pressures of pi = 1 bara and 5 bara in the 6-dm3 vessel and at pi = 10 bara and 30 bara in the 3-dm3 vessel, also to reduce the quantity of gas needed for these experiments. 2.3.1.3 WUT At WUT two different explosion vessels are available. One is a 40-dm3 cylindrical bomb and the second is a semi-spherical chamber of 1.25-m3. The smaller chamber has been prepared to study explosion processes at elevated initial temperature and pressure up to 200 °C and 2 bar, respectively. The scheme of the research stand with 40 dm3 chamber is shown in Fig. 3.

Fig. 3: 40-dm3 cylinder of WUT (D = 340 mm, H = 490 mm; H/D = 1.44). Maximum initial

pressure is 2 bara and the maximum initial temperature is 200 °C. 1 Combustion chamber 2 Ball Valve 3 Ignition electrodes

4 Electrical heater 5 Thermocouple 6 Pressure transducer

In order to keep uniform temperature inside the chamber, the walls of the chamber are heated by means of three electrically powered flat bands located on the external surface of the chamber. It is possible to control the heater temperature. In Fig. 4 the second semi-spherical chamber with a volume of 1.25-m3 is illustrated. For safety reasons the experiments will be carried out only at ambient temperature and pressure.


A

A

A - A

Fig. 4: 1.25-m3 semi-spherical chamber of WUT (D = 1200 mm, L = 1400 mm; D/L = 0.86,

comparable to H/D ratio because the vessel is turned 90°). Maximum initial pressure is 1 bara and the maximum initial temperature is 20 °C.

2.3.1.4 INERIS During the course of this project, INERIS operated three vessels. A few tests with methane air mixtures have been performed in the standard 20-dm3 sphere as already described in Fig. 1. Some refined investigations of the ignition process have been done in an open configuration, with a 8-dm3 transparent tube, specially designed to such purpose (Fig. 5).

the tube gas supply gas control

Fig. 5: 8-dm3 open tube and operating equipment


The explosion chamber is a standard steel tube with a square cross section (100 mm x 100 mm), 850 mm high, equipped with a transparent PMMA window all over the length. A light flap is provided at the upper end and the tube is closed at the bottom end. The ignition source is located 500 mm from the bottom. The gases are introduced at the bottom through nozzles ensuring jet mixing. A number of preliminary tests have proven that the mixture is perfectly homogeneous 200 mm above the nozzles. This devices can also be used to produce and control dust clouds or hybrid mixtures. The gas flows are monitored by means of mass flow meters (BROOKS: 0-100 l/min for air and 0-10 l/min for the combustible gas). The tube is usually swept at a flow rate of the order of 30 l/min e.g. a flow speed of 5 cm/s enabling the turbulence to decay. An additional control of the mixture is performed by sampling the oxygen rate in the vicinity of the ignition source (SERVOMEX device: paramagnetic properties of oxygen). It has been checked that the accuracy of the measurements (O2 content) or calculations (flow rates) of the gas concentration is within ±0.2 vol%. The parametric activity has been done with the large scale 2 m3 vessel available at INERIS (Fig. 6). It is basically a 2 m3 spherical vessel (inner diameter of 1555 mm), made of A52CP steel, 65 mm thick. It is equipped with three 350 mm (inner diameter) flanges, one on the top and the two others on the side 90° apart.

Fig. 6: 2-m3 spherical vessel (D = 1555 mm, official pressure rating: 25 bara at 200 °C)

Three sets of electrical heaters are lining the external wall of the vessel. The whole is thermally insulated with 15 cm rock wool. The heating device is regulated between 25°C up to 200°C. At 200°C set point, the temperature deviation on the external side of the sphere (three measuring points) is not more than 5 K. Inside the sphere, two K-thermocouples are installed, one close to the top and the other close to the bottom: at 200°C set point, the temperature deviation on the internal side of the sphere is not more than 2 K. The gases are introduced from bottles and pressure regulators directly into the sphere through electro pneumatic valves via ducts inserted through one of the side flange (Fig. 7). If we need to prepare an accurate mixture in terms of composition (for flammability limits studies for instance), the quantities of gases need to be precisely monitored. The flow rates are then very small so that the gases stratify. An additional mixing device is operated. It is inserted in the upper flange and consists of two membranes working oppositely as in a membrane compressor. An electrical engine is moving the pistons at a frequency of a few tens of Hz. It can mix a fully stratified (e.g. helium-air) mixture within a few minutes.


Fig. 7: Gas supply of the 2 m3 INERIS vessel

The mixtures are prepared by the method of partial pressures. The vessel is first evacuated down to 50 mbar. The reactants are then introduced to the prescribed pressures. Those pressures are controlled with a piezoresistive KISTLER 0-20 bara transducer (KISTLER 4045A: accuracy ±0.010 bar). A further control of the mixture is performed in the vessel via sampling the oxygen content with an O2 paramagnetic analyser (SERVOMEX). This device is thermo regulated so that the O2 content can be known with an accuracy of 0.01 vol% and the gas concentration is known within ±0.1 vol%. 2.3.1.5 BASF The measurements were carried through in a spherical vessel with 20-dm3 volume. The material of the wall is Inconel 718 (German steel code: 2.4668; composition in units of %-w/w: Cr=18, Al=0.5, Nb=5, Mo=3, Ti=0.9, Ni=50, Fe=22.6). Fig. 8 displays a sectional view of the sphere. The mixtures were prepared according to the partial pressure method. To calculate the volumetric concentrations of one individual gas component in the final mixture on the basis of the pressure increase in the vessel caused by the injection of this component we always assumed ideal gas behaviour. After completing the injection the mixture is stirred for about 30 s. About 10 seconds after having switched off the stirrer the ignition source is activated. The ignition criterion is a pressure rise of more than 5 %, i. e. pex/pi > 1.05.


Fig. 8: 20-dm3 sphere of BASF. The official pressure rating is 600 bara at 500 °C. The electrical

heating jacket that allows to heat the sphere up to 500 °C is omitted in this drawing. All components are drawn to scale.


2.3.2 Data acquisition systems 2.3.2.1 TUD At TUD a new data acquisition programme has been developed in order to record thirteen signals and send four signals with high time precision not higher than 0.25 ms. The recorded signals are:

• two piezoelectric pressure transducers (Kistler, type 7001, maximum operating temperature: 350 °C, maximum operating pressure: 250 bara, location: Fig. 1)

• six thermocouples (arrangement: Fig. 1) • ignition current • ignition voltage • status of fast acting valve • status of the ignition system and • status of additional device, e. g. gas chromatography or flame spectroscopy.

The transmitted signals are

• operation of the fast acting valve • initiation of the ignition device (two channels) • trigger for the external device

For the data collection two PCI-cards, produced by National Instruments are used. The first one is a timer card (type PCI 6602), which is required for the high time precision during the data acquisition. Otherwise the Windows operating system may cause delay times between task switching. The second card (PCI-MIO-16E-4) is required for the data acquisition with a high sampling frequency (max. 500 kHz on one channel, 250 kHz on sixteen channels). This card enables the measurement of analogue voltage signals (unipolar: 0 - 10 V, bipolar: -10 - +10 V), which are converted to and stored as digital signals on a computer. 2.3.2.2 BAM For data acquisition a PCI-multi-channel card manufactured by Keithley (type DAS-1402, 12 bit resolution) is used. This card allows the simultaneous measurement of eight channels with a sampling frequency of 100 kHz (sum of all channels). The initial pressures as well as the pressure-time histories are measured by means of piezoresistive pressure transducer manufactured by Keller (type PAA-10 or PA-10, measuring uncertainty: 0.5 % FS). The transducers are calibrated with digital pressure controller (company GE Druck, type DPI-515, measuring uncertainty: 0.01 % FS). The determination of the initial temperature and the temperature-time histories with respect to the response time in the vessel is carried out by means of coated NiCr-Ni- thermocouples (type K, diameter D = 1.5 mm or 0.5 mm). In the cylindrical vessels they are located approximately 30 mm under the top of the vessel. In the sphere the thermocouple is coming in horizontally. The distance to the vessel wall is min. 30 mm. 2.3.2.3 WUT The research stands are equipped with data acquisition systems which enable the measurement of analogue voltage signals on max. sixteen channels with a multi-channel card, manufactured by ESA Messtechnik GmbH (type ESSAM-3000, sampling frequency: 300 kHz (sum recording rate of all channels)). In both explosion chambers the temperature is measured by using thermocouples. The pressure-time history is measured by means of piezoelectric transducer manufactured by Kistler.


2.3.2.4 INERIS The 20-dm3 sphere is mainly equipped with two piezoresistive pressure gauges mounted rigidly in the wall of the vessel. The 8-dm3 tube has been mainly used to visualize the ignition process and measure the thermal properties of the «hot spot ignition» device described in chapter 2.3.3.3. A standard video camera (24 f/s) has been used to monitor the flame initiation process and the temperature of the hot spot has been estimated with help of a monochromatic IR pyrometer (Kleiber Pyroskop). The instrumentation of the 2 m3 sphere is more complete and summarized in Tab. 1.

Tab. 1: Instrumentation of the 2-m3 spherical vessel

Nature principle range error Temperature thermocouple -273 to 1300 K ±0.5 °C Pressure piezoresistive device 0 to 350 bar ±0.1% range

Gas analysis Oxygen controllers 0 to 100% ±0.1% abs Video Normal and high speed motion 25 to 8000 fps ± 60 µs Flame trajectory in-house ionisation gages 0 to 2000 m/s ±1%

To measure the explosion pressure, two transducers are used:

• One piezoelectric mounted in the wall of the vessel with the sensing element tangential to the interne surface of the sphere. It can measure overpressures up to 250 bar (KISTLER 6052A), supports flash overpressure up to 300 bar and 400°C. The overall accuracy is ±0.2 bar. The cut-off frequency is 120 kHz.

• One piezoresistive 0-100 bar abs (KISTLER 4045A), installed at the extremity of a 800 mm tube (6 mm inner diameter) to limit overheating effects (Fig. 9). The overall accuracy is ±0.1 bar. The cut-off frequency is 180 kHz.

Fig. 9: Mounting of the 0-100 bara piezoresistive pressure gauge


A discrepancy is to expect between the inner and outer explosion pressure sensor. Due to the length of the duct the outer sensor should support a time constant, typically equal to the time for a pressure wave to travel down the connecting duct 2 to 3 times (typically 5 to 10 ms). The corresponding frequency should also be a limit of detection, a cut-off frequency (typically 100 Hz). Available data may help to find those limits. In Fig. 10 three different signals obtained with H2-air mixtures at 10 bara and 200 °C initial conditions are compared.

16 vol% H2 60 vol% H2

45 vol% H2

Fig. 10: Explosion signal recorder with the «inner» and «outer» pressure transducers during H2-air explosions under 200°C and 10 bara initial conditions

It can be seen that the two signals overlap perfectly for the lean mixture (16 vol%), a small discrepancy is visible for the rich mixture (60 vol%) with the appearance of a peak P superposed on the slope of the pressure rise and the traces differ very significantly for the most reactive mixture (45 vol%) with a tremendous peak P (up to 130 bara). The spectrum analysis of the signals reveals that the two signal differ (in terms of maximum pressure rise) as soon as the maximum frequency exceeds 70 Hz, in line with above estimations. Peak P seems to result from the flame rushing down the connecting pipe to the transducer. If the mixture is very reactive it can be expected a volumetric explosion inside. For the present situation, the flame is rushing into the duct when the average pressure has reached 30 bara so that the final explosion pressure could amount to 150 bara in line with the measurements. The signals are recorded on a SEFRAM 8400 (8 channels at 250 kHz/channel max., 16 bits). The sampling frequency need to be defined as function of the maximum frequency of the signals. In several instances, the fundamental mode of vibration of the chamber has been triggered with a maximum frequency of 500-1000 Hz. According to the Shannon theory, we selected a sampling frequency of 10 to 20 kHz. After acquisition, the data can be processed as excel files.


To investigate the flame dynamics, two additional techniques have been implemented. To observe the flame, an air-cooled endoscope (Fig. 11) linked with a high speed video camera (REDLAKE, up to 8000 f/s) has been installed. The flame can be observed from its early beginning up to a diameter of 600 mm. The film may be used to catch an order of magnitude of the flame growth rate.

Fig. 11: Endoscope, cooling tube and high speed camera

But in order to measure more precisely the trajectory of the flame, a set of ionisation gages (six) placed on a rod (Fig. 12) has been used. This device can capture transition to detonation phenomena.

Fig. 12: Ionisation gage rod (top) and typical signals (bottom)


2.3.2.5 BASF At BASF a 12 bit data acquisition card supplied by Geitmann Gmbh (www.geitmann.de; [email protected]) is in use (type Analogue Devices, RTI 860,16 channels, the sum-sampling rate for all channels is 200 kHz, 12 bit resolution). At the beginning of December 2005 a new high speed data acquisition card supplied by the same manufacturer will be used (type: MI3122, 12 bit resolution). This card has 256 Mbyte memory on board (sufficient to store 128 Msamples) and allows the simultaneous measurement of eight channels, each with 10 Mhz sampling frequency. Such a high speed card is very useful for the detection of detonation phenomena. The explosion pressure inside the sphere was recorded with piezoelectric pressure sensors supplied by PCB (PCB-M112A05, 345 bar and PCB-113A03, 1000 bar). The sensors have resonance frequencies greater than 500 kHz. The charge amplifier had a low pass filter with a cut off frequency of 200 kHz. Acquisition of the pressure signal was performed at sampling frequencies of up to 100 000 samples/s. In our experience there is one special effect one has to bear in mind when using piezoelectric pressure sensors. In those cases when a strong detonation peak with pressure pdet occurs in the explosive gas mixture and is registered by the sensor the value for the explosion pressure ratio pex/pinitial of the mixture, which is given by the pressure signal registered immediately after the detonation peak was seen, is sometimes measured smaller than it should be. The explanation is presumably given by the fact that the original signal generated by the piezoelectric sensors consists of an electric charge on the surface of a piezoelectric crystal which is proportional to the pressure that brings about a force on the crystal in one direction. This charge is amplified by special charge amplifiers featuring an extremely high input resistance to avoid any leakage of charge to ground. If, however, the resistance to ground - either in the pressure transducer itself or in the charge amplifier - is not as good as it should be, there will be a small leakage current which is proportional to the charge generated, because the potential difference between ground and the charged crystal surface is proportional to the amount of charge produced. To illustrate why in case of detonative peaks the value for pex/pinitial, which is recorded immediately after the detonative peak was registered by the transducer, may come out too small, we consider the following example:

Let us assume a Propene/O2 mixture with a composition at the lean boundary of the detonative range at pinitial = 5 bara and Tinitial = 25 °C. The true value of pex/pinitial will be about 11. Because of precompression effects the value of pdet will be rather large, e. g. 1000 bar. If 0.5 % of the charge generated by the 1000 bar peak leaks to ground, the error in the peak value is negligible (995 bar instead of 1000 bar would be shown by the sensor). But when the pressure drops down immediately after the detonative pressure acted on the sensor and ends up with the value given by the deflagration pressure ratio of (11*5 bar) – (5 bar) = 50 bar (note that this is the value the sensor will see, because for the piezoelectric sensor the initial pressure at the start of the recording is always zero), the charge that leaked while the pressure was extremely high is still missing and thus only 45 bar are shown by the sensor. This yields a deflagration pressure ratio of only (45 bar + 5 bar)/(5 bar) = 10 instead of 11. The deviation from the true value is thus about 9 %.

If the gas mixture undergoes a purely deflagrative explosion, the values of pex/pinitial are determined correctly. Critical, when attempting to measure explosion pressures with piezoelectric pressure sensors in gaseous mixtures with compositions “deep inside” the explosive range, particularly at elevated initial pressures, is the thermal load on the thin steel membrane under which the piezoelectric crystal is located. If the temperature of the crystal increases while the flame is still propagating through the vessel, the sensor will record smaller pressures than it should (even negative pressures can be indicated!), because all piezoelectric pressure sensors available at present exhibit a pronounced drift to lower pressures with increasing temperature.

http://www.geitmann.de/

mailto:[email protected]


Even if one manages to keep the heat input negligible in the short period of flame propagation (usually less than about 500 ms in laboratory scale apparatuses) and hence determines the correct maximum explosion pressure, there is still the risk that the hot reaction gases (up to 4000 K for stoichiometric combustible/O2 mixtures), which need several seconds to attain thermal equilibrium with the cold wall of the reaction vessel, make the membrane melt or – if there is still sufficient oxygen present – burn. To avoid corruption of the pressure signal and/or destruction of the sensor and still have the sensor mounted “as flush as possible” with the wall, which is required to get realistic pressure recordings in case of detonative combustion, we tested the three different mounting variants displayed in Fig. 13 at pi = 1 bara with a nearly stoichiometric propene/air mixture (The thermal load on the sensor under these conditions is still “moderate” compared to the load brought about by mixtures like stoichiometric propene/oxygen at 5 bara and 10 bara, which are still to be examined in the sphere). As the results shown in Fig. 14 suggest, we chose variant III for all future explosion experiments. The grease in front of the sensor was replaced according to requirements.

Fig. 13: Mounting variants for piezoelectric pressure sensors in the wall of vessels used for gas

phase explosion experiments. The diameter of the pressure sensitive membrane of the sensor is about 6 mm.

0 100 200 300 400 500

0

2.5

5

7.5

10

Time [ms]

Pre

ssur

e [b

ar a

bs]

mounting variant III(best protection against heat)

mounting variant II(good protection for at least 0.2 s)

mounting variant I(sensor output gets corruptedimmediately)

Fig. 14: Pressure recording for three different mounting variants of the piezoelectric pressure

sensor. The sensor was of type PCB 113A03, the gas mixture in all three tests was Propene/Air at pinitial = 1 bara and Tinitial = 27 °C with 4.8 vol.-% Propene and 95.2 vol.-% air. The point with t = 0 ms is arbitrary and hence not correlated with the moment of ignition.


2.3.3 Ignition sources Due to the fact that this type of ignition source was proposed to be the most efficient ignition source especially at elevated initial pressure and that it is recommended in the European standard EN 1839 “B” all participants involved in this work package used the exploding wire igniter. Nevertheless experiments were carried out with different type of igniters, also to examine the influence of the type of igniter respectively the ignition energy on the determined explosion indices. 2.3.3.1 Exploding wire igniter (TUD, BAM, WUT, INERIS, BASF) It was recommended to position the ignition source in the middle of the explosion vessels. This ignition device generates an electric arc by passing an electrical charge along a straight length of the fusing wire connecting two metal rods. The electrical power for melting this wire and generating the arc is supplied by an isolating transformer. The ignition energy delivered by the arc depends on its duration and on the power rating of the isolating transformer. It has a high energy density and a big ignition volume. Through the measurement of the current-time and the voltage-time histories the ignition energy can be determined. The set-up should be adjusted in that way that an ignition energy of 10 J to 20 J is transferred to the gas systems. The exploding wire igniter is very effective for the ignition of gases and gas mixtures especially at elevated initial pressures. A kind of exploding wire device is depicted somehow in EN 13673. Playing on the phase angle between current and voltage may help to limit the energy delivered during the short circuit. INERIS has used such a device for the investigation of the incidence of the ignition mode upon the flammability limits in the laboratory. But the full scale work, an alternative simplified and very robust version of the exploding wire device has been developed. It basically consists of a 0.3 mm diameter, 15 mm long copper wire is mounted on 2 mm diameter steel bases connected via 5 m long wires to the main electrical supply (220 V) (Fig. 15). A simple switch is closed to produce a massive short circuit. Electrical measurements have been performed demonstrating an output energy of 70 J.

Fig. 15: Exploding wire device used for the full scale tests at INERIS

2.3.3.2 Spark ignition (WUT, INERIS) A single or a series of induction sparks between two electrodes is used as ignition source at WUT. Stainless steel is used as material for the electrodes. The electrodes have been positioned in the centre of the explosion vessels. The angle of the tips was (60 ± 3)° and the distance between the tips was (5 ± 0,1) mm. A high voltage transformer, with a root mean square of 13 kV to 16 kV and a short circuit current of 20 mA to 30 mA, was used for producing the ignition spark. The primary winding of the high voltage transformer was connected to the mains via a timer set to the required discharge time. The spark discharge time was adjusted to 0.2 s to 0.5 s. The power of the spark depends on the gas mixture and its pressure. In air at normal atmospheric conditions, such a source shall give a spark with a power of approximately 10 W.


At INERIS a single spark igniter has been used. It basically consists of a set of condensers charged up to 150 V which are discharged through a transformer which increases the voltage by a factor of 100. The spark generator is connected to the igniting rod (Fig. 16), via 5 m long wires. The spark gap is a modified engine spark plug. The ground electrode is mode of a pointed steel 1 mm rod, 2.5 mm apart from the (flat) positive electrode. Electrical measurements have been performed showing a global output of 20 mJ.

Fig. 16: Electrical spark ignition device used for the full scale tests at INERIS

Note two further points. First, the ratio of the energy transferred to the atmosphere to that stored inside the condensers is usually small (a few %) rendering this kind of ignition mode relatively poorly efficient for weakly reactive mixtures. Second, experiments have proven that it may become difficult to produce sparks as soon as the pressure increases. 2.3.3.3 Hot spot igniter (INERIS) The proper choice of the ignition source is particularly important for the determination of flammability limits. Flammability limits should be understood as the limiting composition of a given ATEX for which a self sustained flame is just able to propagate. It is not an ignition limit so that:

1. anything should be done to ignite; 2. but, in the same time, the propagation need to be self-sustained meaning that the flame

should not be forced, artificial in some way. If we think about “spark” type ignition, the energy to transfer to the ATEX near the flammability limits grows exponentially (Fig. 17). If a moderate ignition energy is selected, such as those delivered by electrical sparks (a few mJ), the upper and lower “flammability” limits would be 6 % and 65 % according to Fig. 17. At 10 J ignition energy the same limits will become 5% and 76 % respectively. The latter figures are more in line with expected data than the former, justifying in some way the choice of such a “strong igniter” for standard measurement purposes. But are we sure that such a strong “igniter”, propelling a number of very hot fragments, and large energy delivery will not force the propagation or modify the mixture before flame initiation especially in small vessels? This point has been addressed experimentally with the various ignitions devices used.


Fig. 17: Evolution of the electrical spark ignition energy for H2-air mixtures in standard conditions [1]

Because of this important concern, an alternative igniter has been devised on completely different grounds. The idea comes from previous E.U. sponsored programmes [29][30][31]. It has been found experimentally and theoretically that the ignition of a flammable atmosphere next to a heated body obeys the fundamental laws of flame propagation. The critical parameter is the temperature of the hot spot (provided it is not too small). This temperature can be seen as the threshold above which the combustion zone attached to the hot body is able to extract itself from this area to propagate in a self sustained manner throughout the mixture. This critical temperature is then directly linked to the activation energy of the mixture and thus should not depend greatly on the composition of the ATEX which has been observed experimentally (Fig. 18). Systematic measurements have then demonstrated that the critical hot spot ignition temperature is below 1000 °C for all the tested mixtures (Fig. 18).

Fig. 18: Hot spot ignition temperature as function of the composition for H2-air mixtures in standard

conditions [31] and for other mixtures [29]

In practise, preliminary experiments have shown that that a hot spot held at 1000-1200 °C and 1 cm in size seems able to ignite a very vast spectrum of ATEX within a typical time scale of seconds. An electrical hot spot heater has thus been devised (Fig. 19). A 300 mm long wire (0.2 mm diameter) is wired onto a 3 mm diameter, 14 mm long, alumina tube. This resistance is then covered with a high temperature ceramic cement with 0.5 mm thickness. The electrical contact is provided by two copper flanges equipped with steel electrodes (1.5 mm thick). Theses electrodes can then be plugged on a connector at the extremity of the ignition rod.


Fig. 19: Hot spot igniter (top left), temperatures (top right), calibration (bottom left) and test (bottom

right, 7 vol% CH4-air)

A number of preliminary measurements have been performed, including current, voltage, core and surface temperature. The internal temperature has been measured with a Pt-Pt thermocouple imbedded inside the ceramic rod and the surface temperature with a monochromatic pyrometer. A example is shown on Fig. 19 where is can be seen that in order to reach 1000°C surface temperature, the internal temperature raises up to more than 1500°C. There is a strict linearity indicating that the major heat exchange resistance is due to internal conduction inside the igniter. This means that the skin temperature is to a very large extent dependant on the electrical power and marginally on the external conditions (pressure, temperature,…). The effective control is then mainly performed via the input power after calibration within a precision of ±20 °C. A typical ignition test in the 8-dm3 tube (7 vol% methane-air, 1 bara, 20 °C) is presented showing an ignition at about 920 °C, fully in line with existing data. The input power in this typical test amounts about 60 W over 10 seconds. Note however that more than 80% of this is radiated away and only a few watts are convected inside the mixture. The total energy transferred to the atmosphere until ignition is thus not than the standard exploding wire device but over a much longer period and without any fragments, limiting drastically the perturbation of the atmosphere. In particular pressure disturbances are totally absent. This device is however fragile and has been mainly used for the determination of explosion limits e.g. for weakly reactive mixtures.


2.3.3.4 Pyrotechnical match (INERIS) A further alternative is the pyrotechnical match (Fig. 20), which is a small amount of pyrotechnical composition, ignited by a tiny heated resistance. The energy liberated is about 60 Joules. Note that a significant part of this energy may be absorbed by the solids constituting the igniter.

Fig. 20: Pyrotechnical spark ignition device used for the full scale tests at INERIS

2.4 Raw Data Analysis Because the pressure threshold criterion is used as explosion criterion it is of high importance to ensure a reliable analysis of the pressure-time-histories recorded at each ignition experiment. The peak values of these histories (pex) and the values of the highest rate of pressure rise ((dp/dt)ex) are of special interest. Smoothing will certainly be required, e.g. when the pressure-time-history exhibits oscillations nearby the highest pressure and/or highest rate of pressure rise or to remove noise from the recorded curve. The high pressure oscillations are known to be true pressure effects generated by the explosion inside the test vessel and not resonance artefacts of the pressure gauge. 2.4.1 Explosion pressure pex According to the European standard EN 13673-1 "Determination of the maximum explosion pressure and maximum rate of pressure rise of gases and vapours" the highest pressure of each pressure-time-history can be determined by two different methods: Graphical method From a plot of pressure versus time, which may be the analogue output from a recording instrument, the highest pressure shall be determined. The precision of the data used for the plot and the scale of the graph shall be sufficient to allow the pressures to be resolved to the nearest 0.1 bar. Computational method A computer program may be used to process the pressure-time data to determine the highest pressure. The precision of data used shall be sufficient to allow the pressures to be resolved to the nearest 0.1 bar.


2.4.2 Rate of explosion pressure rise (dp/dt)ex According to the European standard EN 13673-2 "Determination of the maximum explosion pressure and maximum rate of pressure rise of gases and vapours" smoothing of the pressure-time-history can either be achieved by the use of filters in the data acquisition system, to remove specific ranges of frequency from the recorded signal, or by mathematical processing of the recorded data. A mathematical treatment is recommended as this will preserve the raw data for re-analysis. Usage of filters implemented in the data acquisition system results in a loss of information since in such cases the smoothed data would be recorded while the real raw data would be lost. There are a number of mathematical methods that can be used for smoothing data; some examples are averaging, linear regression, filtering (e.g. FFT) or polynomial fitting. Whatever the method used there is at least one parameter, for example the number of data points averaged over, or the degree of polynomial used, which must be carefully selected to achieve the correct amount of smoothing. In Fig. 21 the effect of changing the number of data points for a linear regression on the calculated rate of pressure rise is illustrated schematically.

0 40 80 120 160 200 240 280 320 360 400 440number of raw data points for linear regression

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(dp/

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REGRESSION_DPDTEX | 15.1.2004

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p [b

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number of points: 51

P-T_H25A6021_51 | 16.1.2004

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P-T_H25A6021_11 | 16.1.2004

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p [b

ara]


P-T_H25A6021_401 | 16.1.2004

c)

b)a)

Fig. 21: Rates of pressure rise (dp/dt)ex in dependence of the considered raw data points. In the pressure-time-histories the calculated slope is shown a. The calculated rate of pressure rise is too great in case of too low number of

considered data points. b. The calculated slope shows a good agreement to measured pressure rise. c. The calculated rate of pressure rise is too low in case of too great number of

considered data points.


There is a range of the parameter over which the plot shows a plateau in the rate of pressure rise (in Fig. 21 this plateau is between 41 and 91 considered data points). To obtain the correct value of the rate of pressure rise it is recommended to use a value of the parameter within this range. In some cases the smoothing may have to be applied a number of times to achieve the required degree of smoothing. It has to be mentioned that a supporting software tool has been developed in the SAFEKINEX project (see dev.-rep. no. 3 "Control software for the explosion tests apparatus”). This software consists of the three parts data acquisition, data analysis and databank. In the analysis part two different smoothing methods (linear regression and Savitzky-Golay filter) have been installed, so that it is possible to compare the results of the different smoothing techniques. In most cases the use of different smoothing methods will not result in too different (dp/dt)ex values, but in dependence of the quality of the raw data (e.g. oscillations etc.) the precision of the calculated values will be higher sometimes. 3 Results In this chapter the experimental determined explosion indices of hydrogen, methane and propylene will be presented. The structure of the presentation for the results of hydrogen and methane is divided into three parts since both gases were chosen for validation experiments of the Standard Operating Procedure (SOP) at each participating institute. The first part treats the volume dependence of the determined explosion indices explosion limit, explosion pressure and rate of explosion pressure rise. These experiments were carried out only for a limited number of initial conditions. In the two other parts the results of the pressure and temperature dependencies are shown, which were determined mainly by BAM (hydrogen) and TU Delft (methane). The explosion indices of the third fuel gas propylene were determined by BASF. Therefore the structure of the presentation of the results is different. As mentioned in chapter 2.4.2 the maximum pressure rises of the explosions (dp/dt)ex were determined at each ignition attempt. It is more practical to normalize these values according to the so called the cubic law (KG = (dp/dt)ex * V1/3). These KG-values should be largely volume independent and describes the violence of a gas explosion in a closed vessel. In the following chapters only KG-values are presented and discussed. 3.1 Hydrogen Hydrogen is one of the most hopeful energy carrier of the future. To ensure the safe handling of hydrogen in all situations it is necessary to know the safety related properties over a wide range of operating conditions, which means initial pressures up to 30 bara and initial temperatures up to 250 °C. 3.1.1 Volume dependence In order to determine the volume dependence of explosion limits, explosion pressure ratios and the rates of pressure rise respectively the KG-values only four combinations of initial pressure and temperature were chosen, in fact 1 bara 20 °C, 10 bara 20 °C, 1 bara 200°C and 10 bara 200°C. The amount of experiments could thus be lowered.


3.1.1.1 Explosion limits In Fig. 22 the volume dependence of the explosion limits at an initial pressure of 1 bara and an initial temperature of 20 °C is illustrated. In Tab. 2 the explosion limits are listed. The smaller explosion range determined in vessel volumes of 40-dm3 and 1250-dm3 are caused by the use of a spark igniter instead of an exploding wire igniter (smaller vessels) or a hot spot igniter (2000-dm3 vessel). Therefore no volume dependence of the explosion limits at initial conditions of 20 °C and 1 bara was observed.

Tab. 2: Explosion limits for hydrogen-air mixtures at initial conditions of 1 bara and 20 °C

Vessel volume [dm3] 2.8 6.0 14.0 40.0 1250 2000

LEL [mol-%] 4.1 3.9 4.1 4.7 5.8 4.1

UEL [mol-%] 77.0 77.8 77.8 74.8 75.2 77.2

For several reasons, e.g. limited operating conditions of the explosion vessels or enormous experimental effort in greater volumes, the LEL and UEL at initial conditions of 10 bara and 20 °C, 1 bara and 200 °C as well as 10 bara and 200 °C have been determined only in a few available vessels. The results are listed in Tab. 3, Tab. 4 and Tab. 5.

0

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0,001 0,01 0,1 1 10volume [m3]

H2 [

mol

-%]

BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3

WUT 40-dm^3 WUT 1250-dm^3 INERIS 2000-dm^3

Fig. 22: Volume dependence of the explosion limits of hydrogen/air mixtures at an initial pressure

of 1 bara and an initial temperature of 20 °C


Tab. 3: Explosion limits for hydrogen/air mixtures at initial conditions of 10 bara and 20 °C

Vessel volume [dm3] 2.8 6.0

LEL [mol-%] 4.8 4.9

UEL [mol-%] 71.8 72.8


Vessel volume [dm3] 2.8 6.0 40.0

LEL [mol-%] 3.2 4.0 3.2

UEL [mol-%] 82.4 84.4 82.2


Vessel volume [dm3] 2.8 6.0

LEL [mol-%] 4.1 4.3

UEL [mol-%] 78.0 73.6

The number of explosion limits in different vessel volumes is to low to get the volume dependency at the defined conditions. The listed limits show a good correspondence except the UEL in a 2.8-dm3 vessel at 10 bara and 200 °C. The UEL is 4.4 mol-% higher than the limit determined in a 6.0-dm3 vessel. Perhaps there is a kind of forced flame propagation at these initial conditions by use of the exploding wire igniter that causes a pressure increase high enough to exceed the explosion criterion of 5%. This effect is known when too small vessels are used for the determination of explosion limits at several initial conditions, but usually this leads to a shift of the EL to the safer side [1]. 3.1.1.2 Explosion pressure ratios In the next figures the explosion pressure ratios of hydrogen/air mixtures for initial conditions of 1 bara and 20 °C (Fig. 23), 10 bara and 20 °C (Fig. 24), 1 bara and 200 °C (Fig. 25) as well as 10 bara and 200 °C (Fig. 26) are shown. The symbols at the lowest respectively the highest amount of hydrogen in the mixtures indicate the explosion limits and therefore a pressure ratio of < 1.05. In general the ratios determined in the different vessel volumes show a good correspondence to each other. No significant volume dependence was observed at all initial conditions. The slightly higher explosion pressure ratios sometimes determined in different vessels are caused especially by slightly lower initial temperatures (WUT, Fig. 23: 10 °C to 15 °C instead of 20 °C; BAM, Fig. 25: 190 °C instead of 200 °C). The explosion pressures are directly dependent on the initial pressure and temperature and also small changes can lead to differences up to 0.5 bar in the determined explosion pressures. In chapter 3.1.2.2 and 3.1.3.2 the dependence on pressure and temperature is shown when the initial conditions are changed in great steps.


0,0

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i [-]

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BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3WUT 40.0-dm^3 WUT 1250-dm^3 INERIS 2000-dm^3 "hot wire"INERIS 2000-dm^3 "spark"

Fig. 23: Explosion pressure ratios of hydrogen/air mixtures at an initial pressure of 1 bara and an

initial temperature of 20 °C, determined in different volumes

When the LEL is exceeded the explosion pressure ratios increase moderately independent of the initial conditions. Up to an amount of 10 mol-% hydrogen the determined pressure ratios are below a value of 4. The highest pressure ratios were determined for mixtures close to stoichiometric. Remarkable is the abrupt increase of the pressure ratios when decreasing the amount of hydrogen only 0.4 mol-% below the UEL. At concentrations slightly below the UEL results in an explosion pressure 3 - 4 times higher than the initial pressure. The type of igniter seems to influence the pressure ratios at low hydrogen contents. INERIS used two igniters for a mixture with 8 mol-% hydrogen. The explosion pressure ratio determined by use of a hot wire igniter was higher than the ratio determined by use of a spark igniter. Since these experiments were single ignition attempts this influence could not be confirmed. At higher hydrogen contents comparable pressure ratios were determined for the spark and the exploding wire igniters.


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BAM 6.0-dm^3

INERIS 2000-dm^3 "spark"



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BAM 6.0-dm^3




3.1.1.3 KG-values The Fig. 27 to Fig. 30 illustrate the determined KG-values and their dependence on the hydrogen content in the mixtures at the different initial conditions. The KG-values should be largely volume independent, but as it can be seen in Fig. 27 and Fig. 28 the KG-values at an initial temperature of 20 °C grow with increasing volume. Only the KG-values determined in a 40-dm3 and a 1250-dm3 vessel are lower than the values determined in the other volumes. The reason is to find in the shape of the vessels. Both vessels are cylindrical, but the H/D-ratio (in case of 1250-dm3 vessel it is the D/L-ratio) are not suitable for the determination of KG-values of gas explosions. If one assumes spherical flame propagation from the ignition source only a small volume is taken by the flame when it reaches the vessel walls. In case of the 40-dm3 vessel the volume of the spherical flame at this stage is 20-dm3, so that only half of the volume reacted. Due to the precompression of the unburnt mixture the amount of the burnt mixture is less than the half. The heat losses when the flame touches the walls and the low amount of burnt gas mixture lead to low KG-values. Such effects do not occur in spherical vessels and also not in cylindrical vessels with a H/D-ratio of about 1. Nevertheless the general volume dependence (increasing KG-value with increasing volume) is found if one compares only the KG-values determined in the 40-dm3 and the 1250-dm3 vessel.


0

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0 10 20 30 40 50 60 70 80 9H2 [mol-%]

KG [b

ar m

/s]

0

BAM 2.8-dm^3 BAM 6.0-dm^3BAM 14.0-dm^3 WUT 40.0-dm^3WUT 1250-dm^3 INERIS 2000-dm^3 "spark"

Fig. 27: KG-values of hydrogen/air mixtures at an initial pressure of 1 bara and an initial

temperature of 20 °C, determined in different volumes

At a higher initial temperature of 200 °C this volume dependence reversed. The KG-values determined in a 2.8-dm3, a 6.0-dm3, a 40-dm3 and a 2000-dm3 vessel decrease with increasing volume (Fig. 29 and Fig. 30). The behaviour of the KG-values dependence on the hydrogen content in the mixtures is comparable to that for the explosion pressure ratios. The highest values were found close to stoichiometric mixtures. Only at the UEL the slope of the course of the KG-values is significant lower than for the explosion pressure ratios.


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3.1.2 Pressure dependence 3.1.2.1 Explosion limits In order to determine the pressure influence on the explosion limits of hydrogen/air mixtures the LEL and UEL were determined at initial pressures of 1 bara, 5 bara, 10 bara and 30 bara. In Fig. 31 the experimentally determined LEL and UEL are shown for an initial temperature of 20 °C. The values are listed in Tab. 6 and Tab. 7. With increasing initial pressure, the LEL slightly increase whereas the UEL decrease. This behaviour of hydrogen is contrary to that of most other flammable gases of the hydrocarbon type. The effect is larger at the UEL. This pressure dependence lead to smaller explosion ranges with increasing pressure. If we presuppose that there is no volume influence on the explosion limits for higher initial pressures (see chapter 3.1.1.1) the same tendencies were also determined for initial temperatures of 100 °C (Fig. 32, Tab. 8), 200 °C (Fig. 33, Tab. 9) and 250 °C (Fig. 34, Tab. 10).


0

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0 5 10 15 20 25 30 35initial pressure [bara]

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mol

-%]

BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3WUT 40-dm^3 WUT 1250-dm^3 INERIS 2000-dm^3

Fig. 31: Pressure dependence of the explosion limits of hydrogen/air mixtures at an initial


Tab. 6: LEL of hydrogen/air mixtures at different initial pressures and an initial temperature of 20 °C, determined in different vessel volumes

p0 [bara] Vessel volume [dm3] 2.8 6.0 14.0 40.0 1250 2000

1 4.1 3.9 4.1 4.7 5.8 4.1 5 - 4.6 4.6 - - -

10 4.8 4.9 - - - - 30 5.2 - - - - -

Tab. 7: UEL of hydrogen/air mixtures at different initial pressures and an initial temperature of 20 °C, determined in different vessel volumes

p0 [bara] Vessel volume [dm3] 2.8 6.0 14.0 40.0 1250 2000

1 77.0 77.8 77.2 74.8 75.2 77.2 5 - 73.8 74.4 - - -

10 71.8 72.8 - - - - 30 72.2 - - - - -


0

10

20

30

40

50

60

70

80

90

100


H2 [

mol

-%]

BAM 2.8-dm^3 BAM 6.0-dm^3



Tab. 8: Explosion limits of hydrogen/air mixtures at different initial pressures and an initial temperature of 100 °C, determined in different volumes

Vessel volume [dm3] 2.8 6.0 2.8 6.0

p0 [bara] LEL [mol-%] UEL [mol-%] 1 - 3.8 - 81.4 5 - 4.2 - 77.0

10 4.5 - 74.2 - 30 5.1 - 75.4 -




p0 [bara] LEL [mol-%] UEL [mol-%] 1 3.2 4.0 84.4 81.4 5 - 3.9 - 80.0

10 4.1 4.3 78.0 73.6 30 4.6 - 78.8 -

0

10

20

30

40

50

60

70

80

90

100


H2 [

mol

-%]





0

10

20

30

40

50

60

70

80

90

100


H2 [

mol

-%]






p0 [bara] LEL [mol-%] UEL [mol-%] 1 - 3.4 - 85.6 5 - 3.6 - 81.2

10 3.9 - 78.4 - 30 4.5 - 80.4 -

3.1.2.2 Explosion pressure ratios In Fig. 35 to Fig. 38 it is evident that the explosion pressure ratios do not depend significantly on the initial pressure. Particularly in the stoichiometric range, close to the maximum explosion pressure, there is a slight influence of the initial pressure. In this range an increase in initial pressure leads to slightly higher explosion pressure ratios. Also clear in the figures is the pressure dependence of the UEL. With increasing initial pressure the UEL is shifted to lower hydrogen concentrations. The widest explosion range is therefore reached at the lowest initial pressure.


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

11,0

0 10 20 30 40 50 60 70 80 9

H2 [mol-%]

p ex/p

i [-]

0

BAM 6.0-dm^3 1 bara BAM 6.0-dm^3 5 baraBAM 2.8-dm^3 10 bara BAM 2.8-dm^3 30 baraINERIS 2000-dm^3 1 bara "hot wire" INERIS 2000-dm^3 1bara "spark"INERIS 2000-dm^3 10 bara "spark"

Fig. 35: Explosion pressure ratios of hydrogen/air mixtures at different initial pressures and an


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

0 10 20 30 40 50 60 70 80 9

H2 [mol-%]

p ex/p

i [-]

0

BAM 6.0-dm^3 1 baraBAM 6.0-dm^3 5 baraBAM 2.8-dm^3 10 baraBAM 2.8-dm^3 30 bara




0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

6,0

0 10 20 30 40 50 60 70 80

H2 [mol-%]

p ex/p

i [-]

90




0,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

4,0

4,5

5,0

5,5

0 10 20 30 40 50 60 70 80

H2 [mol-%]

p ex/p

i [-]

90





3.1.2.3 KG-values The pressure influence on the rates of explosion pressure rise (dp/dt)ex and therefore on the KG-values is extremely large. The determined KG-values are plotted in Fig. 39 (20 °C), Fig. 40 (100 °C), Fig. 41 (200 °C) and Fig. 42 (250 °C). At all initial temperatures the KG-values increase substantially. For instance at an initial temperature of 20 °C (Fig. 39) the highest KG-value for a mixture with 35 mol-% H2 increase from 770 bar/m s at p0 = 1 bara, 3144 bar/m s at p0 = 5 bara, 5610 bar/m s at p0 = 10 bara up to 10940 bar/m s at p0 = 30 bara. It has to be mentioned that these values have been determined by single ignitions. Therefore it is not reasonable to use the plotted values e.g. for the calculation of venting areas. For such purposes much more ignitions are necessary in order to determine the KG-value at fixed initial conditions including the mixture composition more precisely.

0

2000

4000

6000

8000

10000

12000

14000

0 10 20 30 40 50 60 70 80

H2 [mol-%]

K G [b

ar m

/s]

90

BAM 6.0-dm^3 1 bara BAM 6.0-dm^3 5 baraBAM 2.8-dm^3 10 bara BAM 2.8-dm^3 30 baraINERIS 2000-dm^3 1 bara "spark" INERIS 2000-dm^3 10 bara "spark"

Fig. 39: KG-values of hydrogen/air mixtures at an initial temperature of 20 °C, determined at

different initial pressures and in different volumes


0

2000

4000

6000

8000

10000

12000

14000

0 10 20 30 40 50 60 70 80

H2 [mol-%]

KG [b

ar m

/s]

90

BAM 6.0-dm^3 1 bara

BAM 6.0-dm^3 5 bara

BAM 2.8-dm^3 10 bara




0

2000

4000

6000

8000

10000

12000

14000

0 10 20 30 40 50 60 70 80

H2 [mol-%]

K G [b

ar m

/s]

90

BAM 6.0-dm^3 1 bara

BAM 6.0-dm^3 5 bara






0

2000

4000

6000

8000

10000

12000

0 10 20 30 40 50 60 70 80

H2 [mol-%]

K G [b

ar m

/s]

90

BAM 6.0-dm^3 1 bara

BAM 6.0-dm^3 5 bara





3.1.2.4 Normalized KG-values In order to present another effect of increasing initial pressures the KG-values shown in chapter 3.1.2.3 are normalized by division through the initial pressure. In Fig. 43 to Fig. 46 these KG/pI-values are plotted for the different hydrogen/air mixtures at different initial temperatures. In Fig. 43 it can be seen that the normalized KG-values decrease with increasing initial pressure at an initial temperature of 20 °C. This means that the KG-values increase with increasing pressure, but the combustion reaction and therefore the rate of pressure rise is getting slower relative to the reaction at an initial pressure of 1 bara. A similar tendency was observed at an initial temperature of 100 °C (Fig. 44). Due to the determination of the rates of explosion pressure rise by only single ignitions the value at an initial pressure of 5 bara and 35 mol-% hydrogen is too low. At initial temperature of 200 °C (Fig. 45) and 250 °C (Fig. 46) the above mentioned tendency was not found. The highest normalized KG/pI-values were found at an initial pressure of 10 bara (Fig. 45) respectively 5 bara (Fig. 46) and can not be explained by carrying out only single experiments for the determination. An explanation might be also an anomaly as it is known for the pressure dependence of the explosion limits of hydrogen (see chapter 3.1.5.1).


0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 8H2 [mol-%]

KG/p

i [ba

r m/s

bar

]

0


Fig. 43: Normalized KG-values of hydrogen/air mixtures at an initial temperature of 20 °C,

determined at different initial pressures and in different volumes

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 80

H2 [mol-%]

KG/p

i [ba

r m/s

bar

]

90

BAM 6.0-dm^3 1 bara

BAM 6.0-dm^3 5 bara






0

100

200

300

400

500

600

700

800

0 10 20 30 40 50 60 70 80

H2 [mol-%]

KG/p

i [ba

r m/s

bar

]

90

BAM 6.0-dm^3 1 bara

BAM 6.0-dm^3 5 bara





0

100

200

300

400

500

600

0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00

H2 [mol-%]

KG

/pi [

bar m

/s b

ar]

BAM 6.0-dm^3 1 bara

BAM 6.0-dm^3 5 bara






3.1.3 Temperature dependence In this chapter mainly the same explosion indices are shown as it was done in chapter 3.1.2, but the presentation of diagrams and tables is now aligned to the temperature dependence. 3.1.3.1 Explosion limits The temperature dependence of the explosion limits at initial pressures of 1 bara, 5 bara, 10 bara and 30 bara is illustrated in Fig. 47 to Fig. 50. For all initial pressures a decreasing LEL and increasing UEL was observed with an increasing initial temperature. The decrease at the LEL is less than the increase at the UEL. In order to represent more particularly the small change at the LEL the explosion limits are listed again in Tab. 11 to Tab. 15.

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140 160 180 200 220 240 260initial temperature [°C]

H2 [

mol

-%]

BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3WUT 40-dm^3 WUT 1250-dm^3 INERIS 2000-dm^3

Fig. 47: Temperature dependence of the explosion limits of hydrogen/air mixtures at an initial

pressure of 1 bara, determined in different vessel volumes

Tab. 11: LEL of hydrogen/air mixtures at different initial temperatures and an initial pressure of 1 bara, determined in different vessel volumes

T0 [°C] Vessel volume [dm3] 2.8 6.0 14.0 40.0 1250 2000

20 4.1 3.9 4.1 4.7 5.8 4.1 100 - 3.8 - 3.8 - - 200 3.2 4.0 - 3.2 - - 250 - 3.4 - - - -


Tab. 12: UEL of hydrogen/air mixtures at different initial temperatures and an initial pressure of 1 bara, determined in different vessel volumes

T0 [°C] Vessel volume [dm3] 2.8 6.0 14.0 40.0 1250 2000

20 77.0 77.8 77.2 74.8 75.2 77.2 100 - 81.4 - 78.5 - - 200 82.4 84.4 - 82.2 - - 250 - 85.6 - - - -

0

10

20

30

40

50

60

70

80

90

100


H2 [

mol

-%]

BAM 6-dm^3 BAM 14-dm^3



Tab. 13: Explosion limits of hydrogen/air mixtures at different initial temperatures and an initial pressure of 5 bara, determined in different vessel volumes

Vessel volume [dm3] 6.0 14.0 6.0 14.0

T0 [°C] LEL [mol-%] UEL [mol-%] 20 4.6 4.6 73.8 74.4

100 4.2 - 77.0 - 200 3.9 - 80.0 - 250 3.6 - 81.2 -


0

10

20

30

40

50

60

70

80

90

100


H2 [

mol

-%]




0

10

20

30

40

50

60

70

80

90

100


H2 [

mol

-%]

BAM 2.8-dm^3


pressure of 30 bara, determined in a 2.8-dm3 vessel


Tab. 14: Explosion limits of hydrogen/air mixtures at different initial temperatures and an initial pressure of 10 bara, determined in different vessel volumes


T0 [°C] LEL [mol-%] UEL [mol-%] 20 4.8 4.9 71.8 72.8

100 4.5 - 74.2 - 200 4.1 4.3 78.0 73.6 250 3.9 - 78.4 -

Tab. 15: Explosion limits of hydrogen/air mixtures at different initial temperatures and an initial pressure of 30 bara (2.8-dm3 vessel)

T0 [°C] LEL [mol-%] UEL [ mol-%] 20 5.2 72.2

100 5.1 75.4 200 4.6 78.5 250 4.5 82.2

3.1.3.2 Explosion pressure ratios With increasing initial temperature the explosion pressure ratios decrease (Fig. 51 to Fig. 54). This statement is valid for all examined initial pressures and is most obvious for hydrogen/air mixtures close to the stoichiometric composition. Furthermore the temperature dependence of the explosion limits particularly the UEL is shown in another way. With increasing initial temperature the explosion range is getting broader.


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

11,0

0 10 20 30 40 50 60 70 80 9H2 [mol-%]

p ex/p

i [-]

0

BAM 6.0-dm^3 20 °C BAM 6.0-dm^3 100 °C BAM 6.0-dm^3 200 °C

BAM 6.0-dm^3 250 °C WUT 40.0-dm^3 20 °C WUT 40.0-dm^3 100 °C

WUT 40.0-dm^3 200 °C

Fig. 51: Explosion pressure ratios of hydrogen/air mixtures at an initial pressure of 1 bara,

determined at different initial temperatures and in different volumes

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

0 10 20 30 40 50 60 70 80

H2 [mol-%]

p ex/p

i [-]

90

BAM 6.0-dm^3 20 °C

BAM 6.0-dm^3 100 °C

BAM 6.0-dm^3 200 °C

BAM 6.0-dm^3 250 °C


determined at different initial temperatures


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

11,0

0 10 20 30 40 50 60 70 80 9

H2 [mol-%]

p ex/p

i [-]

0

BAM 2.8-dm^3 20 °C BAM 2.8-dm^3 100 °C BAM 2.8-dm^3 200 °C

BAM 2.8-dm^3 250 °C INERIS 2000-dm^3 20 °C INERIS 2000-dm^3 200 °C


determined at different initial temperatures and in different volumes

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

0 10 20 30 40 50 60 70 80

H2 [mol-%]

p ex/p

i [-]

90

BAM 2.8-dm^3 20 °C

BAM 2.8-dm^3 100 °C

BAM 2.8-dm^3 200 °C

BAM 2.8-dm^3 250 °C


determined at different initial temperatures


3.1.3.3 KG-values In Fig. 55 to Fig. 58 the KG-values for different hydrogen/air mixtures at initial pressures of 1 bara, 5 bara, 10 bara and 30 bara are shown for initial temperatures of 20 °C, 100 °C, 200 °C and 250 °C. An increase of the initial temperature leads to a decrease of the KG-value. This clear tendency was only determined for hydrogen/air mixtures in the range of the stoichiometric composition. Outside of this region (LEL 25 mol-% H2 respectively 45 mol-% H2 UEL) more or less the same KG-values were observed at the different initial temperatures (same initial pressure).

0

100

200

300

400

500

600

700

800

900

0 10 20 30 40 50 60 70 80

H2 [mol-%]

K G [b

ar m

/s]

90

BAM 6.0-dm^3 20 °CBAM 6.0-dm^3 100 °CBAM 6.0-dm^3 200 °CBAM 6.0-dm^3 250 °CWUT 40.0-dm^3 20 °CWUT 40.0-dm^3 100 °CWUT 40.0-dm^3 200 °C

Fig. 55: KG-values of hydrogen/air mixtures at an initial pressure of 1 bara, determined at different

initial temperatures and in different volumes


0

500

1000

1500

2000

2500

3000

3500

0 10 20 30 40 50 60 70 80 9

H2 [mol-%]

KG [b

ar m

/s]

0

BAM 6.0-dm^3 20 °C

BAM 6.0-dm^3 100 °C

BAM 6.0-dm^3 200 °C

BAM 6.0-dm^3 250 °C

Fig. 56: KG-values of hydrogen/air mixtures at an initial pressure of 5 bara, determined at different

initial temperatures

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 10 20 30 40 50 60 70 80

H2 [mol-%]

K G [b

ar m

/s]

90

BAM 2.8-dm^3 20 °C BAM 2.8-dm^3 100 °C

BAM 2.8-dm^3 200 °C BAM 2.8-dm^3 250 °C

INERIS 2000-dm^3 20 °C "spark" INERIS 2000-dm^3 200 °C "spark"

Fig. 57: KG-values of hydrogen/air mixtures at an initial pressure of 10 bara, determined at

different initial temperatures and in different volumes


0

2000

4000

6000

8000

10000

12000

14000

0 10 20 30 40 50 60 70 80

H2 [mol-%]

KG [b

ar m

/s]

90

BAM 2.8-dm^3 20 °C

BAM 2.8-dm^3 100 °C

BAM 2.8-dm^3 200 °C

BAM 2.8-dm^3 250 °C

Fig. 58: KG-values of hydrogen/air mixtures at an initial pressure of 30 bara, determined at

different initial temperatures

3.1.4 Flame propagation: dependence on hydrogen concentration The flame propagation especially close to the explosion limits is always of great interest. At Warsaw University of Technology it is possible to take high speed videos in the 40-dm3 chamber. Therefore a window of XXX mm diameter is installed in a height of 245 mm. In Fig. 59 to Fig. 61 the flame propagation in hydrogen/air mixtures at initial conditions of 1 bara and 20 °C are shown. A mixture with 6.2 mol-% hydrogen, which is close to the LEL of 4.7 mol-%, it can be seen that after the spark ignition a small and pale flame is slightly growing while rising up to the top due to the convection (Fig. 59). An ignition of a mixture which contains 42.5 mol-% hydrogen is shown in Fig. 60. At this mixture composition the highest burning velocity was determined. After the spark ignition already 40 ms later the flame took the whole volume. A special effect can be seen 20 ms after the ignition. There are small and brighter areas spread over the volume. It looks like flying particles which are definitely not in the vessel and can not be generated during a hydrogen/air explosion. This effect was also observed in ignitions with ammonia/air mixtures. It has to be checked whether the formation of NOx might be a reason.


t=0 20 ms 40 ms

60 ms 80 ms 100 ms

Fig. 59: Flame propagation in a hydrogen/air mixture with 6.2 mol-% H2, close to the LEL of 4.7 mol-% (spark ignition, 40-dm3 vessel). The pictures were recorded with 50 fps.

t=0 20 ms 40 ms

Fig. 60: Flame propagation in a hydrogen/air mixture with 42.5 mol-% H2. At this hydrogen concentration the maximum flame velocity was determined (spark ignition, 40-dm3 vessel). The pictures were recorded with 50 fps.

The flame propagation in a hydrogen/air mixture close to the UEL is different to the propagation at lower hydrogen contents. One example is given in Fig. 61. During the first 60 ms after the ignition a small flame detaches from the ignition source, but this flame is not growing. Than, 20 ms later, the whole vessel volume is getting brighter. Another 20 ms later the vessel is alight. The colour of the pictures is an effect of the recording system and not the true colour of the flame. This flame propagation is comparable to the propagation shown in Fig. 60, but much slower and not so intensive. Another important question is how it looks when an ignition occurred, given it was not an explosion according to the defined explosion criterion and was it the right criterion for the determination of the explosion indices [11]. Two examples for those cases are shown in Fig. 62 and Fig. 63. In Fig. 62 six pictures of an ignition attempt carried out with a mixture of 6.0 mol-% hydrogen in air are shown. At first the flame propagation looks like that one shown in Fig. 59, but in the time period of 80 ms to 100 ms the small pale flame is extinguished. The heat release up to this moment was not enough to produce a pressure increase greater than the 5% pressure criterion although the fuel gas concentration was inside the explosion range.


t=0 20 ms 40 ms

60 ms 80 ms 100 ms

120 ms 140 ms 160 ms

Fig. 61: Flame propagation in a hydrogen/air mixture with 71.0 mol-% H2, close to the UEL of 74.8 mol-% (spark ignition, 40-dm3 vessel). The pictures were recorded with 50 fps.

t=0 20 ms 40 ms

60 ms 80 ms 100 ms

Fig. 62: Quenching process in a hydrogen/air mixture with 6.0 mol-% H2, close to the LEL of 4.7 mol-%, but no explosion was determined according to the ignition criterion (spark ignition, 40-dm3 vessel). The pictures were recorded with 50 fps.


An ignition attempt carried out with a hydrogen concentration 0.5 mol-% higher than the determined UEL is presented in Fig. 63. It is obvious that no flame propagation occurred. The flame initially produced by the ignition source did not detach the ignition electrodes.

t=0 20 ms 40 ms

Fig. 63: Quenching process in a hydrogen/air mixture with 75.3 mol-% H2, 0.5 mol-% higher than the UEL. No explosion was determined according to the ignition criterion (spark ignition, 40-dm3 vessel)

3.1.5 Discussion The explosion indices of hydrogen/air mixtures have been determined over a wide range of initial conditions, so that the volume, pressure and temperature dependence of explosion limits, explosion pressures, respectively explosion pressure ratios, as well as rates of pressure rise, respectively KG-values, could be pointed out. The dependency of each parameter for its own was already presented in the chapters 3.1.1, 3.1.2 and 3.1.3. At this point a more combined view will be given. 3.1.5.1 Explosion limits In Fig. 64 and Fig. 65 the temperature dependency of the explosion limits of hydrogen in air is shown for different initial pressures and different volumes. The greatest influence on the LEL (Fig. 64) and the UEL (Fig. 65) is caused by temperature and pressure increase. The volume dependence on the explosion limits of hydrogen can be neglected. A temperature increase leads to a decreasing trend for the LEL and an increasing one for the UEL. Therefore the explosion range gets broader with increasing temperature. An additional pressure increase, has the opposite effect. For instance the explosion range gets smaller when the initial conditions increase from 1 bara, 20 °C to 5 bara, 100 °C and to 10 bara, 200 °C in the 6.0-dm3 vessel. This statement is not valid in general due to the pressure anomaly of the UEL of hydrogen. After decreasing for initial pressures up to 20 bara, the UEL increases again (Fig. 66) [8] [9]. Therefore the above mentioned statement is not valid for temperature increases and a pressure increase higher than 20 bara at the same time. The LEL is more or less constant when increasing the initial conditions from 1 bara, 20 °C to 10 bara, 100 °C and 30 bara, 200 °C in the 2.8-dm3 vessel. However the UEL decreases first and increases again, and in sum the explosion range also.


3

3,5

4

4,5

5

5,5

6

6,5


H2 [

mol

-%]

BAM 2.8-dm^3 1 bar BAM 2.8-dm^3 10 bar BAM 2.8-dm^3 30 bar

BAM 6.0-dm^3 1 bar BAM 6.0-dm^3 5 bar BAM 6.0-dm^3 10 bar

WUT 40-dm^3 1 bar

Fig. 64: Temperature dependence of LEL of hydrogen/air mixtures, determined at different initial

pressures and in different volumes

70

72

74

76

78

80

82

84

86

88

90


H2 [

mol

-%]

BAM 2.8-dm^3 1 bar BAM 2.8-dm^3 10 bar BAM 2.8-dm^3 30 barBAM 6.0-dm^3 1 bar BAM 6.0-dm^3 5 bar BAM 6.0-dm^3 10 barWUT 40-dm^3 1 bar

Fig. 65: Temperature dependence of UEL of hydrogen/air mixtures, determined at different initial

pressures and in different volumes


0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150pI [bara]

3

4

5

6

70

72

74

76

78

80H

2 [m

ol-%

]

Fig. 66: Pressure dependence of the explosion limits of hydrogen in air at an initial temperature of

20 °C [8] [9]

3.1.5.2 Explosion pressure ratios No significant volume dependence on the explosion pressure ratios was determined. But there are some dependencies on pressure and temperature, especially for hydrogen/air mixtures close to stoichiometric. In Tab. 16 the highest determined pressure ratios are listed for the quoted initial conditions of pressure and temperature. In most cases the highest values were determined for a mixture with 30 mol-% hydrogen. It has to be remembered that the step distance of the mixture variation was 5 mol-%. It has to be mentioned that these values are the result of single experiments and were not determined according to existing standards [10] for the determination of the maximum explosion pressure pmax.

Tab. 16: Highest explosion pressure ratios at different initial pressures and temperatures, determined by single ignition attempts in a 2.8-dm3 (p0 = 10 bara and 30 bara) and a 6.0-dm3 (p0 = 1 bara and 5 bara) cylindrical steel vessel

initial pressure [bara]

explosion pressure ratio pex/pi [-]

20 °C 100 °C 200 °C 250 °C 1 7.91 6.20 5.26 4.65 5 8.04 6.49 5.27 4.82

10 8.33 6.74 5.46 4.85 30 7.69 6.65 5.60 5.16


The pressure dependence of the highest explosion pressure ratios at a fixed initial temperature can be seen reading the Tab. 16 from the top to the bottom. A slight increase for the values can be seen. For the temperature dependence at a fixed initial pressure one has to compare the values from the left to the right. A significant decrease in the pressure ratios with increasing initial temperature is obvious. A comparison of the experimental determined explosion pressure ratios to the thermodynamically calculated ratios [13] was done. One example is shown in Fig. 67 for the pressure ratios at an initial pressure of 10 bara. The course of the explosion pressure ratios are in good correspondence to each other except outside from the explosion range. Up to now it is not possible to calculate the explosion range respectively the explosion limits only on the basis of thermodynamics.

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

0 10 20 30 40 50 60 70 80 90 1H2 [mol-%]

p ex/p

i [-]

00

BAM 2.8-dm^3 20 °CBAM 2.8-dm^3 100 °CBAM 2.8-dm^3 200 °CBAM 2.8-dm^3 250 °Ccalculation 20°Ccalculation 100°Ccalculation 200°Ccalculation 250°C

Fig. 67: Experimental and calculated explosion pressure ratios of hydrogen/air mixtures at an

initial pressure of 10 bara and different initial temperatures

Usually the calculated ratios are a bit higher than the experimentally determined ratios. The reason is mainly as a result of heat losses during the experiments. Nevertheless there are some experimental values in excess over the calculated trend of explosion pressure ratios (e.g. Ti = 200 °C). Here the reason is probably in the analysis of the p-t-curves. Oscillations were observed during the experiments in the hydrogen concentration range of 25 mol-% to 50 mol-% (Fig. 68). Despite the fact that a sampling frequency of 100 kHz was chosen only 10 to 25 pressure points were recorded on an oscillation peak what makes a precise analysis difficult.


0 0.01 0.02 0.03 0.04 0.050

102030405060708090

p [b

ara]

25 mol-% H2

0 0.01 0.02 0.03 0.04 0.050

102030405060708090

30 mol-% H230 mol-% H2

0 0.01 0.02 0.03 0.04 0.05t [s]

0102030405060708090

p [b

ara]

35 mol-% H235 mol-% H2

0 0.01 0.02 0.03 0.04 0.05t [s]

0102030405060708090

50 mol-% H250 mol-% H2

4_P_T_H2AIR_28L_10BARA_200oC | 5.1.2006 Fig. 68: Pressure-time histories of ignition attempts with different hydrogen/air mixtures at an initial

pressure of 10 bara and an initial temperature of 200 °C (2.8-dm3 vessel)

3.1.5.3 KG-values The KG-value is the explosion index of all determined which shows the strongest dependence on the vessel volume and its shape. According to the cubic law it should be largely independent but in fact the KG-values depend in many cases on the vessel volume, because the flame propagation is often disturbed and not laminar in large vessels. With increasing initial pressure at the same initial temperature, the KG-values increase considerably. For instance, at an initial temperature of 20 °C the highest KG-value at 30 bara is about 14 times, at 10 bara about 7 times and at 5 bara still 4 times higher than at 1 bara (Tab. 17). Nevertheless relative to an initial pressure of 1 bara the combustion reaction is getting slower with increasing pressure, as it was shown by the normalized KG-values in chapter 3.1.2.4. With increasing initial temperature these factors grow, although the KG-values decrease. In order to identify the highest experimentally determined KG-values are listed in Tab. 17 for the different initial conditions. In the brackets are given the factors how many times this KG-value is higher than at p0 = 1 bara at the same initial temperature. Because the listed values were determined by single ignition attempts there are some values that deviate, mainly the values at Ti = 20 °C and pi = 10 bara and 30 bara as well as the value at Ti = 100 °C and pi = 10 bara. The measured oscillations as they were presented in chapter 3.1.5.2 did not influence the determination of the KG-values. The oscillations always occurred behind the first pressure increase after ignition. In contrast to the explosion pressure ratios, the highest KG-values were determined at 35 mol-% hydrogen in the mixtures. This shows that both maximum explosion indices do not inevitably occur at the same mixture composition.


Tab. 17: Highest KG-value at different initial pressures and temperatures, determined by single ignitions in a 2.8-dm3 (p0 = 10 bara and 30 bara) and a 6.0-dm3 (p0 = 1 bara and 5 bara) cylindrical steel vessel


KG-value [bar/m s]

20 °C 100 °C 200 °C 250 °C 1 770 775 570 450 5 3144 (4.0) 3029 (3.9) 2786 (4.9) 2704 (6.0)

10 5610 (7.3) 6135 (7.9) 6772 (11.9) 4548 (10.1) 30 10940 (14.2) 12780 (16.4) 11436 (20.0) 11170 (24.8)

3.2 Hydrogen/oxygen mixtures As additional experiments, the explosion indices for hydrogen/oxygen systems were determined at initial conditions of 1 bara and 20 °C. Due to the energetics of the explosion reactions of that gas system it was not possible to carry out experiments at higher initial pressures and temperatures without a lot of modifications of the experimental setup. The explosion limits of the hydrogen/oxygen system were determined at 3.9 mol-% (LEL) and 95.8 mol-% (UEL). In Fig. 69 the measured explosion pressure ratios are shown and compared to the explosion pressure ratios of hydrogen/air mixtures at the same initial conditions. The difference of the ratios of hydrogen/air and hydrogen/oxygen mixtures is only about 2 bar for mixtures close to stoichiometric composition (29.5 mol-% H2 in air, 66.7 mol-% H2 in oxygen). Nevertheless, these higher pex/pi ratios do not correspond to the expected values when considering the system energy only. Also shown are thermodynamically calculated explosion pressure ratios of hydrogen/air and hydrogen/oxygen mixtures (Fig. 69). Therefore the SAFEKINEX software tool “Explosion pressure” (version 1.3/2004) was used [13]. Both calculated curves show a very good correspondence to the measured values inside the explosion range. Of course, the deviations outside the explosion range are obvious since explosion reactions are thermodynamically possible. Within a hydrogen concentration range of 30 mol-% to 80 mol-% in mixture with pure oxygen oscillations in the pressure–time-histories were detected which make an analysis of these histories difficult (chapter 2.4.1). In Fig. 70 three p-t-curves are shown for ignition attempts with 60 mol-%, 65 mol-% and 70 mol-% hydrogen in the mixture. In these cases the strongest oscillations have been detected. The sampling frequency was 100 kHz for every ignition attempt, so that every 0.01 ms a pressure was recorded. Nevertheless only 5 to 15 points are on the oscillation peaks. In the context of the above mentioned oscillations, the precision of the explosion pressure is ± 0.5 bar for mixtures with 30 mol-% up to 80 mol-% hydrogen. This may be the reason for the deviation of the different hydrogen concentrations for the measured and calculated maximum explosion pressures. The same analysis problems occur in the case of the determination of (dp/dt)ex- and KG-values. The determined KG-values for hydrogen/oxygen mixtures are shown in Fig. 71 and compared the values of hydrogen/air mixtures. For hydrogen concentrations up to 20 mol-% the KG-values are comparable to the values of hydrogen/air mixtures. Higher hydrogen concentrations result in KG-values which are 5 times higher than for hydrogen/air mixtures (close to stoichiometric composition).


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

11,0

0 10 20 30 40 50 60 70 80 90 100H2 [mol-%]

p ex/p

i [-]

BAM 6.0-dm^3 H2/air

BAM 6.0-dm^3 H2/O2

calculation H2/air

calculation H2/O2

Fig. 69: Explosion pressure ratios of hydrogen/oxygen mixtures compared to those of hydrogen-air

mixtures at an initial pressure of 1 bara and an initial temperature of 20 °C. Also shown are explosion pressure ratios calculated with the SAFEKINEX software tool “Explosion pressure” (version 1.3/2004).

0 0.01 0.02 0.03 0.04 0.050

2

4

6

8

10

12

14

p [b

ara]

60 mol-% H2

0 0.01 0.02 0.03 0.04 0.050

2

4

6

8

10

12

14

65 mol-% H265 mol-% H2

0 0.01 0.02 0.03 0.04 0.05t [s]

0

2

4

6

8

10

12

14

p [b

ara]

70 mol-% H270 mol-% H2

3_P_T_H2O2_6L_1BARA_20oC | 5.1.2006 Fig. 70: Pressure-time histories of ignition attempts with three different hydrogen/oxygen mixtures


0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 10 20 30 40 50 60 70 80 90 100

H2 [mol-%]

KG [b

ar m

/s]

BAM 6.0-dm^3 H2/air

BAM 6.0-dm^3 H2/O2

Fig. 71: KG-values for hydrogen/oxygen mixtures at an initial pressure of 1 bara and an initial

temperature of 20 °C


3.3 Methane Methane is one of the most known and used fuel gases. It is the main component of natural gas. Although it might be the best examined gas regarding explosion safety, but until now not all dependencies and combustion behaviour are known. To ensure also the safe handling of methane in all situations it is important to know the safety related properties at all necessary conditions. Therefore it was examined as well as hydrogen over a wide range of operating conditions. 3.3.1 Volume dependence In order to determine the volume dependence of the explosion indices the same combinations of initial pressure and temperature were chosen as it was done for hydrogen 3.3.1.1 Explosion limits The experimental results for the explosion limits of methane/air mixtures at the different initial conditions are shown in Fig. 72 to Fig. 75. At all examined initial conditions the LEL decreases slightly with increasing volume. The volume dependence of the UEL is much greater. At an initial pressure of 1 bara and an initial temperature the explosion limits decreases dramatically from 23.6 mol-% methane in a 2.8-dm3 vessel to 14.8 mol-% methane in a 2000-dm3 vessel (Fig. 72). Because of the great difference between the determined UEL in a 2.8-dm3 vessel and the 6.0-dm3 vessel one can deduce that at an initial pressure of 1 bara an overdriven initiation caused by the ignition source is possible. This over initiation causes a forced flame propagation which is not self-sustaining.

0

5

10

15

20

25

30

35

0,001 0,01 0,1 1 10volume [m3]

CH

4 [m

ol-%

]

BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3TU Delft 20-dm^3 BASF 20-dm^3 WUT 40-dm^3WUT 1250-dm^3 INERIS 2000 dm^3 (hot spot) INERIS 20-dm^3 (expl. wire)INERIS 8.0-dm^3 (expl. wire) INERIS 8.0-dm^3 (hot spot)

Fig. 72: Volume dependence of the explosion limits of methane/air mixtures at an initial pressure



INERIS determined the explosion limits in a 8.0-dm3 vessel with two different ignition sources. The differences are obvious at the UEL. By use of the exploding wire igniter the UEL was determined at 16.5 mol-% methane, with a hot spot the UEL was found at 14.9 mol-% methane. It is remarkable that nearly the same LEL and UEL was found in a 2000-dm3 vessel when the hot spot igniter was used. At all other initial conditions (10 bara 20 °C Fig. 73, 1 bara 200 °C Fig. 74, 10 bara 200 °C Fig. 75) the same volume dependence of the explosion limits was found with two exceptions. The first one is that the overdriven initiation effect was found in the 2.8-dm3 volume only at initial pressure of 1 bara, independent of the initial temperature. The second one a higher UEL in a 20-dm3 vessel only at an initial pressure of 10 bara. At initial conditions of 1 bara and 200 °C (Fig. 74) the UEL determined in the 6.0-dm3 vessel might be influenced by overdriven initiation effects, too. Nevertheless the volume dependency is the same as described above. The over initiation effects of the ignition source will become more obvious when the pressure and temperature dependencies are presented (chapters 3.3.2.1 and 3.3.3.1).

0

5

10

15

20

25

0,001 0,01 0,1 1 10volume [m3]

CH

4 [m

ol-%

]

BAM 2.8-dm^3 BAM 6.0-dm^3 TU Delft 20-dm^3 INERIS 2000-dm^3




0

5

10

15

20

25

30

0,001 0,01 0,1 1volume [m3]

CH

4 [m

ol-%

]

BAM 2.8-dm^3BAM 6.0-dm^3TU Delft 20-dm^3WUT 40-dm^3



0

5

10

15

20

25

30

0,001 0,01 0,1 1volume [m3]

CH

4 [m

ol-%

]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3




3.3.1.2 Explosion pressure ratios There was no volume dependence found for the explosion pressure ratios of methane/air mixtures at the examined initial conditions. The differences which can be seen in Fig. 76 to Fig. 79 are mainly caused by the different explosion limits determined in the different vessels. The trends of the pressure ratios in the different volumes are comparable for methane concentrations close to the UEL. One fact already mentioned in chapter 3.3.1.1 can be seen again in Fig. 76: the overdriven initiation effect by the exploding wire igniter in the 2.8-dm3 vessel. Over a range from 18.0 mol-% to the UEL at 23.6 mol-% no real pressure rise was detected for the different methane concentrations. The explosion pressure ratios were always between 1.06 and 1.12. High enough to overstep the defined explosion criterion, but there has to be some doubt that there was a self-sustaining flame propagation.

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

p ex/p

i [-]

BAM 2.8-dm^3

BAM 6.0-dm^3

BAM 14.0-dm^3

TU Delft 20-dm^3

BASF 20-dm^3

WUT 40-dm^3

WUT 1250-dm^3

INERIS 20-dm^3

INERIS 2000-dm^3

Fig. 76: Explosion pressure ratios of methane/air mixtures at an initial pressure of 1 bara and an



0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

0,0 5,0 10,0 15,0 20,0 25,0CH4 [mol-%]

p ex/p

i [-]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3

INERIS 2000-dm^3



0,0

1,0

2,0

3,0

4,0

5,0

6,0

0,0 5,0 10,0 15,0 20,0 25,0 30,0CH4 [mol-%]

p ex/p

i [-]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3

WUT 40-dm^3

INERIS 2000-dm^3




0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

0,0 5,0 10,0 15,0 20,0 25,0 30,0CH4 [mol-%]

p ex/p

i [-]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3



3.3.1.3 KG-values The Fig. 80 to Fig. 83 illustrate the determined dependence of the KG-values on the methane content in the mixtures at the different initial conditions. As already mentioned in chapter 3.1.1.3, the KG-values should be largely volume independent, but in fact they grow with increasing volume at all examined conditions. Only the KG-values determined in a 40-dm3 and a 1250-dm3 vessel seem to be lower than the values determined in the other volumes. But the KG-values of the 1250-dm3 are also higher than those of the 40-dm3 vessel. The reasons for the lower KG-values in the both vessels were already given in chapter 3.1.1.3, where comparable results were obtained for hydrogen. The KG-values determined in the 2.8-dm3 vessel give also a reason to see the high UEL as a fact of overdriven initiation effects from the exploding wire igniter. Comparable to the explosion pressure ratios the determined KG-values in the range from 18.0 mol-% to the UEL at 23.6 mol-% are about 0 bar/m s to 0.5 bar/m s (Fig. 80).


0

20

40

60

80

100

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

KG [b

ar m

/s]

BAM 2.8-dm^3BAM 6.0-dm^3BAM 14.0-dm^3TU Delft 20-dm^3BASF 20-dm^3WUT 40-dm^3WUT 1250-dm^3INERIS 20-dm^3INERIS 2000-dm^3

Fig. 80: KG-values of methane/air mixtures at an initial pressure of 1 bara and an initial


0

500

1000

1500

2000

2500

3000

3500

0,0 5,0 10,0 15,0 20,0 25,0CH4 [mol-%]

KG [b

ar m

/s]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3

INERIS 2000-dm^3




0

10

20

30

40

50

60

70

80

90

100

0,0 5,0 10,0 15,0 20,0 25,0 30,0CH4 [mol-%]

KG [b

ar m

/s]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3

WUT 40-dm^3

INERIS 2000-dm^3



0

100

200

300

400

500

600

0,0 5,0 10,0 15,0 20,0 25,0 30,0CH4 [mol-%]

KG [b

ar m

/s]

BAM 2.8-dm^3

BAM 6.0-dm^3

TU Delft 20-dm^3




3.3.2 Pressure dependence 3.3.2.1 Explosion limits The explosion limits of methane/air mixture were determined at initial pressures of 1 bara, 5 bara and 10 bara at five different initial temperatures. The results are plotted in Fig. 84 to Fig. 88. There was no pressure dependence detected at the LEL. In contrast to this, the UEL increases with increasing initial pressure. This trend was determined at all initial temperatures. The assumption of overdriven initiation effects in too small volumes at an initial pressure of 1 bara and initial temperatures of 20 °C (Fig. 84) and 200 °C (Fig. 87) can be validated in these presentations and are also visible in the presentation of the temperature dependence (chapter 3.3.3.1).

0

5

10

15

20

25

30

0 2 4 6 8 10initial pressure [bara]

CH

4 [m

ol-%

]

12

BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3 TUD 20-dm^3BASF 20-dm^3 WUT 40-dm^3 WUT 1250-dm^3 INERIS 2000 dm^3

Fig. 84: Pressure dependence of the explosion limits of methane/air mixtures at an initial



0

2

4

6

8

10

12

14

16

18

20

22

24

26


CH

4 [m

ol-%

]

12

TUD 20-dm^3

WUT 40-dm^3



0

2

4

6

8

10

12

14

16

18

20

22

24

26


CH

4 [m

ol-%

]

12

TUD 20-dm^3




0

5

10

15

20

25

30

35


CH

4 [m

ol-%

]

12

BAM 2.8-dm^3 BAM 6.0-dm^3 TUD 20-dm^3 WUT 40-dm^3



0

5

10

15

20

25

30


CH

4 [m

ol-%

]

12

TUD 20-dm^3




3.3.2.2 Explosion pressure ratios In order to present the pressure dependence of the explosion pressure ratios only, the results of TU Delft are shown in Fig. 89 to Fig. 93. They carried out the experiments for methane according to the Detailed Project Plan (DPP) [1]. There was no pressure dependence found for methane/air mixture with methane concentration between the LEL and the concentration where the maximum explosion pressure ration was found. For higher methane concentrations the course of the pressure ratios at the different initial pressures is that what can be expected due to the pressure dependence of the UEL. But at initial pressures of 5 bara and 10 bara there was no sharp decrease to a pressure ratio below 1.05 (explosion criterion for EL). The ratios decrease to values of about 1.5 and are stable over a large concentration range. This range increases with increasing initial pressure. At an initial temperature of 20 °C and 5 bara initial pressure the concentration range with such pressure ratios is to find between methane concentrations of 16.5 mol-% to the UEL of 19.1 mol-%. At 10 bara the concentration range is found between 18.0 mol-% to the UEL of 22.0 mol-% (Fig. 89). The higher the initial temperature the smaller are these concentration ranges (Fig. 90, Fig. 91 and Fig. 92), until such ranges were not found at an initial temperature of 240 °C (Fig. 93). The same behaviour can also be seen in chapter 3.3.3.2 where the temperature dependence of the pressure ratios is shown. These deductions have to be used carefully because the exact concentration ranges are strongly dependent to the step size of the variation of the methane concentration.

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0CH4 [mol-%]

p ex/p

i [-]

TUD 1 bar

TUD 5 bar

TUD 10 bar

Fig. 89: Explosion pressure ratios of methane/air mixtures at an initial temperature of 20 °C,

determined at different initial pressures (20-dm3 vessel)


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

p ex/p

i [-]

TUD 1 bar

TUD 5 bar

TUD 10 bar



0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

p ex/p

i [-]

TUD 1 bar

TUD 5 bar

TUD 10 bar




0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

p ex/p

i [-]

TUD 1 bar

TUD 5 bar

TUD 10 bar



0,0

1,0

2,0

3,0

4,0

5,0

6,0

3,0 6,0 9,0 12,0 15,0 18,0 21,0 24,0 27,0 30,0CH4 [mol-%]

p ex/p

i [-]

TUD 1 bar

TUD 5 bar

TUD 10 bar




3.3.2.3 KG-values The pressure influence on the rates of explosion pressure rise (dp/dt)ex and therefore on the KG-values is strong. The determined KG-values are plotted in Fig. 94 to Fig. 98 for the different examined initial temperatures. At all initial temperatures the KG-values increase with increasing initial pressure. For instance, at an initial temperature of 100 °C (Fig. 95) the highest KG-value for a mixture with 10 mol-% methane increase from 75.2 bar/m s at p0 = 1 bara and 237.9 bar/m s at p0 = 5 bara up to 411.6 bar/m s at p0 = 10 bara. Similarly for the KG-values of hydrogen/air mixtures these values have been determined by single ignitions. Therefore it is not reasonable to use the plotted values e.g. for the calculation of venting areas. For such purposes far more ignitions are necessary in order to determine the KG-value at fixed initial conditions including the mixture composition more precisely.

0

50

100

150

200

250

300

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 1 bar

TUD 5 bar

TUD 10 bar

Fig. 94: KG-values of methane/air mixtures at an initial temperature of 20 °C, determined at

different initial pressures (20-dm3 vessel)


0

50

100

150

200

250

300

350

400

450

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 1 bar

TUD 5 bar

TUD 10 bar



0

100

200

300

400

500

600

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 1 bar

TUD 5 bar

TUD 10 bar




0

100

200

300

400

500

600

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 1 bar

TUD 5 bar

TUD 10 bar



0

100

200

300

400

500

600

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 1 bar

TUD 5 bar

TUD 10 bar




3.3.2.4 Normalized KG-values In chapter 3.1.2.4 the normalized KG-values of hydrogen and their pressure dependence were shown. The same will be done is this chapter. Similar to the behaviour for hydrogen/air mixtures the KG/pi-values decrease with increasing initial pressure at an initial temperature of 20 °C (Fig. 99). This means that the KG-values increase with increasing pressure, but the combustion reaction and therefore the rate of pressure rise is getting slower relative to the reaction at an initial pressure of 1 bara. In contrast to hydrogen/air mixtures the same tendency was observed at all other initial temperatures (100 °C Fig. 101, 120 °C Fig. 102, 200 °C Fig. 103 and 240 °C Fig. 105). Thereby the difference between the values determined at 1 bara and 5 bara is much greater than between the values of 5 bara and 10 bara.

0

10

20

30

40

50

60

70

80

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]

TUD 1 bar

TUD 5 bar

TUD 10 bar

Fig. 99: Normalized KG-values of methane/air mixtures at an initial temperature of 20 °C,


This tendency was determined in every volume (e.g. Fig. 104) except one. In the 2000-dm3 vessel a different behaviour is detected and plotted in Fig. 100. Beside the heavy increase of the normalized KG-values with increasing initial pressure there is an other remarkable fact: the spike in the course of the normalized KG-values determined in the 2000-dm3 vessel at 10 mol-% methane for an initial pressure of 1 bara. This spike is not seen in the smaller volume of 20-dm3. But it has to be mentioned that the value of the 20-dm3 vessel from INERIS (Fig. 100) is lower than the value at the same conditions determined in the 20-dm3 vessel from TU Delft (Fig. 99).


0

50

100

150

200

250

300

350

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]

INERIS 20-dm3 1 bar

INERIS 2000-dm^3 1 bar

INERIS 2000-dm^3 10 bar



0

10

20

30

40

50

60

70

80

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]

TUD 1 bar

TUD 5 bar

TUD 10 bar




0

10

20

30

40

50

60

70

80

90

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]TUD 1 bar

TUD 5 bar

TUD 10 bar



0

10

20

30

40

50

60

70

80

90

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]

TUD 1 bar

TUD 5 bar

TUD 10 bar




0

10

20

30

40

50

60

70

80

90

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]TUD 20-dm^3 1 bar

TUD 20-dm^3 5 bar

TUD 20-dm^3 10 bar

BAM 2.8-dm^3 1 bar

BAM 2.8-dm^3 10 bar

BAM 6-dm^3 1 bar

BAM 6-dm^3 10 bar


determined at different initial pressures and in different vessel volumes

0

10

20

30

40

50

60

70

80

90

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]

TUD 1 bar

TUD 5 bar

TUD 10 bar




3.3.3 Temperature dependence 3.3.3.1 Explosion limits In Fig. 106 to Fig. 108 the temperature dependence of the explosion limits of methane/air mixtures at initial pressures of 1 bara, 5 bara and 10 bara is shown. An increase in initial temperature leads to an enlargement of the explosion ranges as a result of a decreasing LEL and a increasing UEL. This dependence is valid for 1 bara initial pressure (Fig. 106) as well as for 5 bara (Fig. 107) and 10 bara (Fig. 108). It has to be mentioned that TU Delft, who carried out the methane experiments according to the Detailed Project Plan (DPP) [1], examined the influence of the ignition energy on the explosion limits, but using the same ignition source. Therefore they carried out the experiments at initial temperatures of 120 °C and 240 °C with ignition energies of about 3 J ± 0.3 J in opposite of 11.5 J ± 1.5 J at the other temperatures. This influences the UEL at all examined initial pressures. The UEL determined with lower energies are lying under the UEL course in dependence of the initial temperature which was found with higher ignition energies. The higher the initial pressure the greater is the influence of the ignition energy. There is also a slight influence on the LEL.

0

5

10

15

20

25

30


CH

4 [m

ol-%

]

BAM 2.8-dm^3 BAM 6.0-dm^3 BAM 14.0-dm^3 TUD 20-dm^3BASF 20-dm^3 WUT 40-dm^3 WUT 1250-dm^3 INERIS 2000 dm^3

Fig. 106: Temperature dependence of the explosion limits of methane/air mixtures at an initial

pressure of 1 bara, determined in different volumes


0

2

4

6

8

10

12

14

16

18

20

22

24

26


CH

4 [m

ol-%

]

TUD 20-dm^3



0

5

10

15

20

25

30

35


CH

4 [m

ol-%

]

BAM 2.8-dm^3 BAM 6.0-dm^3 TUD 20-dm^3 INERIS 2000 dm^3




3.3.3.2 Explosion pressure ratios In Fig. 109 the explosion pressure ratios for methane/air mixtures at an initial pressure of 1 bara and different initial temperatures and volumes are shown. Increasing the initial temperature leads to decreasing pressure ratios, especially for mixtures close to the stoichiometry. The ratios determined at the same temperatures, but in different vessel volumes are also in a good correspondence. In this presentation it can easily be seen that a temperature increase also leads to greater explosion ranges. As already mentioned in chapter 3.3.2.2 at initial pressures of 5 bara (Fig. 110) and 10 bara (Fig. 111) the pressure ratios close to the UEL show an interesting behaviour. Over concentration ranges of several mol-% the pressure ratios are only about 1.5 and lower. With increasing initial temperature the concentration ranges gets smaller, but with increasing initial pressure larger.

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0CH4 [mol-%]

p ex/p

i [-]

TUD 20 °CTUD 100 °CTUD 120 °CTUD 200 °CTUD 240 °CWUT 20 °CWUT 100 °CWUT 200 °C

Fig. 109: Explosion pressure ratios of methane/air mixtures at an initial pressure of 1 bara

(TUD: 20-dm3 sphere; WUT: 40-dm3-vessel)


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

p ex/p

i [-]

TUD 20 °C

TUD 100 °C

TUD 120 °C

TUD 200 °C

TUD 240 °C

Fig. 110: Explosion pressure ratios of methane/air mixtures at an initial pressure of 5 bara,

determined at different initial temperatures (20-dm3 vessel)

0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

3,0 6,0 9,0 12,0 15,0 18,0 21,0 24,0 27,0 30,0CH4 [mol-%]

p ex/p

i [-]

TUD 20 °C

TUD 100 °C

TUD 120 °C

TUD 200 °C

TUD 240 °C

Fig. 111: Explosion pressure ratios of methane/air mixtures at an initial pressure of 10 bara,

determined at different initial temperatures (20-dm3 vessel)


3.3.3.3 KG-values In Fig. 112 to Fig. 114 the KG-values for different methane/air mixtures at initial pressures of 1 bara, 5 bara and 10 bara are shown for initial temperatures of 20 °C, 100 °C, 120 °C, 200 °C and 240 °C. An increase of the initial temperature leads to an increase of the KG-value. This tendency is most obvious for an initial pressure of 10 bara (Fig. 114). Remarkable is the fact that especially for fuel rich mixtures at an initial temperature of 200 °C higher KG-values were obtained than for the other temperatures. The reason is to find in the higher ignition energy which was used for the experiments.

0

10

20

30

40

50

60

70

80

90

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 20 °CTUD 100 °CTUD 120 °CTUD 200 °CTUD 240 °CWUT 20 °CWUT 100 °CWUT 200 °C

Fig. 112: KG-values of methane/air mixtures at an initial pressure of 1 bara, determined at

different initial temperatures (TUD: 20-dm3 sphere; WUT: 40-dm3-vessel)


0

50

100

150

200

250

300

3,0 6,0 9,0 12,0 15,0 18,0 21,0 24,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 20 °C

TUD 100 °C

TUD 120 °C

TUD 200 °C

TUD 240 °C


different initial temperatures (20-dm3 vessel)

0

100

200

300

400

500

600

3,0 6,0 9,0 12,0 15,0 18,0 21,0 24,0 27,0 30,0CH4 [mol-%]

KG [b

ar m

/s]

TUD 20 °C

TUD 100 °C

TUD 120 °C

TUD 200 °C

TUD 240 °C


different initial temperatures (20-dm3 vessel)


3.3.4 Flame propagation: dependence on methane concentration As was done for hydrogen (chapter 3.1.4), the dependence of flame propagation on methane concentration was examined for several conditions [14]. Based on the pressure time traces, three different combustion regimes can be identified. The regimes depend on the initial mixture composition and the given initial conditions (pressure, temperature, ignition energy, volume etc.), as illustrated in Fig. 115. In the first one, marked as 1, the pressure increases quickly and smoothly to the maximum value, after ignition. This type of pressure development is seen for mixtures close to stoichiometric. In the second regime, marked as 2, the pressure trace has a distinct, shaped appearance. This type of pressure development is present in a very narrow concentration range with fuel lean mixtures, and in a wider concentration range with fuel rich mixtures. In the third regime, marked as 3, the pressure increase is similar to that in the first region, but the pressure increase is much lower and slower. Examples of pressure time traces of the three regimes are presented in Fig. 116.

Pexp

Methane mole fraction [%]

3

12

3 Stoichiometric point

2

Pmax

Fig. 115: Scheme of the course of explosion pressures in dependence of the methane mole

fraction in order to show the three different combustion regimes


5,0

10,0

15,0

20,0

25,0

30,0

35,0

40,0

45,0

0,0 1,0 2,0 3,0 4,0 5,0 6,0 7,0 8,0time [s]

P [b

ara]

26.5% CH4, region 3

25.0% CH4, region 2

23.0% CH4, region 2

12.0% CH4, region 1

ignition

Fig. 116: Experimental examples of p-t-curves in the three different combustion regions, determined at initial conditions of 10 bara and 240 °C.

As presented in chapter 2.3.1.1, the 20-dm3 sphere at TU Delft is equipped with six thermocouples, three horizontal and three vertical directed (Fig. 1). Hence it is possible to determine the flame propagation not only by use of video records (chapter 3.1.4 and 3.3.5), also by the temperature history measured during the explosion. The temperature time traces of these thermocouples show that in the first combustion regime at stoichiometric concentration after ignition the flame propagates evenly in all directions in the vessel. At off-stoichiometric compositions it is almost evenly propagating (buoyancy affected). The surface area of the reaction front increases evenly, thus the pressure, with the progressing explosion to the moment the flame front reaches the walls of the vessel. In the second combustion regime, after ignition, the density difference between the hot burnt gas and the cold unburnt gas induces the buoyancy force, which contributes to the upwards flame front propagation in the upper half of the explosion sphere. The upwards-propagating flame front is the sum of the burning velocity and the buoyancy-induced velocity, while the buoyancy force opposes the downward propagation of the flame. If the burning velocity is too low to oppose the buoyancy force, the flame rises upwards to the top of the vessel.


In the second regime the flame is able to propagate downwards after some delay causing the signal increase of the horizontal thermocouples. In some cases the upward movement of the flame, just after ignition, passes the first, vertically placed thermocouple T4, without increasing its temperature as illustrated in Fig. 117. The surface of the reaction front is strongly affected by the buoyancy force, and does not develop evenly over time. Consequently the amount of energy released changes in time causing the observed uneven pressure time profile. It is plausible that the buoyancy force enhances the contact time between reacting gas and the vessel surface thereby enabling wall catalysis and contributes to abundant soot formation during the downward propagation of the flame. In the third combustion regime, after ignition, the flame propagates upwards since the reactivity (laminar burning velocity) of the mixtures is too low to cause downward flame propagation. Consequently the flame is suppressed at the top of the vessel. In even less reactive mixtures the flame is suppressed during its upward movement. Only the temperatures of the vertical thermocouples rise and the temperature of the horizontal ones are almost unchanged. This observation is in agreement with a recent study of Takahashi [17]. At certain conditions, a sharp explosion pressure discontinuity appears between the second and the third combustion regime. This is clearly visible e. g. in Fig. 111 at 17 % of methane at 20 °C, at 21.5 % at 120 °C and at 26 % at 240 °C. The discontinuity is usually caused by the inability of the flame for downward propagation. A smaller amount of the test mixture is consumed yielding a lower explosion pressure. The combustion regimes are in agreement with a comprehensive study of Cashdollar carried out at ambient conditions in different volumes of the test vessels [15].

4,0

6,0

8,0

10,0

12,0

14,0

7,0 8,0 9,0 10,0 11,0 12,0time [s]

P [b

ara]

100

200

300

400

500

600

700

800

900

T [°

C]

T5 T6 T4 T3

T2

T1

ignition

P1 and P2

Fig. 117: Experimental example of p-t- and T-t-curves for the explosion development in a mixture with 18.5 mol-% methane in air according to combustion region 2, determined at initial conditions of 5 bara and 120 °C.

Some experiments have been carried out to visualize the above mentioned flame propagation regimes by taking high-speed videos together with the pressure-time histories of different methane/air mixtures at initial conditions of 10 bara and 20 °C. These experiments were carried out in a 2.0-dm3 windowed vessel (cylindrical, 90° turned; D = 180 mm, L = 150 mm).


This windowed vessel can not be heated and at an initial temperature of 20 °C the course of the explosion pressure ratio does not show the effect of a sudden decrease at high fuel concentrations (see Fig. 89). Therefore the experiments were carried out at elevated initial pressure. In Fig. 119 the sequences of the high-speed videos for the experiments with the different methane/air mixtures are shown. The corresponding pressure-time histories are plotted in Fig. 118.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2t [s]

0

10

20

30

40

50

60

70

p [b

ara]

15 mol-% CH4: pex/pi = 6.219 mol-% CH4: pex/pi = 1.420 mol-% CH4: pex/pi = 1.0

P_T_WINDOW_CH4_1BARA_15%_19%_20% | 8.3.2006 Fig. 118: Pressure-time histories of methane/air explosions with different fuel concentrations at

initial conditions of 10 bara and 20 °C. The exploding wire igniter (Ei ≈ 10 J) was located in the center of a 2-dm3 cylindrical, windowed vessel.

These pressure-time histories show a very good correspondence to the histories for the combustion regions 2 and 3 plotted in Fig. 116. The measured explosion pressure ratios are also in a good agreement to the results determined at the same conditions in the 20-dm3 sphere (Fig. 89). An ignition in a mixture with 15 mol-% methane in air resulted in a pressure-time history according to combustion region 2. For this region one could expect that the flame propagates through the whole vessel volume (top sequence in Fig. 119). There was a short time period between 408 ms and 608 ms, where a stationary flame occurred before the downwards propagation started. The pressure-time history of the ignition in a mixture with 19 mol-% methane was like that one of combustion region 3. In this case no downwards flame propagation was expected, but in Fig. 119 it can be seen that the propagation is comparable to that in a mixture with 15 mol-% methane. The only difference is that the flame was not so bright. The reason for this unexpected downwards flame propagation is to explain by the very small volume of the windowed vessel, where a kind of forced flame propagation took place. Further experiments for more precise examinations on flame propagations in dependence of fuel concentration etc. are planned in a new windowed vessel with a volume of about 11-dm3.


The flame propagation in the mixture with 20 mol-% (bottom sequence in Fig. 119) is typical for a flame when the fuel concentration is outside of the explosion range. A small flame ball with a constant volume initiated by the ignition source rises to the top and is extinguished latest at the vessel walls. No remarkable pressure increase was recorded for this ignition.

8 ms 108 ms 208 ms 308 ms 408 ms 608 ms 708 ms 808 ms



Fig. 119: Flame propagation in methane/air mixtures with 15 mol-% CH4 (top), 19 mol-% CH4 (middle) and 20 mol-% CH4 (bottom) at initial conditions of 1 bara and 20 °C when ignited in the center of a 2-dm3 cylindrical vessel (90° turned) by use of an exploding wire (Ei ≈ 10 J). The pictures were recorded with 250 fps.

The three different regimes of flame propagation are result of the ignition of the mixture in the centre of the vessel and the physical phenomena associated with flame propagation like buoyancy and species diffusivity. Mashuga [16] pointed out that analysis of the pressure time curves in the second combustion regime cause practical problems. Because two distinctly different pressures rises can be observed thought was given as to which pressure rise is appropriate and should be further analysed for the determination of the maximum rate of pressure rise. The maximum rate of pressure rise is the maximum slope of the pressure time curve. Although not discussed in this paper it is a very important explosion severity parameter used for proper design and sizing of mitigation measures against explosions. 3.3.5 Flame propagation: dependence on initial pressure and location of the

ignition source In the following section experiments for the detection of flame propagations in methane/air mixtures at different initial pressures are described. These experiments were also carried out in the 2.0-dm3 windowed vessel in order to find reasons for the above mentioned change of combustion regions what can influence the explosion indices, e.g. the drop in the course of the explosion pressure ratios for fuel rich mixtures at different initial pressures (Fig. 111). One hope was to find different flame structures and propagations.


In Fig. 120 and Fig. 121, sequences of high-speed videos of explosion reactions in a mixture with 15.0 mol-% methane in air at initial pressures of 1 bara, 5 bara, 7 bara and 10 bara are shown. The difference of the sequences is used to find in the location of the ignition source. The first series of experiments (Fig. 120) were done with the exploding wire in the middle of the vessel, and for the second series the ignition source was located at the bottom of the vessel (Fig. 121). Below the frames the time of record are mentioned. At 1 bara initial pressure the flame was not very bright so that the flame front was shown by the white lines. At higher initial pressures the flame is brighter and the flame front well to see. The corresponding p-t-curves are shown in Fig. 122 respectively Fig. 123. According to the above mentioned statements (3.3.4) the p-t-curves show courses which belong to combustion region 2 for a mixture with 15 mol-% methane in air at initial conditions of 1 bara and 20 °C. The highest explosion pressure ratios and rates of pressure rise as well as the times of these two explosion indices are listed in Tab. 18 and Tab. 19.





Fig. 120: Flame propagation in a methane/air mixture with 15 mol-% CH4 at initial pressures of 1 bara (first sequence, for better illustration of the flame front), 5 bara (second sequence), 7 bara (third sequence) and 10 bara (fourth sequence) when ignited in the center of a 2-dm3 cylindrical vessel (90° turned) by use of an exploding wire (Ei ≈ 10 J). The pictures were recorded with 250 fps.


After ignition (first picture in the line) a flame ball propagates to the top of the vessel due to natural convection. The higher the distance to the top of the vessel the more the flame is able to grow. When the mixture is ignited in the center and the upper volume of the vessel is taken by the flame the slope of the corresponding p-t-curve decreases slightly. At this time the heat losses of the system are great. By further propagation downwards the slope increases again. Usually the time period between the pictures which were chosen for the presentation is constant except the series shown at an initial pressure of 7 bara and 10 bara. In these series there is one picture missing at 508 ms. At this time step a stationary flame front was detected for min. 100 ms, so that the picture at 508 ms is equal to that recorded at 408 ms. If one looks to the corresponding p-t-curves for these initial pressures in this time period the slope of the curve decreases for a moment. After this period the slope increases again and the highest rate of explosion rise is reached (Tab. 18). One can assume that at the time of the stationary flame there is a balance between the natural convection (buoyancy) and the downward flame propagation. Due to increasing pressure and temperature in the system the flame propagation is accelerated and the flame front goes down again. Afterwards the whole mixture in the volume was burned the highest explosion pressure was detected. For initial pressures of 1 bara, 5 bara and 7 bara there was no picture available at this time, the volume was already dark. Only at an initial pressure of 10 bara the whole volume was alight. When the mixture was ignited at the bottom (Fig. 121), the flame ball after the ignition is able to grow much more than by an ignition in the center. Therefore much more of the mixture can react and the slope of the corresponding p-t-curves is higher compared to those when ignited in the center. In this first part of the p-t-curves the highest rates of pressure rise were determined (Tab. 19). A slight decrease in the slope was detected when the brightest flame takes the greatest volume of the vessel. At 10 bara initial pressure this was close to the time when the highest explosion pressure ratio was determined.


8 ms 84 ms 160 ms 236 ms 312 ms 388ms 464 ms 540 ms



Fig. 121: Flame propagation in a methane/air mixture with 15 mol-% CH4 at initial pressures of 1 bara (first sequence, for better illustration of the flame front), 5 bara (second sequence), 7 bara (third sequence) and 10 bara (fourth sequence) when ignited at the bottom of a 2-dm3 cylindrical vessel (90° turned) by use of an exploding wire (Ei ≈ 10 J). The pictures were recorded with 250 fps.


In a few pictures are glowing particles visible caused by the exploding wire igniter. But these glowing particles do not force secondary ignitions in the areas of unburned mixtures. The explosion pressure ratios correspond very well to the ratios already presented (e.g. chapter 3.3.2.2) and determined in a 20-dm3 sphere. Furthermore the presented dependencies are valid for the two sets of experiments, too. Due to the smaller volume the KG-values are lower than those presented for instance in Fig. 94. With increasing initial pressure the explosion pressure ratio increases (Fig. 89), the same to the KG-values (Fig. 94).

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4t [s]

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

p [b

ara]

pi = 1 barapi = 5 barapi = 7 barapi = 10 bara

P_T_WINDOW_CH4_MITTE | 4.1.2006 Fig. 122: Pressure-time histories of methane/air explosions at initial pressures of 1 bara, 5 bara,

7 bara and 10 bara. The exploding wire igniter (Ei ≈ 10 J) was located in the center of a 2-dm3 cylindrical, windowed vessel.


0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4t [s]

0

0.5

11.5

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

p [b

ara]

pi = 1 barpi = 5 barpi = 7 barpi = 10 bar

P_T_WINDOW_CH4_UNTEN | 4.1.2006 Fig. 123: Pressure-time histories of methane/air explosions at initial pressures of 1 bara, 5 bara,

7 bara and 10 bara. The exploding wire igniter (Ei ≈ 10 J) was located at the bottom of a 2-dm3 cylindrical, windowed vessel,

Tab. 18: Explosion pressure ratios and rates of pressure rise of a mixture with 15 mol-% methane in air, determined at different initial pressures in the 2-dm3 vessel, ignited in the center of the vessel

pi [bara] pex/pi [-] tpex [ms] (dp/dt)ex [bar/m s] t(dp/dt)ex [ms] KG [bar/m s] 1 3.23 1041 12.1 323 1.5

5 5.44 998 79.1 854 10.0

7 6.00 820 57.5 755 7.2

10 6.17 742 84.0 657 10.6

Tab. 19: Explosion pressure ratios and rates of pressure rise of a mixture with 15 mol-% methane in air, determined at different initial pressures in the 2-dm3 vessel, ignited at the bottom of the vessel

pi [bara] pex/pi [-] tpex [ms] (dp/dt)ex [bar/m s] t(dp/dt)ex [ms] KG [bar/m s] 1 4.17 737 19.8 356 2.5

5 5.67 610 82.3 335 10.4

7 6.00 522 105.0 337 13.2

10 6.36 483 140.0 297 17.6


The assumed change in the flame structure with increasing initial pressure could not be clearly identified. There is a change from a laminar flame front to a turbulent, but it is hardly observed in the pictures. In general there is an increase of turbulence in the system when the pressure is increased. As it can be seen in Fig. 120 and Fig. 121 the flame propagation is strongly dependent on the distance between the ignition source and the vessel wall especially to the top, and therefore strongly dependent to the vessel volume. In order to start a more systematic examination a new cylindrical vessel, 90° turned and equipped with one or two windows (V = 11-dm3, D = 225 mm, L = 275 mm) was constructed. If the ignition source is located in the center the distance to the top is nearly the same as in the 6.0-dm3 vessel. Therefore a good comparison between the experimental results in the both vessels should be possible and will be reported in Deliverable Report No. 13 or through choice in a new report no. 10. 3.3.6 Discussion 3.3.5.1 Explosion limits The explosion limits of methane are strongly dependent on the vessel volume. It was shown that a 2.8-dm3 vessel was too small for the determination of the UEL at 1 bara. A 6.0-dm3 vessel is the smallest useful volume for such purposes, but for temperatures equal or higher than 200 °C at 1 bara the volume has to be also larger (Fig. 74 and Fig. 87). The dependence of the explosion limits on initial pressure and temperature can easily be seen in Fig. 124, where the limits are shown for different temperatures, determined at 1 bara, 5 bara and 10 bara initial pressure. The ignition energy was always about 11.5 J ± 1.5 J.

0

5

10

15

20

25

30

35

0 20 40 60 80 100 120 140 160 180 200 220initial temperature [°C]

CH

4 [m

ol-%

]

TUD LEL 1 bar TUD UEL 1 bar TUD LEL 5 barTUD UEL 5 bar TUD LEL 10 bar TUD UEL 10 bar

Fig. 124: Temperature dependence of the explosion limits of methane/air mixtures, determined

at different initial pressures (20-dm3 vessel, ignition energy: 11.5 J ± 1.5 J)


With increasing initial temperature, the explosion range gets broader. An increase in initial pressure leads to a further increase of the explosion range. Thereby the slope of the explosion limits with increasing temperature is the same at every initial pressure. In Fig. 125 two additional LEL and UEL are added to the diagram of Fig. 124, so that the influence of the ignition energy is obvious. The lower the ignition energy the smaller is the explosion range. Especially the UEL is decreases strongly. With increasing initial pressure and temperature this influence is getting more serious.

0

5

10

15

20

25

30

35


CH

4 [m

ol-%

]

TUD LEL 1 bar TUD UEL 1 bar TUD LEL 5 barTUD UEL 5 bar TUD LEL 10 bar TUD UEL 10 bar

Fig. 125: Temperature dependence of the explosion limits of methane/air mixtures, determined

at different initial pressures (20-dm3 vessel, ignition energy: 11.5 J ± 1.5 J, respectively 3.1 J ± 0.3 J at 120 °C and 240 °C)

3.3.5.2 Explosion pressure ratios In order to show the volume, pressure and temperature dependence on the explosion pressure ratios Fig. 126 was prepared. The first view is a bit confusing, but some results are clearly visible. The full symbols are measured pressure ratios at an initial temperature of 20 °C, the empty symbols demonstrate the values determined at 200 °C. At both initial temperatures there is no volume and pressure influence visible for methane concentrations from the LEL to 12 mol-% methane (Ti = 20 °C) respectively 14 mol-% methane (Ti = 200 °C). The temperature influence in the same concentration range is also visible. The LEL is found at lower mole fractions of methane and the highest explosion pressure ratios, determined close to stoichiometric mixtures, are lower at higher temperature. For higher fuel gas concentrations all examined influences come together: volume, pressure and temperature dependencies which lead to a chaotic view of explosion pressure ratios in this range. Nevertheless the drops in the ratios caused by a change of the combustion region (chapter 3.3.4 and 3.3.5) for elevated conditions are also obvious


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

8,0

9,0

10,0

3,0 6,0 9,0 12,0 15,0 18,0 21,0 24,0 27,0 30,0CH4 [mol-%]

p ex/p

i [-]

TUD 20-dm^3 1 bar 20 °CTUD 20-dm^3 10 bar 20 °CBAM 6-dm^3 1 bar 20 °CBAM 6-dm^3 10 bar 20 °CTUD 20-dm^31 bar 200 °CTUD 20-dm^3 10 bar 200 °CBAM 6-dm^3 1 bar 200 °CBAM 6-dm^3 10 bar 200 °CWUT 40-dm^3 1 bar 20 °C WUT 40-dm^3 1 bar 200 °C INERIS 2000-dm^3 1 bar 20 °CINERIS 2000-dm^3 10 bar 20 °C

Fig. 126: Explosion pressure ratios for methane/air mixtures, determined at different initial

pressures and temperatures and in different vessel volumes.

In Tab. 20 the highest explosion pressure ratios determined in the 20-dm3 vessel are listed for the different initial temperatures. With increasing initial temperature at the same initial pressure the explosion pressure ratios decreases. Furthermore the ratios obtained for the same initial temperature increase with increasing initial pressure. This result is also achieved by calculating the ratios with the SAFEKINEX “Explosion Pressure” software [13] and shown in Fig. 127. There the pressure ratios are plotted for 200 °C initial temperature and pressures of 1 bara and 10 bara. The highest calculated explosion pressure ratios differ a bit from each other whereby the higher value is calculated for 10 bara initial pressure. An agreement of the experimental and the calculated results is only obtained for mixtures with 7 mol-% to 12.0 mol-% methane. An interesting fact is that the tendency at 10 bara initial pressure for higher mole fractions up to 24 mol-% methane is the same. May be that in this region the influence especially of the experimental parameters is sufficiently large to cause these differences.

Tab. 20: Highest explosion pressure ratios at different initial pressures and temperatures, determined for a mixture of 10 mol-% methane in air by single ignition attempts in a 20.0-dm3 spherical vessel


pex/pi [-]

20 °C * 100 °C * 120 °C ** 200 °C * 240 °C ** 1 8.30 6.68 6.34 5.31 4.95 5 8.57 6.97 6.54 5.61 5.02

10 8.69 6.89 6.70 5.73 5.16 * ignition energy: 11.0 J ± 1.5 J ** ignition energy: 3.0 J ± 0.5 J


0,0

1,0

2,0

3,0

4,0

5,0

6,0

7,0

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0 27,0 29,0CH4 [mol-%]

p ex/p

i [-]

TUD 1 bar

TUD 10 bar

pex calculated (1 bara)

pex calculated 10 bara

Fig. 127: Experimental and calculated explosion pressure ratios of methane/air mixtures at an

initial temperature of 200 °C and initial pressures of 1 bara and 10 bara

3.3.5.3 KG-values The KG-values of methane/air mixtures are dependent on each parameter examined. With increasing volume and/or pressure and/or temperature the KG-values increase very strongly in some respects. In Tab. 21 the KG-values are listed which have been determined in the 20-dm3 sphere at different initial pressures, temperatures and also ignition energies. Beside the above mentioned dependencies it can be seen that the pressure dependence is larger than the temperature dependence. The influence of the ignition energy can not be clearly identified. Only at an initial pressure of 5 bara the values obtained with lower energies (TI = 120 °C and 240 °C) are lower than the values at the directly lower or higher initial temperature. At 10 bara it is the opposite. Further studies appear to be necessary for the influence of ignition energy.

Tab. 21: Highest KG-value at different initial pressures and temperatures, determined for a mixture of 10 mol-% methane in air by single ignition attempts in a 20.0-dm3 spherical vessel


KG-value [bar/m s]

20 °C * 100 °C * 120 °C ** 200 °C * 240 °C ** 1 75.3 75.2 77.2 78.9 77.1 5 194.3 237.9 214.9 285.2 236.2

10 266.9 411.6 494.5 485.2 506.0 * ignition energy: 11.0 J ± 1.5 J ** ignition energy: 3.0 J ± 0.5 J


3.3.5.4 Normalized KG-values In chapter 3.3.2.4 it was shown that the normalized KG-values decrease with increasing pressure except in a 2000-dm3 sphere. In Fig. 128 the normalized KG-values of methane/air mixture at 20 °C are shown for the different volumes and initial pressure of 1 bara and 10 bara. In this presentation it is clearly seen that the KG/pi-values determined in the 2000-dm3 vessel are higher, in case of 10 bara initial pressure even a factor 3 (1 bara, 2000-dm3) respectively a factor 3.75 (1 bara, 20-dm3). In such a big vessel the turbulence forced by the combustion inside the vessel accelerate the flame propagation and therefore the (dp/dt)ex-value. A comparable effect was already shown in chapter 3.3.5 where experiments in a 2.0-dm3 windowed vessel were presented and the turbulence increase with increasing initial pressure. When it is possible to detect such turbulence and their effect on the rates of pressure rise in that small vessel one can imagine that the turbulence in a 1000 times greater volume are much more extensive.

0

50

100

150

200

250

300

350

3,0 5,0 7,0 9,0 11,0 13,0 15,0 17,0 19,0 21,0 23,0 25,0CH4 [mol-%]

KG/p

i [ba

r m/s

bar

]

TUD 20-dm^3 1 barTUD 20-dm^3 5 barTUD 20-dm^3 10 barBAM 2.8-dm^3 1 barBAM 2.8-dm^3 10 barBAM 6-dm^3 1 barBAM 6-dm^3 10 barWUT 40-dm^3 1 barWUT 1250-dm^3 1 barINERIS 20-dm3 1 barINERIS 2000-dm^3 1 barINERIS 2000-dm^3 10 bar


determined at different initial pressures and in different vessel volumes


3.4 Propylene The work presented below deals with the ternary mixture propene/O2/N2. This mixture was selected as an industrial very important system. It can be expected that alkanes and alkenes with more than 2 C atoms in the molecule will exhibit, in principle, a similar combustion behaviour. At 1 bara the auto ignition temperature of Propene in air is 485 °C 0. The LEL und UEL in air at 25 °C and 1 bara are 1.8 vol.-% and 11.2 vol.-%, respectively 0. In Tab. 22 the basic oxidation reactions of propene are summarized

Tab. 22: Basic types of oxidation reactions of Propene and stoichiometric Propene concentrations in Propene/Air and Propene/O2 mixtures for these reactions. The mole number ratio denotes the number of moles after the oxidation reaction divided by the number of moles before the oxidation reaction. For air we assumed a composition of 21 vol.-% O2 and 79 vol.-% N2.

Basic types of oxidation reactions oxidant is air oxidant is O2 C3H6 content in

stoichiometric C3H6/Air mixture

[vol.-%]

mole number

ratio

C3H6 content in stoichiometric

C3H6/O2 mixture [vol.-%]

mole number

ratio

C3H6 + 4.5 O2 → 3 CO2 + 3 H2O 4.458 1.02 18.18 1.09 C3H6 + 3 O2 → 3 CO + 3 H2O 6.54 1.13 25 1.5 C3H6 + 1.5 O2 → 3 CO + 3 H2 12.28 1.43 40 2.4 3.4.1 Explosion limits In Tab. 23 all explosion limits and LOC-values are summarized. The corresponding explosion diagrams are presented in Fig. 129 to Fig. 136. In all diagrams crosses denote investigated compositions which could be ignited (i.e. a self-sustaining flame propagation could be triggered by the ignition), circles denote mixtures that turned out to be non-explosive. For pinitial = 10 bara and 30 bara we had to refrain from conducting measurements of pex and KG in the central part of the explosive range of the binary mixture propene/O2 in view of the fact that the expected pressures might have significantly exceeded the official design pressure of our reaction vessel. For the same reason we also did not conduct measurements for high propene concentrations on the stoichiometric line. For all propene/O2 mixtures soot formation was observed at propene concentrations exceeding 40 vol.-%. All diagrams do not only disclose the range of explosive mixtures, but have this range subdivided into sections where the explosion

I: proceeds as a normal deflagration (red coloured range), II: makes a transition from a deflagration to a thermal explosion (yellow coloured range) and III: makes a transition from deflagration to detonation (magenta coloured range).

The discussion of this topic is postponed to chapter 3.4.6.


Tab. 23: Summary of the explosion limits of propene/O2/N2 at 25 °C and 200 °C for different initial

pressures. The step size for variation of the propene content was 0.25 vol.-% at the LEL and 1 vol.-% at the UEL. To determine the LOC, the step size for variation of the O2 content was 1 vol.-%.

* Extrapolation based on UEL in air, because the vapour pressure of propene is too low at Tinitial = 25 °C.

** Measurement at Tinitial = 55 °C to provide sufficient vapour pressure of propene. *** This is the threshold value for stoichiometric propene/O2/N2 mixtures (with respect to

formation of CO2 and H2O). When exceeding this value the mixtures become explosive. Because at the high initial pressure of 30 bara self decomposition comes into play more and more with increase of the propene content, the LOC value is presumably 0.

**** Due to safety considerations (electrodes of ignition source, which were insulated by a PTFE-packing, were sometimes blown out at Tinitial = 200 °C) we could not perform experiments at propene concentrations exceeding about 50 vol.-% (secondary explosion outside the vessel would become too violent).

Explosion limits and LOC-values [vol.-%] at different initial temperatures and pressures [bara]

Tinitial = 25 °C Tinitial = 200 °C 1 5 10 30 1 5 10 30 LEL in O2 [vol.-%] 2 2 2 1.75 1.5 1.5 1.5 1.25 UEL in O2 [vol.-%] 58 70 75 92.5* 61 77 87 **** LEL in air [vol.-%] 2 2 2 1.75 2 1.5 1.5 1.25 UEL in air [vol.-%] 11 14 19 65** 15 22 28 **** LOC [vol.-%] 11.5 11.5 11.5 9.6 10 9 9 8***

0 10 20 30 40 50 60 70 80 90 100Propene C3H6 [Vol.-%]

0

10

20

30

40

50

60

70

80

90

100

O2 [Vol.-%

]

0

10

20

30

40

50

60

70

80

90

100

N 2 [V

ol.-%

]

propene/air-mixtures

e of defl ative explosion,

bar abs, 25 °C

rangagr

1

yellow range:possibly heat explosion

range of detonative

explosion

stoichiometric C 3H 6 +

1.5 O 2 -> 3 CO + 3 H 2

stoich

iometr

ic C 3H

6 + 3

O 2 -> 3

CO + 3 H 2O

stoich

iometr

ic C 3H

6 + 4.

5 O2 -

> 3 C

O 2 + 3

H 2O

Fig. 129: Explosion diagram of propene/O2/N2 at pinitial = 1 bara and Tinitial = 25 °C.



0 10 20 30 40 50 60 70 80 90 100Propene C3H6 [Vol.-%]

0

10

20

30

40

50

60

70

80

90

100

O2 [Vol.-%

]

0

10

20

30

40

50

60

70

80

90

100

N 2 [V

ol.-%

]

range of deflagrative explosion, 5 bar abs, 25 °C

range of detonative

explosion

stoichiometric

C 3H 6 + 1.5 O 2 -

> 3 CO + 3 H 2sto

ichiom

etric

C 3H6 +

3 O 2 -

> 3 CO + 3

H 2O

stoich

iometr

ic C 3H

6 + 4.

5 O2 ->

3 CO 2 +

3 H 2O

soot is formed here

(43 vol.-% up to 69 vol.-%

)




0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

20

30

40

50

60

70

80

90

100

Propene C3H6 [Vol.-%]

N 2 [V

ol.-%

]

O2 [Vol.-%

]stoich

iometr

ic ra

tio P

ropen

e : O

2 =

1 : 4.

5



range of detonative explosion(boundary at propeneconc. > 10 vol.-% only estimated)



UEL in O2 extrapolatedbased on theUEL in air

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

20

30

40

50

60

70

80

90

100


O2 [Vol.-%

]

N 2

[Vol

.-%]



stoich

iometr

ic C 3H

6 +

4.5 O

2 ->

3 CO 2 +

3 H 2O

conceivable range of detonative explosion


0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

20

60

70

90

100


N 2

[Vol

.-%] O

2 [Vol.-%]


80

50

30

40

stoichiometric C 3H 6 +

1.5 O 2 -> 3 CO + 3 H 2

stoich

iometr

ic C 3H

6 + 3

O 2 -> 3

CO + 3 H 2O

stoich

iometr

ic C 3

H 6 + 4.

5 O2 -

> 3 C

O 2 +

3 H 2

O


range of detonative explosion

r

ange of

deflagrative explosion,

5 bar abs, 200 °C




0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

30

40

100


N 2 [V

ol.-%

] O2 [Vol.-%

]

stoich

iometric C 3H 6 +

1.5 O 2 -> 3 CO + 3 H 2

stoich

iometr

ic C 3H

6 + 3

O 2 -> 3

CO +

3 H2O

stoich

iometr

ic C 3H

6 + 4.

5 O2 ->

3 CO 2 +

3 H 2O


range of detonative

explosion

soot is formed here

(41 vol.-% up to 75 vol.-%

)


90

50

60

70

80

20


0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

30

40

100


N 2 [V

ol.-%

] O2 [Vol.-%

]

stoichiometric

C3H 6 + 1.5 O 2 -

> 3 CO + 3 H 2

stoich

iometr

ic C 3

H 6 + 3

O 2 -> 3

CO +

3 H2O

stoich

iometr

ic C 3H

6 + 4.

5 O2 -

> 3 C

O 2 + 3

H 2O



90

50

60

70

80

20

range of detonative

explosionyellow range:

possibly heat explosion



Important:Measurement ofexplosion diagramonly up to 26 vol.-%Propene. UEL in O2 could ly between 94 vol.-% and 100 vol.-%.

0 10 20 30 40 50 60 70 80 90 1000

10

20

30

40

50

60

70

80

90

1000

10

20

30

40

50

60

70

80

90

100


O2 [Vol.-%

]

N 2 [V

ol.-%

]



stoich

iometr

ic C 3H

6 + 4.

5 O2 -

> 3 C

O 2 + 3

H 2O

conceivable range of detonative explosion

This course of the boundary in case of UEL = 100 vol.-% is tentative.


3.4.2 Explosion pressure ratios Fig. 137 to Fig. 142 display the explosion pressure ratios. Fig. 143 to Fig. 148 show the KG-values normalized to 1 bara. Whenever a deflagration to detonation transition occurred in the course of the reaction, the normalized KG-value was set to 30000 bar*m/s. This was only done to facilitate plotting. Naturally, KG is meaningless in case of detonative explosions. Fig. 149 to Fig. 153 show the combustion times. Fig. 154 shows the explosion pressure ratios found at pinitial = 1 bara and Tinitial = 25 °C for propene/O2-mixtures, stoichiometric propene/O2/N2 mixtures and propene/air mixtures.


0

1

2

3

4

5

6

7

8

9

10

11

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70Propene concentration in Propene/Air-mixture [vol.-%]

p ex/p

initi

al

1 bar abs, 25 °C

5 bar abs, 25 °C

10 bar abs, 25 °C

30 bar abs, 25 °C

0

1

2

3

4

5

6

7

8

9

10

11

0,0 2,5 5,0 7,5 10,0 12,5 15,0 17,5 20,0Propene concentration in Propene/Air-mixture [vol.-%]

p ex/p

initi

al

1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 °C30 bar abs, 25 °C

4.46

vol

.-%: s

toic

hiom

etric

com

posi

tion

for

C3 H

6 + 4

.5 O

2 ->

3 C

O2 +

3 H

2 O

6.54

vol

.-%: s

toic

hiom

etric

com

posi

tion

for

C3 H

6 + 3

O2 -

> 3

CO

+ 3

H2 O

12.2

8 vo

l.-%

: sto

ichi

omet

ri cco

mpo

sitio

n fo

rC

3 H6 +

1.5

O2 -

> 3

CO

+ 3

H2

Fig. 137: Explosion pressure ratios for propene/Air mixtures at 25 °C and different initial pressures. Top diagram: plot over full range of propene concentrations. Bottom diagram: only small propene concentrations


0

1

2

3

4

5

6

0,0 2,5 5,0 7,5 10,0 12,5 15,0 17,5 20,0 22,5 25,0 27,5 30,0Propene concentration in Propene/Air-mixture [vol.-%]

p ex/p

initi

al

1bar abs, 200 °C5 bar abs, 200 °C

10 bar abs, 200 °C30 bar abs, 200 °C

Fig. 138: Explosion pressure ratios for propene/Air mixtures at 200 °C for different initial

pressures.

0

1

2

3

4

5

6

7

8

9

10

11

0,0 2,5 5,0 7,5 10,0 12,5 15,0 17,5 20,0 22,5 25,0 27,5 30,0Propene concentration in Propene/Air-mixture [vol.-%]

p ex/p

initi

al

1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 °C30 bar abs, 25 °C1bar abs, 200 °C5 bar abs, 200 °C10 bar abs, 200 °C30 bar abs, 200 °C

Fig. 139: Explosion pressure ratios for propene/Air mixtures at 25 °C and 200 °C for different

initial pressures.


0

5

10

15

20

25

0 10 20 30 40 50 60 70 80 9Propene concentration in Propene/O2-mixture [vol.-%]

p ex/p

initi

al

0

1 bar, 25 °C5 bar, 25 °C10 bar, 25 C

Fig. 140: Explosion pressure ratios at Tinitial = 25 °C for propene/O2 mixtures.

0

5

10

15

20

25


p ex/p

initia

l

0

1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 C1 bar abs, 200 °C5 bar abs, 200 °C10 bar abs, 200 °C

18.1

8 vo

l.-%

: sto

ichi

omet

ric c

ompo

sitio

n fo

r C

3H6 +

4.5

O2 -

> 3

CO

2 + 3

H2O

25 v

ol.-%

: sto

ichi

omet

ric c

ompo

sitio

n fo

r C

3H6 +

3 O

2 ->

3 C

O +

3 H

2O

40 v

ol.-%

: sto

ichi

omet

ric

com

posi

tion

for

C3H

6 + 1

.5 O

2 ->

3 C

O +

3 H

2

Fig. 141: Explosion pressure ratios at Tinitial = 25 °C and Tinitial = 200 °C for propene/O2 mixtures.


0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20Propene conc. in (Propene:O2=1:4.5)/N2 mixture [vol.-%]

p ex/p

initi

al

1 bar, 25 °C5 bar, 25 °C10 bar, 25 °C1 bar abs, 200 °C5 bar abs, 200 °C10 bar, 200 °C

Fig. 142: Explosion pressure ratios for stoichiometric (with respect to formation of CO2 and H2O)

propene/O2/N2 mixtures at 25 °C and 200 °C.

3.4.3 Normalized KG-values

0,1

1,0

10,0

100,0

1000,0

0 5 10 15 20 25 30 35 40 45 50 55 60 65Propene concentration in Propene/Air-mixture [vol.-%]

(dp/

dt) e

x /V1/

3 * 1

bar/p

initi

al

[bar

*m/s

] 1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 °C30 bar abs, 55 °C

Fig. 143: KG/pi-values of propene/Air mixtures at 25 °C, normalized to 1 bar initial pressure.


0,0

100,0

200,0

300,0

400,0

500,0

0 1 2 3 4 5 6 7 8 9 10 11 1Propene concentration in Propene/Air-mixture [vol.-%]

(dp/

dt) e

x /V1/

3 * 1

bar/p

initi

al

[bar

*m/s

]

2

1 bar abs, 25 °C

5 bar abs, 25 °C

10 bar abs, 25 °C

30 bar abs, 55 °C


Interestingly, the normalized KG –values of the measurements at pinitial = 5, 10 and 30 bara are larger than those at pinitial = 1 bar close to stoichiometric propene concentrations, but smaller at concentrations far off from the stoichiometric value.

0,1

1,0

10,0

100,0

1000,0

0 5 10 15 20 25Propene concentration in Propene/Air-mixture [vol.-%]

(dp/

dt)ex

/V1/

3 * 1

bar/P

initia

l [b

ar*m

/s]

1 bar abs, 200 °C

5 bar abs, 200 °C

10 bar abs, 200 °C

30 bar abs, 200 °C



0,1

1,0

10,0

100,0

1000,0

0 5 10 15 20 25Propene concentration in Propene/Air-mixture [vol.-%]

(dp/

dt) e

x /V1/

3 * 1

bar/P

initi

al

[bar

*m/s

]

1 bar abs, 25 °C

5 bar abs, 25 °C

10 bar abs, 25 °C

30 bar abs, 55 °C

1 bar abs, 200 °C

5 bar abs, 200 °C

10 bar abs, 200 °C

30 bar abs, 200 °C

Fig. 146: Comparison of KG/pi-values found for all investigated initial conditions.

0

1

10

100

1000

10000

100000


(dp/

dt)e

x *V1/

3 * 1

bar/

Pini

tial [

bar*

m/s

]

30000 means

1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 °C1 bar abs, 200 °C5 bar abs, 200 °C10 bar abs, 200 °C

0

Fig. 147: KG/pi-values of propene/O2 mixtures at 200 °C. The KG values are normalized to 1 bara initial pressure. Whenever a deflagration to detonation transition occurred in the course of the reaction, the normalized KG-value was set to 30000 bar*m/s.


0

1

10

100

1000

10000

100000


(dp/

dt) e

x *V1/

3 * 1

bar/p

initi

al [

bar*

m/s

]

1 bar, 25 °C

5 bar, 25 °C

10 bar, 25 °C

1 bar abs, 200 °C

5 bar abs, 200 °C

10 bar abs, 200 °C

30000 means

Fig. 148: KG/pi-values of stoichiometric (with respect to formation of CO2 and H2O) propene/O2/N2

mixtures at all investigated initial conditions. The KG-values are normalized to 1 bara initial pressure. Whenever a deflagration to detonation transition occurred in the course of the reaction, the normalized KG-value was set to 30000 bar*m/s.

3.4.4 Combustion times

10,0

100,0

1000,0

10000,0

100000,0

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Propene concentration in Propene/Air mixture [vol.-%]

Com

bust

ion

time

[ms

1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 °C30 bar abs, 25 °C1 bar abs, 200 °C5 bar abs, 200 °C10 bar abs, 200 °C30 bar abs, 200 °C

Fig. 149: Combustion time of propene/air mixtures at all investigated initial conditions.


10

100

1000

10000

0 5 10 15 20 25


Com

bust

ion

time

[ms]

5 bar abs, 25 °C

5 bar abs, 200 °C

10

100

1000

10000

0 5 10 15 20 25


Com

bust

ion

time

[ms]

10 bar abs, 25 °C

10 bar abs, 200 °C

10

100

1000

10000

0 5 10 15 20 25


Com

bust

ion

time

[ms]

30 bar abs, 25 °C

30 bar abs, 200 °C

10

100

1000

10000

0 5 10 15 20 25


Com

bust

ion

time

[ms]

1 bar abs, 25 °C

1 bar abs, 200 °C

Fig. 150: Combustion time of propene/air mixtures at all investigated initial conditions.

0,1

1,0

10,0

100,0

1000,0

10000,0


Com

bust

ion

time

[ms]

s

0

1 bar abs, 25 °C

5 bar abs, 25 °C10 bar abs, 25 °C

1 bar abs, 200 °C5 bar abs, 200 °C

10 bar abs, 200 °C

1 bar abs, 25 °C (10 - 35 vol.-%)

detonative range

5 bar abs, 25 °C (6.5 - 42.5 vol.-%)

10 bar abs, 25 °C (5 - 42 vol.-%)

1 bar abs, 200 °C (11 - 36.5 vol.-%)

5 bar abs, 200 °C (6.5 - 41.5 vol.-%)

10 bar abs, 200 °C (4.5 - 42.5 vol.-%)

Fig. 151: Combustion time of propene/O2 mixtures at all investigated initial conditions.


0,1

1,0

10,0

100,0

1000,0

10000,0

30 35 40 45 50 55 60 65 70Propene concentration in Propene/O2-mixture [vol.-%]

Com

bust

ion

time

[ms]

1 bar abs, 25 °C

5 bar abs, 25 °C

10 bar abs, 25 °C

1 bar abs, 200 °C

5 bar abs, 200 °C

10 bar abs, 200 °C

0,1

1,0

10,0

100,0

1000,0

10000,0

2 4 6 8 10 12 14Propene concentration in Propene/O2-mixture [vol.-%]

Com

bust

ion

time

[ms]

1 bar abs, 25 °C

5 bar abs, 25 °C

10 bar abs, 25 °C

1 bar abs, 200 °C

5 bar abs, 200 °C

10 bar abs, 200 °C

Fig. 152: Combustion times of propene/O2 mixtures shown in Fig. 151 plotted with different scales.

0,1

1,0

10,0

100,0

1000,0

10000,0


Com

bust

ion

time

[ms]

1 bar abs, 25 °C5 bar abs, 25 °C10 bar abs, 25 °C1 bara bs, 200 °C5 bar abs, 200 °C10 bar abs, 200 °C

Fig. 153: Combustion times for stoichiometric (with respect to formation of CO2 and H2O)

propene/O2/N2 mixtures at all investigated initial conditions.


0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18 20Propene concentration [vol.-%]

pex/p

initi

al

1 bar abs, 25 °C, propene/O2 line

1 bar abs, 25 °C, propene/O2/N2stoichiometric line1 bar abs, 25 °C, propene/air line

Fig. 154: Explosion pressure ratios of propene/O2/N2-mixtures at 1 bara, 25 °C.

3.4.5 Discussion 3.4.5.1 Explosion limits The increase of the UEL’s , the decrease of the LEL’s and the decrease of the LOC’s with increasing pressure and increasing temperature is in the usual range. One exception is the extraordinary increase of the LEL when increasing the pressure from 10 bara to 30 bara (see Fig. 131 and Fig. 132). Presumably decomposition reactions come into play at these high initial pressures. When raising the initial temperature to 200 °C, already in the explosion diagram recorded for pinitial = 10 bara (Fig. 135) there are clear signs of decompositions reactions (the O2-concentration at the UEL in air is higher than the O2-concentration at the UEL in oxygen. Considering the formation enthalpies of propene, methane and ethane (+20.41 kJ/mol, -74.87 kJ/mol and -83.8 kJ/mol, respectively [24]), decomposition reactions as listed below would thermodynamically be possible:

a) C3H6 → 3 C + 3H2 + 20.41 kJ/mol b) C3H6 → CH4 + 2C + H2 + 95.28 kJ/mol

c) 2 C3H6 → 3 CH4 + 3C + 132.7 kJ/mol d) C3H6 → C2H6 + C + 104.2 kJ/mol

It is reported that self-sustaining decomposition reactions can be triggered in propene at pressures of several kilobars [25]. Tab. 24 gives the composition of the reaction gases for a number of propene/air mixtures ignited at pinitial =30 bara in our 20 dm3 sphere. Furthermore, soot was absent at 10 vol.-% propene in the propene/air mixture, little soot was found at 16 vol.-% propene and large amounts were present at 20 vol.-% and higher propene concentrations. The soot was loose and not sticky, hence polymerisation reactions of propene can probably be excluded. These observations and the gas analyses suggest that b) is presumably the main decomposition reaction.


Tab. 24: Results of the GC-analysis of the reaction gases generated by combustion of propene/air mixtures at pinitial = 30 bara and 25°C ≤ Tinitial ≤ 55 °C. The samples were drawn from the reaction vessel about 5 min after the explosion. C4, C5 and C6 compounds were in the ppm-range and are not included in this table. In the fourth row from the top the O2-content means the oxygen brought into the propene/air mixture by the air.

Gas Unit Composition of the propene/air mixture before ignition, pinitial = 30 bara C3H6 vol.-% 2,5 3,5 4,46 9 10 16 16 20 26 30Air vol.-% 97,52 96,52 95,52 91 90 84 84 80 74 70O2 vol.-% 20,48 20,27 20,06 19,11 18,90 17,64 17,64 16,80 15,54 14,70test no.

194 196 198 202 201 200 200 199 219 220

Composition of the reaction products H2 vol.-% 0,255 0,207 0,927 15,3 17,5 16,1 20,89 25,8 24,1 22,3O2 vol.-% 8,64 5,86 <0,03 0,803 0,795 4,82 0,01 <0,03 0,05 0,527N2 vol.-% 81,2 81,4 82,1 60,2 57,3 57,6 34,08 45,6 42,4 40,1CO vol.-% 0,297 0,370 1,67 18,6 20,7 15,4 19,99 17,7 15,6 13,7CO2 vol.-% 8,32 10,6 13,9 2,94 1,95 0,945 1,23 1,06 1,30 1,22CH4 vol.-% 0,167 0,27 0,198 0,324 0,469 2,77 3,59 7,40 13,8 18,2C2H6 ppm 33 61 36 172 202 726 942 656 1780 2110C2H4 ppm 21 27 25 79 82 511 663 388 493 510C3H8 vol.-% 0,000 0,001 0,001 0,004 0,005 0,048 0,06 0,029 0,076 0,088C3H6 vol.-% 0,032 0,035 0,051 0,165 0,144 0,789 1,02 0,754 0,965 1,83

3.4.5.2 Explosion pressure ratios pex/pinitial The explosion pressure ratios determined at same initial temperature are almost identical for all initial pressures which is to be expected. Naturally, a comparison between the data determined at different initial pressures can only be carried through in the range of propene concentrations where the explosive ranges overlap. Only the explosion pressure ratios found for fuel-rich propene/air mixtures (i. e. propene concentrations exceeding the stoichiometric content with respect to CO2 and H2O formation of 4.46 vol.-%) at pinitial = 30 bara and Tinitial = 25 °C are higher than the values measured at pinitial = 1, 5 and 10 bara. At Tinitial = 200 °C these higher values are already found for pinitial ≥ 10 bara. This is in line with the assumption expressed in chapter 3.4.5.1 that decomposition of propene comes into play at high initial pressures. Concerning pex, this reaction can only become “visible” in the fuel rich range, because for fuel lean mixtures the entire combustible is converted to H2O and CO2. Even if a propene molecule decomposed, its decomposition products would then be oxidized to CO2 and H2O and the net effect in terms of energy release and mole number ratio would be the same as if the molecule had directly been oxidized to CO2 and H2O without having – formally - undergone a decomposition reaction beforehand. In the fuel rich range, however, the presence of decomposition will bring about higher explosion pressure ratios than if decomposition had not occurred. Interesting is the variation of the explosion pressure ratio with propene content in propene/O2 mixtures. In the literature one finds pmax-values for combustible/O2 mixtures at Tinitial = 25 °C of about 17*pinitial. As Fig. 140 suggests, 17 is the explosion pressure ratio at stoichiometric (with respect to CO2 and H2O formation) propene content whereas the maximum explosion pressure pmax of the binary mixture propene/O2 at 25 °C is about 24*pinitial. The maximum of pex/pinitial is attained at propene concentrations of about 32 vol.-% which is still higher than the stoichiometric propene content with respect to formation of CO and H2O (see Tab. 22).


To collect information about the chemistry of the oxidation reaction, GC-analyses were made of the reaction product gases of propene/O2 mixtures, see Tab. 25. Concerning soot formation, a rather sharp onset of soot formation was found when exceeding a propene content of 40 vol.-%. This observation and the GC-data suggest a simplified reaction scheme as depicted in Fig. 156. The variations in the mole number ratio are also reflected by the pressure of the cold reaction gases (see Fig. 155). Calculations of the explosion pressure of propene/O2 with CHEMKIN were carried through in [26]. These calculations nicely reproduce the measured data for all propene concentrations below the onset point of soot formation, at higher concentrations there are some deviations. These calculations also reveal the molar fractions of the main species formed in the course of the combustion. Fig. 157 shows the results, which are in line with Fig. 156. The variation of the explosion pressure ratio with temperature (see Fig. 137 to Fig. 142) is compatible with the usual approximative 1/T-dependence. At 200 °C the explosion pressure ratios should attain only 100 %*(273 K + 25 °C)/(273 K + 200 °C) = 63% of the values at 25 °C which is what we observe.

18.18 vol-%

0 5000 10000 150000

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sure

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]

28 vol.-%

38 vol.-%

33 vol-%

Propene content in the Propene/O2 mixtureat pinitial = 5 bar abs, Tinitial = 25 °C:

Fig. 155: Pressure of the almost cold reaction gases as function of propene content in the

propene/O2 mixture (pinitial = 5 bara, Tinitial = 25 °C). When raising the propene content from 0 up to 40 vol.-% the mole number ratio increases and so does the pressure of the cold reaction gases. A quantitative analysis would have to account for the fact that most of the generated water does not contribute to the pressure due to condensation.


Tab. 25: Results of the GC-analysis of the reaction gases generated by combustion of propene/O2 mixtures at pinitial = 5 and 10 bara and Tinitial = 25 °C. The samples were drawn from the reaction vessel about 5 min after the explosion. C4, C5 and C6 compounds were in the ppm-range and are not included in this table. From the reaction gases generated by the explosion with 28 vol.-% propene two samples were drawn and sent to the lab under different names to check the reproducibility of the measurements. The content of O2 and N2 in the reaction gases is presumably due to bad vacuum in the sample cylinder before filling. H2O is absent in the analyses, since it condenses in the vessel.

Gas Unit Composition and initial pressure of the propene/O2 mixture before ignition C3H6 vol.-% 18,2 23 28 28 33 38 43 48 53 52,5 55 60 70 74 75O2 vol.-% 82 77 72 72 67 62 57 52 47 47,5 45 40 30 26 25pinitial bara 5 5 5 5 5 5 5 5 5 10 10 10 10 10 10test no. 130 131 132 132 133 134 139 138 140 159 158 156 155 154 153 Composition of the reaction products H2 vol.-% 7,97 22,6 34,9 33,1 42,5 43,3 50,6 55 56,6 48,3 42,1 41,7 50.7 44,6 42,2O2 vol.-% 0,068 0,368 0,03 1,06 0,117 1,29 0,03 0,094 0,282 1,06 1,23 0,703 0,1 0,2 0,1N2 vol.-% 0,249 1,5 0,404 4 0,431 4,62 0,635 0,833 0,837 4,19 4,59 2,98 0,4 0,8 0,4CO vol.-% 18,1 38,1 48,9 46,2 51,8 49,6 48,3 42,7 36,1 31,6 26,3 31,2 21,70 18 17,96CO2 vol.-% 72 35,8 15,5 14,8 5,97 1,33 0,135 0,274 0,554 0,912 1,29 0,397 0,821 0,926 0,904CH4 vol.-% 0,076 0,070 0,056 0,05 0,233 0,531 1,61 2,08 4,5 12,2 15,9 19,2 23,9 30,4 34,12C2H6 ppm 74 74 38 36 98 207 35 219 562 2260 3060 3160 2199 2820 2657C2H4 ppm 167 158 28 27 < 2 201 32 143 523 551 924 1890 115 566 1084C2H2 ppm 11 7 < 2 < 2 < 2 5 < 2 < 2 5 3 7 18 < 2 < 4 7C3H8 ppm 7 12 6 6 15 60 4 18 241 45 332 750 369 406 314


0

510

15

20

2530

3540

45

5055

6065

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80

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Upper explosion limit = 70 vol.-%

Pro

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con

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ratio

n in

Pro

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/O2-m

ixtu

re [v

ol.-%

]

18.18 vol.-% = stoich. composition for C3H6 + 4.5 O2 -> 3 CO2 + 3 H2O

25 vol.-% = stoich. composition for C3H6 + 3 O2 -> 3 CO + 3 H2O

mol number ratio: 6/5.5 = 1.09

mol number ratio: 6/4 = 1.5

40 vol.-% = stoich. composition for C3H6 + 1.5 O2 -> 3 CO + 3 H2

mol number ratio: 6/2.5 = 2.4

fuel lean range, excess of oxygen, combustion of C and H to CO2 and H2O

soot formation range, lack of oxygen, combustion to CO, H2, C (soot), CH4

oxygen sufficient to convert all C and H to CO2, CO and H2O

oxygen not sufficient to convert all C and H to CO and H2O;priority: converting C to CO; if O2 remains then this will convert H to H2O, the rest H remains as H2

C, C

H4

{

{{

I

II

III

IVno longer formation of H2O

products of combustion reaction

H2O, CO2

H2O CO

in

crea

se

H2O

de

crea

se

H2

incr

ease

CO

CO, H2

incr

ease

CO

2 de

crea

se

Fig. 156: Schematic diagram of the main reaction paths for explosions in propene/O2 mixtures.

(The explosive range depicted here is for pinitial = 5 bara and Tinitial = 25 °C).


0

0.1

0.2

0.3

0.4

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0.6

0 10 20 30 40 50 60 7propene mole fraction [%]

mol

e fra

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n [-]

0

CO

CO2

C(S)H2O

O2 H2

CH4

Fig. 157: Calculated molar fractions of the main species generated by the combustion of propene in Oxygen at pinitial = 5 bara and Tinitial = 25 °C as function of the propene content in the mixture. (The figure is reproduced from ref. [26])

3.4.5.3 KG-values Since pex is proportional to pinitial, KG will also be proportional to pinitial. In order to directly deduce, at least qualitatively, the pressure dependence of the laminar burning velocity by comparing the KG-values measured at different initial pressures, all plots show KG divided by pinitial. Whenever a deflagration to detonation transition (DDT) occurs in the course of the combustion, specifying KG is no longer meaningful. For the ease of data handling, KG was set to 30000 bar*m/s in those cases. If the combustion developed in a way which is interpreted as a thermal explosion (chapter 3.4.6.2), the KG value was derived from that part of the pressure time trace preceding the thermal explosion peak. The dependence of KG on temperature (see Fig. 143 to Fig. 148) is as follows: For those mixtures, which are very reactive in terms of laminar burning velocity (propene/air close to stoichiometric, propene/O2 over almost the entire explosive range, stoichiometric propene/O2/N2 mixtures with O2-concentrations exceeding the value in stoichiometric propene/air (20.07 vol.-%)), KG decreases with increasing initial temperature. For the rest of the explosive mixtures, which burn rather slowly the temperature increase brings about a slight increase in KG.


3.4.5.4 Combustion times When plotting the combustion times over the propene content of a binary mixture, the position of the minimum value gives the most reactive mixture composition in terms of laminar burning velocity. In case of deflagrative explosions this minimum should coincide with or should at least be very close to those mixtures that exhibit the largest KG value. This is actually what is observed (compare Fig. 146 with Fig. 149). In case of detonative explosions there is a wide range of propene concentrations where KG cannot be defined. Here the combustion time is a valuable parameter that reveals for which mixture composition the DDT requires least time to occur. This mixture should be the most reactive one. As Fig. 151 shows, the most reactive propene/O2 mixture is the one with stoichiometric composition with respect to formation of CO and H2O, i. e. at 25 vol.-% propene. There is a second aspect in case of detonative explosions which makes it worthwhile to have the combustion time measured. When linearly increasing the propene content in the propene/O2 mixture, both pdet (Chapman-Jouguet pressure) and pex/pinitial (explosion pressure ratio recorded in mixture after Chapman-Jouguet peak was „seen“) should vary linearly as well. But actually, both parameters – determined with piezoelectric pressure transducers - exhibit some experimental errors such that pdet and pex/pinitial will sometimes scatter and hence are often not in line with what should be expected when linearly varying the mixture composition. The reasons for scattering are the twofold: a) pdet will scatter slightly, because the sampling rate we applied (maximum sampling rate was

100 kHz) is still too slow to really “catch” the maximum value of the detonation peak. This means that the measured value will always be smaller or – at maximum – equal to the true value

b) pex/pinitial will scatter sometimes, if a huge detonative pressure peak was recorded beforehand. This problem has already been elaborated in chapter 2.3.2.5.

Scattering values of pdet and pex/pinitial could also be a consequence of errors in the mixture composition. If this were the case, the combustion times would also scatter. As Fig. 149 to Fig. 153 shows, the combustion times always varied monotonously when varying the composition of the two component mixtures. Thus, determination of the combustion time represents an internal cross check for the quality of the mixture preparation. 3.4.6 Thermal explosion and detonation To explain the detonation and thermal explosion phenomena observed with propene/O2/N2 we will discuss in detail the pressure-time traces found for propene/O2/N2 at pinitial = 5 bara and Tinitial = 25 °C (Fig. 158 and Fig. 160). For the other values of pinitial and Tinitial the characteristic effects are the same, only the location of the boundary between the deflagrative range, the thermal explosion range and the detonative range is different. Fig. 161 and Fig. 162 give examples for propene/O2/N2 at pinitial = 5 bara and Tinitial = 200 °C 3.4.6.1 Experimental observations Fig. 158 displays all pressure-time traces recorded over the central section of the explosive range of propene/O2 at pinitial = 5 bara and Tinitial = 25 °C. When increasing the propene content to values slightly above the LEL there is a purely deflagrative explosion in the vessel. For 2.25 vol.-% propene the pressure-time trace is shown.


The double-peak structure of the leading edge is a phenomenon observed close to the explosion limits, where the laminar flame speed is so low that flame propagation is markedly influenced by buoyancy effects: after being triggered in the center of the sphere, the flame first propagates upwards through the upper half of the sphere and then – with even slower speed – burns the rest of the mixture in the lower half of the sphere. When shifting mixture compositions away from the limits to the center of the explosive range flame propagation becomes spherically symmetric and the double-peak structure in the leading edge of the pressure-time trace disappears. When increasing the propene content to 3 vol.-% the pressure-time trace looks like a normal deflagration except for the extra peak at t = 172.5 ms (Fig. 158). The value of pheat_initial is about equal to pex = 33 bara. Also the next tests with 4, 5 and 6 vol.-% propene exhibit an extra peak that emerges at the upper part of the leading edge of the pressure-time trace. Following facts characterizing these peaks are important to memorize and are also observed for propene/O2 at other values of pinitial and Tinitial: a) The precompression ratios pheat_initial /pinitial associated with these peaks are large (of the order of

the explosion pressure ratio pex/pinitial). We never observed precompression ratios less than about 65% of the explosion pressure ratio (see Fig. 163).

b) The width of the peaks at half height is about 100 – 200 µs. c) In the leading edge of these peaks there are usually several sampling points. d) The occurrence of this extra peak does not bring about additional oscillations in the pressure-

time trace. e) There is a tendency that the precompression ratio decreases slightly as the propene content is

increased and thus more reactive mixtures are produced. f) For explosions producing extra peaks like these the noise heard outside the sphere is like a

“click”. In anticipation of what is explained later these peaks are attributed to the occurrence of thermal explosions. The value of the highest point in these thermal explosion peaks should probably not be taken too seriously for two reasons: 1) The sampling rate of 100 kHz is still small for such fast varying pressure signals, i.e. the true

maximum value is usually missed. 2) Experience from other experiments show that when extremely fast pressure variations with large

maximum pressure values like in detonation peaks act on the silicone grease in front of the sensor, the pressure might be recorded too large by up to a factor 2. A possible reason is the compressibility of the grease, which allows it to be accelerated in direction to the membrane. When finally the “flow” of grease comes to a standstill the deceleration acts as an additional force on the membrane.

Further increase of the propene concentration to 7 vol.-% and higher also gives rise to the occurrence of sharp peaks emerging from the leading edge of the pressure-time trace (Fig. 158). These peaks differ from the thermal explosion peaks in the following respects: a) The precompression ratios pdet_initial/pinitial associated with these peaks can attain all values

between 1 (i.e. no precompression) and the explosion pressure ratio (see Fig. 163). b) The width of the peaks at half height is of the order of 50 µs and less. (This is the width we

measure with the same pressure sensors for usual detonations of hydrocarbon/O2 mixtures in long tubes).

c) In the leading edge of these peaks there is usually only one or no sampling point, i.e. the pressure rise is faster than in the thermal explosion peaks.

d) The occurrence of this extra peak triggers very strong and fast pressure variations in the sphere. These pressure variations sometimes look “irregular” and sometimes they seem to contain only one single frequency (mostly about 2 to 2.2 kHz), see Fig. 159.

e) In the fuel lean range the precompression ratio decreases monotonously with increasing propene concentration, reaches 1 and thereafter, in the fuel rich range, rises monotonously when further increasing the propene content. On the average, the maximum heights of these peaks are correlated with the precompression ratio.


f) For explosions producing extra peaks like these the noise heard outside the sphere is like a

prolonged whistle. g) On the average these peaks are about a factor 2 to 4 higher than the thermal explosion peaks. In anticipation of what is explained later these peaks are attributed to the occurrence of detonations. Concerning the maximum value of these peaks the same applies as was said in context with the thermal explosion peaks. When increasing the propene concentration from 42 vol.-% to 43 vol.-% the detonation ceases to occur and one is left with a normal deflagration. This remains so until the UEL is reached. As Fig. 129, Fig. 133 and Fig. 134 show, there is sometimes also in the fuel rich range a rather small interval of propene concentrations between the detonative and the deflagrative regime, where a thermal explosion turns up. When rising the initial temperature to 200 °C one observes in principle the same (see Fig. 161) as discussed above for the case with Tinitial = 25 °C, only the propene concentrations that mark the transition points between the different explosion regimes are slightly different. When we move on the stoichiometric line (with respect to CO2 and H2O formation) from low propene concentrations (less reactive mixtures) to high propene concentrations (more reactive mixtures), we also pass from a purely deflagrative regime into a concentration interval with the occurrence of thermal explosions and finally end up in the detonative regime (see Fig. 160 and Fig. 162). Again, in the detonative regime the precompression drops with increasing reactivity of the mixture. Tab. 26 summarizes the above-mentioned observations.

Tab. 26: Summary of experimental observations in the range of thermal explosions and detonations.

course of explosion deflagration thermal explosion detonation

presence of pressure pulse

no yes yes

width of pressure pulse an half height [µs]

(not applicable) 100 - 200 < 50

height of pressure pulse divided by pressure in mixture just before its occurrence

(not applicable) typically 4 to 7 (equals deflagration pressure ratio of the

precompressed unreacted mixture)

typically 15 to 50 (due to too small

sampling rate the true maximum was

presumably mostly missed)

precompression ratio at the moment when pressure pulse occurs

(not applicable) 5 – 10 (largest value equals

pex/pinitial of the corresponding gas

mixture)

1 – 20 (largest value equals

pex/pinitial of the corresponding gas

mixture) "oscillations" in reaction gases after occurrence of pressure pulse

no oscillations anyway

no oscillations massive oscillations (due to shock front

bouncing backwards and forwards in reaction gases)

acoustic sound heard outside the vessel

nothing “click” prolonged whistle


171 171.5 172 172.5 173 173.5 1740

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Fig. 158: Pressure-time traces recorded for propene/O2 mixtures at pinitial = 5 bara and

Tinitial = 25 °C. The plots are arranged with increasing propene content: preceding page top left (lowest propene content) -> bottom left -> top right -> bottom right -> this page top left -> bottom left -> top right -> bottom right (largest propene content). The moment of ignition was always at t = 0 ms. All plots with a quadratic frame show in appropriate resolution the thermal explosion or detonation peak contained in the pressure-time trace plotted in the rectangular frame left to it.

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Experiment: Propene=18.18 vol.-%, O2=81.82 vol.-%,1 bar abs, 200 °C, 20 l sphere

Fig. 159: Pressure oscillations recorded after detonative explosion in propene/O2 (pinitial = 1 bara,

Tinitial = 200 °C, 18.18 vol.-% propene, 81.82 vol.-% O2). The lower plot represents the continuation of the signal depicted in the upper plot (note the different vertical scale). The cycle time T of the oscillation increases by about 30 % over a time period of 80 ms due to cooling of the reaction gases (speed of sound is proportional to 1/T1/2).


Fig. 160: Pressure-time traces recorded for stoichiometric (with respect to CO2 and H2O

formation) propene/O2/N2 mixtures at pinitial = 5 bara and Tinitial = 25 °C. The plots are arranged with increasing propene content: top left (lowest propene content) -> bottom left -> top right -> bottom right (largest propene content). For two propene concentrations the reproducibility of the thermal explosion peaks is shown: The second test with 6 vol.-% was carried through directly after the first test, the second test with 7.7 vol.-% was made 2 months after the first one.

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File: PROP216A-60%O2.DAT


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Propene = 5 vol.-%,

O2 = 95 vol.-%, 5 bar abs, 200 °C

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3 3.5 4 4.5 5 5.5 60

50

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300

Time [ms]

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pene

=8

vol.-

%,

O2

=92

vol.-

%,

5ba

rabs

,200

°C

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20L-Kugel

-0.5 0 0.5 1 1.5 2 2.50

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Time [ms]

Prop

ene

=13

vol.-

%,

O2

=87

vol.-

%,

5ba

rabs

,200

°C

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File: PROP59A;Propen=18.18, O2=81.82Vol.%;5bar abs;200C.DAT


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File: V26A-Propen=35.5,O2=64.5 vol.-%, 5 bar , 200 C.DAT

3.5 4 4.5 5 5.5 6 6.50

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pene

=35

.5vo

l.-%

,O

2=

64.5

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%,

5ba

rabs

,200

°C

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abs]


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=38

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%,

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=62

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%,

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rabs

,200

°C


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=40

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%,

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=60

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%,

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rabs

,200

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Fig. 161: Pressure-time traces recorded for propene/O2 mixtures at pinitial = 5 bara and

Tinitial = 200 °C. The plots are arranged with increasing Propene content: preceding page top left (lowest propene content) -> bottom left -> top right -> bottom right -> this page top left -> bottom left -> top right -> bottom right (largest propene content). The moment of ignition was always at t = 0 ms. All plots with a quadratic frame show in appropriate resolution the thermal explosion or detonation peak contained in the pressure-time trace plotted in the rectangular frame left to it.


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File: PROP35A;Propen=6.9;N2=62.1; O2=31Vol.%;5bar abs;200C.DAT

Propene = 6.9 vol.-%,O2 = 31.05 vol.-%, N2 = 62.05 vol.-%5 bar abs, 200 °C

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6 6.5 7 7.5 8 8.5 90

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File: PROP81A;Propen=10;N2=45; O2=45Vol.%;5bar abs;200C.DAT

Pro

pene

=10

vol.-

%,

O2

=45

vol.-

%,

N2

=45

vol.-

%5

bara

bs,2

00°C

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(second test)

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sure

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abs]


20L-Kugel

Propene = 15.6 vol.-%,O2 = 70.2 vol.-%, N2 = 85.8 vol.-%5 bar abs, 200 °C(first test)

0 0.5 1 1.5 2 2.5 30

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File: PROP59A;Propen=18.18, O2=81.82Vol.%;5bar abs;200C.DAT

Propene = 18.18 vol.-%,O2 = 81.82 vol.-%, N2 = 0 vol.-%5 bar abs, 200 °C

-1 -0.5 0 0.5 1 1.5 20

25

50

75

100

125

Time [ms]

Fig. 162: Pressure-time traces recorded for stoichiometric (with respect to CO2 and H2O

formation) propene/O2/N2 mixtures at pinitial = 5 bara and Tinitial = 200 °C. The plots are arranged with increasing propene content: top left (lowest propene content) -> bottom left -> top right -> bottom right (largest propene content). For propene concentrations of 7.8, 13.3 and 15.6 vol.-% the reproducibility of the course of the combustion is shown.


0

5

10

15

20

25

0 10 20 30 40

Propene concentration in Propene/O2-mixture [vol.-%]

phea

t_in

itial

/pin

itial

or p

det_

initi

al/p

initi

al

50

5 bar abs, 25 °C

5 bar abs, 200 °C

0

5

10

15

0 5 10 15 20Propene conc. in (Propene:O2=1:4.5)/N2-mixture [vol.-%]

phea

t_in

itial

/pin

itial

or p

det_

initi

al/p

initi

al 5 bar abs, 25 °C

5 bar abs, 200 °C

Fig. 163: Top diagram: Precompression factors found for propene/O2 at pinitial = 5 bara and

Tinitial = 25 °C and 200 °C. The values are taken from the pressure-time diagrams shown in Fig. 158 and Fig. 161.

Bottom diagram: Precompression factors found for stoichiometric propene/O2/N2 at pinitial = 5 bara and Tinitial = 25 °C and 200 °C. The values are taken from the pressure-time diagrams shown in Fig. 160 and Fig. 162. The orange filled symbols stand for thermal explosions, the blue and read symbol for detonations. On the stoichiometric line at 200 °C one test at 7.8 vol.-% propene was only deflagrative.


3.4.6.2 Suggested explanation for the experimental observations The most reactive mixtures of the ternary system propene/O2/N2 will be close to the stoichiometric propene/O2 mixture (with respect to CO and H2O formation). In this region of the explosion triangle the laminar burning velocity will be largest, the ignition temperature will be lowest and the ignition delay times for thermal explosions will be lowest. Moving away from this region will reduce the burning velocity and increase the ignition temperature and the ignition delay time. All explosive mixtures close to the boundary of the explosive range and in particular at low oxygen concentrations are mixtures of low reactivity and, after having triggered a self-sustaining flame front by a thermal ignition source in these mixtures, the flame front propagates as a deflagration through the entire volume. All explosive mixtures lying in the yellow range in Fig. 129 to Fig. 136 still have some distance to the point of highest reactivity, but, by virtue of the high O2 content, should be more reactive than stoichiometric propene/air mixtures. When a deflagration is triggered in the centre of the sphere the flame front moves outwards in radial direction. The reaction gases produced act as a spherically expanding piston on the yet unreacted mixture located between the flame front and the wall of the vessel. Upon compression, this mixture is heated up. As Tab. 27 shows, temperatures well above 200 °C may be attained when starting at 35 °C and assuming precompression factors between 5 to 10 as found in Fig. 158, Fig. 160, Fig. 161 and Fig. 162. Bearing in mind that the ignition temperatures of hydrocarbon/air mixtures drop drastically to values between 200 °C and 250°C when rising the initial pressure to 10 bar [9, 10], it can be expected that the ignition temperature of the mixtures in the yellow range are even lower at the moment the thermal explosion occurs, because their reactivity is higher due to the higher oxygen content and because they are precompressed to pressures between 25 and 50 bara (in case of pinitial = 5 bara). If the actual temperature of the unreacted gas mixture attained by precompression is some tens degrees higher than its actual ignition temperature (i.e. in the precompressed state), the ignition delay time may drop down to values of 0.1 ms and the shell of the unreacted precompressed mixture in front of the wall of the sphere may undergo a thermal explosion. The pressure in the shell should rise by a factor of 4 to 7 which is the explosion pressure ratio of the unreacted mixture at the initial temperature attained by the precompression. This pressure rise is recorded by the sensor in the wall as thermal explosion peak, because the time needed to establish again pressure equilibration with the inner volume of the sphere filled with the reaction gas of the initial deflagration and having a pressure of pheat_initial, will be of the order 0.17 m/1100 m/s = 0.154 ms (0.17 m is the radius of the sphere, 1100 m/s is roughly the speed of sound in the hot reaction gases) and this time is presumably longer than the duration of the thermal explosion. When the mixture composition crosses the border between the yellow range and the magenta range in Fig. 129 to Fig. 136, the mixtures become even more reactive. Of course, thermal explosions can still occur (even at lower precompression ratios), but obviously the flame speed has become so high that the spherically expanding piston of reaction gases moves outwards so fast that there is no longer pressure equilibrium attained over the whole thickness of the shell of unreacted gas (see Fig. 164), but a thin layer of unreacted mixture directly ahead of the flame front is compressed to much higher values than the unreacted mixture further ahead of the flame front (i.e. close to the wall of the vessel) and consequently this thin layer attains temperatures far above the ignition temperature of the gas mixture such that ignition delay times presumably drop into the range of microseconds. Since these are conditions as present in detonations, a detonation now starts and propagates outwards through the rest of the unreacted shell of precompressed gas. Of course, a thermal explosion in the rest of the unreacted shell could still occur in principle, but obviously the detonation front has already passed through before the ignition delay time has elapsed.


024

1086

12

radial position r/R0 0 1

hot reaction gases

flam

e fro

ntpr

ecom

pres

sed

unre

acte

d m

ixtu

re

pres

sure

/pin

itial

024

1086

12

radial position r/R0 0 1

hot reaction gases

flam

e fro

nt

pres

sure

/pin

itial

prec

ompr

esse

dun

reac

ted

mix

ture

Fig. 164: Schematic representation of the assumed instantaneous radial pressure distribution

inside the 20 dm3 sphere during the course of an explosion in a gas mixture with pinitial = 1 bara (r = radial position, R0 = 0.17 cm for the 20 dm3 sphere). Note that the pressure we recorded in our experiments was always the pressure at r/R0 = 1.

left plot: shortly before a thermal explosion occurs in the unreacted precompressed mixture; the pressure does not vary with radial position, because the burning velocity is still slow enough for to allow for instantaneous pressure equilibration.

right plot: shortly before a DDT occurs at the interface between flame front and unreacted mixture. The flame speed is so high that pressure equilibrium over the whole sphere can no longer be attained.

Tab. 27: Temperature increase in a gas mixture brought about by adiabatic compression. The

initial pressure and temperature of the mixture is denoted by pinitial and Tinitial. The temperature reached after compression to pfinal is denoted by Tfinal. γ stands for cp/cv . The formula to calculate Tfinal is given below:

( )γ

γ 1−

⎟⎠⎞

⎜⎝⎛⋅=

initialfinal

initialfinal ppTT

pfinal/pinitial Tfinal [°C] resulting for different values of � in case that Tinitial = 35 °C = 308 K

Tfinal [°C] resulting for different values of � in case that Tinitial = 200 °C = 473 K

� = 1,4 � = 1,3 � = 1,2 � = 1,1 � = 1,4 � = 1,3 � = 1,2 � = 1,11,5 72,8 65,2 56,5 46,6 258,1 246,4 233,1 217,82 102,5 88,4 72,7 55,0 303,6 282,0 257,9 230,83 148,6 123,9 96,9 67,3 374,4 336,5 295,0 249,74 184,7 151,1 115,1 76,4 429,9 378,3 322,9 263,55 214,8 173,5 129,8 83,5 476,1 412,7 345,5 274,56 240,9 192,7 142,2 89,5 516,2 442,2 364,6 283,77 264,0 209,6 153,0 94,6 551,7 468,1 381,2 291,58 284,9 224,7 162,6 99,1 583,8 491,3 395,9 298,49 304,0 238,4 171,2 103,1 613,1 512,4 409,2 304,610 321,7 251,0 179,1 106,7 640,2 531,7 421,3 310,112 353,5 273,5 193,0 113,1 689,1 566,3 442,7 319,914 381,7 293,3 205,2 118,5 732,4 596,7 461,3 328,216 407,1 311,0 215,9 123,3 771,5 623,9 477,8 335,618 430,4 327,1 225,6 127,6 807,2 648,6 492,7 342,120 451,9 341,9 234,4 131,4 840,2 671,3 506,3 348,1


3.4.6.3 Pressure load on the vessel It is very important to point out that the maximum pressure load on the wall of the vessel is not produced by a stoichiometric (with respect to CO and H2O formation) propene/O2-mixture but by mixtures which are located on the circumference of the detonative range. This is because the precompression ratio pdet_initial/pinitial rises strictly linearly when varying the mixture composition on a straight line starting from the stoichiometric point given by 25 vol.-% propene and 75 vol.-% O2 and leading to any point on the boundary of the detonative range (see Fig. 163). On this boundary the precompression ratios are identical with the deflagration pressure ratios pex/pinitial of the mixtures. Fig. 165 shows an example. In the experiments with mixtures lying on or very close to the boundary line we did not always observe precompression factors as given here, which represent the largest values possible, but sometimes found smaller values. The largest value possible is only reached if the DDT happened directly in front of the wall. If it should happen a little earlier, i.e. already some millimetres in front of the wall, there is still a considerable quantity of unreacted gas such that the pressure in the sphere at the moment the DDT occurred is still much less than the explosion pressure ratio pex/pinitial of the mixture.

0 10 20 30 40 50 60 70 80 90 100Propene C3H6 [Vol.-%]

0

10

20

30

40

50

60

70

80

90

100

O2 [Vol.-%

]

0

10

20

30

40

50

60

70

80

90

100

N 2 [V

ol.-%

]


range of detonative

explosion

stoich

iometric C 3H 6 +

1.5 O 2 -> 3 CO + 3 H 2

stoich

iometr

ic C 3H

6 + 3

O 2 -> 3

CO + 3 H 2O

stoich

iometr

ic C 3H

6 + 4.

5 O2 ->

3 CO 2 +

3 H 2O


11

20

11

14 17

1

Fig. 165: Precompression factors (encircled numbers in colour rust-brown) on the boundary

(curve in rust-brown) of the detonative range, shown for the system propene/O2/N2 at pinitial = 5 bara, Tinitial = 25 °C. The values between the apex of the detonative range at about 8.4 vol.-% propene and 38 vol.-% O2 and the point with a precompression factor of 20 are obtained by linear interpolation.


To give an estimate of the detonation pressure we expect for pinitial = 5 bar abs on the boundary between the detonative and the heat explosion range, we apply the formula for the Chapman-Jouguet pressure pdet:

pdet /pinitial = (γ ∗M²+1)/(γ+1) ≈ (γ∗ M²)/(γ+1) = (D² *MW)/ (R* Tinitial *(γ+1))

where

M = D/c Mach number D propagation speed of detonation [m/s] c = (γ ∗ pinitial /ρ)0.5 speed of sound in the unreacted gas mixture [m/s] ρ density of unreacted gas mixture [kg/m3] MW mean molar mass of unreacted gas mixture [kg/mol] R=8.314 universal gas constant [J/(mol K)] Tinitial, pinitial initial temperature [K], initial pressure [Pa] of gas mixture γ cp/cv, for diatomic gases γ = 1.4 In a coarse approximation, we neglect that M will vary over the boundary line and simply assume M = 7.2 at Tinitial = 35 °C for all mixtures lying on the boundary. For γ we assume γ = 1.3. Then the Chapman-Jouguet pressure is 29.7*pinitial. Choosing a precompression factor of 10 and consequently an initial temperature of 251 °C, the Chapman-Jouguet pressure on the boundary line will be: pdet = (5 bar abs) * 10 * 29.7 * (273+35)/(273+251) = 872 bar abs. (A preciser calculation would have to account for the variation of M and the precompression factor over the boundary line). For the stoichiometric (CO and H2O formation) propene/O2 mixture we assume M = 7.6 at Tinitial = 35 °C and γ = 1.3. Since there is no precompression, pdet = (5 bara) * 33 = 165 bara, which is much less than the value found for the boundary line. It should be pointed out, that the pressure sensor and also the wall of the vessel experience the reflected pressure of the detonation and not the side-on pressure, i.e. the pressure signal should be about twice as high as the calculated Chapman-Jouguet pressure. 3.4.6.4 Origin of pressure oscillations observed in case of detonations In case of detonations of stoichiometric ethane/O2 mixtures which we triggered in different vessels (volumes: 100 dm3, 500 dm3 and 2500 dm3) as shown in Fig. 166, we observe that the shockwave of the detonation, after having reached the upper torospherical head of the vessel for the first time, is reflected downwards into the hot reaction gases and travels many times downwards and upwards through the vessel with decreasing amplitude. Fig. 167 shows two examples. The time difference ∆t between two successive reflections becomes ever larger because the reaction gas cools down and consequently the speed of sound decreases.


s = 34 wal

l th

ickn

ess

s =

30

346

175

4021

3040 (cylindirical section)3718

s = 40

332128

3866

P2

1938

loca

tion

of ig

nitio

nw

eld

neck

flan

geN

W15

0, P

N16

0

wel

d ne

ck fl

ange

NW

300,

PN

160

torospheri-cal head

torospheri-cal head

oute

r dia

met

er φ

o = 1

016

inne

r dia

met

er φ

i = 9

56

1928

P1 P3L = 3644

Fig. 166: Dimensions of the 2500 dm3 vessel, design pressure = 63 bara.

L/Φi = 3644mm/956mm = 3.8. P1, P2 and P3 denote the positions of piezoelectric pressure sensors mounted flush with the inner wall of the vessel. In the experiments the vessels were put in upright position, the thermal ignition source being at the bottom. The smaller vessels with 500 dm3 and 100 dm3 were constructed in the same way (cylindrical section with torospherical heads welded to both ends, design pressure 63 bara) and had almost the same L/Φi –ratios: V = 500 dm3: L = 2146 mm, Φi = 570 mm, L/Φi = 3.76, distance P1-P3: 2309 mm; V = 100 dm3: L = 1309 mm, Φi = 327.2 mm, L/Φi = 4.0, distance P1-P3: 1343 mm.

The retonation wave should be almost absent, because for the mixture used the predetonation distance will be less than 10 cm (in analogy to the results presented in chapter 3.4.6.7). If we assume a spherical flame propagation after ignition the retonation wave is generated on a spherical shell (or half-shell since ignition was at the end of the vessel) with radius less than 10 cm. Since the detonation wave later encompasses the whole cross section of the vessel, the wave seen in Fig. 167 is only the shock wave of the former detonation. In the 20 dm3 sphere the shockwave will undergo in principle the same reflections and bounces inwards and outwards with ever decreasing amplitude. Due to the small dimensions there is no pressure plateau in the pressure-time trace between two successive impacts of the shockwave on the pressure sensor as we observed in the large vessels (Fig. 167). The fact that sometimes a rather “irregular” variation of the pressure is recorded (Fig. 158, Fig. 160, Fig. 161 and Fig. 162) and sometimes an almost monofrequent oscillation (Fig. 159) presumably depends on the location of the shockwave when it emerged for the first time. If it had the shape of a spherical shell and had its center coinciding with the center of the 20 dm3 sphere, a monofrequent oscillation pattern should result. If it had its center not coinciding with the center of the sphere, or if it did not extend over a full spherical shell but only part of it, an “irregular” pattern should result.


65 70 75 80 85 900

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Time [ms]

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sure

[bar

abs

]

File: EFOV108K.DAT

Experiment:Ethane=22.2 vol.-%, O2=77.8 vol-%, Pinitial = 5 bar abs,Tinitial = 25 °C,100 l vessel, L/D = 4

v = ∆s/∆t = (2*1.343m)/2 ms = 1343 m/s

∆t = 2ms

65 70 75 80 85 900

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]

File: EFOV150K.DAT

Experiment:Ethane=11.4 vol.-%, O2=40 vol.-%, N2=48.6 vol.-%,Pinitial = 8 bar abs,Tinitial = 25 °C,500 l vessel, L/D = 3.8∆t = 3.66ms

v = ∆s/∆t = (2*2.309m)/3.66ms = 1261m/s

Fig. 167: Pressure-time diagrams measured in vessels with L/D ca. 3.8 after detonations in

ethane/O2 and ethane/O2/N2 mixtures by the piezoelectric senor P3. The peaks are due to the shockwave of the detonation which after having reached the one end of the vessel is reflected backwards into the hot reaction gas, travels until reaching the opposite torospherical head of the vessel, is again reflected and so on.

The time difference ∆t between two successive impacts of the shockwave of the original detonation on the pressure sensor in the wall should be given by the time it takes to travel from the wall to the center and back to the wall. Assuming a speed as observed in the vessel experiments (see Fig. 167) one finds ∆t = ∆s/v = 0.34m/1343m/s = 0.25ms, which corresponds to 4 kHz. In few cases with almost monofrequent pattern we find 5 kHz, but in most cases 2 to 2.2 kHz are observed giving a time difference ∆t of 0.45 to 0.5 ms, which is twice as long as the value calculated above. This discrepancy could be resolved if one assumed that the DDT happened only over the upper part of a spherical shell. Then, when the pressure sensor located at the bottom of the sphere records a peak for the first time, this would not be the detonation peak but the impact of the shock wave that was reflected when the detonation front arrived at the upper section of the wall of the sphere. The distance ∆s travelled by the shock wave between two successive impacts on the pressure sensor is now 2 times the diameter of the sphere and hence ∆t = 2*0.34m/1343m/s = 0.5ms corresponding to 2 kHz.


3.4.6.5 Detonative range at pinitial = 30 bara and Tinitial = 25 °C For these initial conditions a DDT might have resulted in major damage to our sphere. Therefore we did not determine the boundary line between the range of deflagrative explosions and the range of thermal explosions. We even skipped the critical concentration range on the propene/air line between 4.46 vol.-% and 9 vol.-% propene, because at 4.46 vol.-% the value for KG normalized to 1 bara was already twice as high as it was at the same propene content for pinitial = 10 bara. Hence it is not known so far, whether in the explosion triangle for pinitial = 30 bara and Tinitial = 25 °C the thermal explosion range or the detonative range might even reach down to the propene/air line. In a conservative approach we have drawn in Fig. 132 the conceivable range of detonative explosion such that it touches the air line. 3.4.6.6 Variation of the detonative range with vessel volume For all practical applications it is of interest to determine how the range with thermal explosions and the range with detonations changes with vessel volume. We conducted experiments at Tinitial = 25 °C with stoichiometric ethane/O2/N2 mixtures (ethane:O2 = 1:3.5) at pinitial = 4 bara and 8 bara in the 20 dm3 sphere and in three different vessels (100 dm3, 500 dm3 and 2500 dm3). The design of the vessels is given by Fig. 166. The inside of the vessels was free of any turbulence enhancing elements. Tab. 28 summarizes the results of the experiments. The transition from deflagration to detonation occurs between O2 = 30 vol.-% and O2 = 40 vol.-% and is obviously independent of the vessel volume, which varied over more than two orders of magnitude. These results must be interpreted as preliminary, since the number of conducted experiments is still small and tests with Ethane/O2 mixtures are still missing. Also, the small number of tests does not yet allow a final statement whether the precompression observed in the larger vessels is the same as in the 20 dm3 sphere or less.

Tab. 28: Compilation of experiments to investigate the transition between deflagrative and detonative explosions in vessel-like geometry as function of vessel volume. All tests were carried through with stoichiometric (with respect to CO2 and H2O formation) Ethane/O2/N2-mixtures at 25 °C. All vessels were put upright. Ignition was in the center in case of the sphere and at the bottom in case of the vessels.

course of the explosion pinitial = 4 bara pinitial = 8 bara

O2 concentration in the stoichiometric mixture [vol.-%]

20 dm3 sphere

2500 dm3 cylinder, L/D = 3.8

20 dm3 sphere

100 dm3 cylinder, L/D = 4



30 deflagrative deflagrative deflagrative deflagrative, deflagrative 35 thermal

explos. detonation

40 detonation detonation detonation detonation detonation


3.4.6.7 Predetonation distance of propene/O2-mixtures in long tubes To investigate whether the precompression of the unreacted mixture brought about by the initial deflagration helped to trigger the DDT or whether the DDT is mainly due to the fast burning velocity which makes the spherically expanding piston of the reaction gases moving outwards so fast that the unreacted gas directly in front of it is pressurised because of its inertia, we started first experiments to determine the predetonation distance of propene/O2-mixtures at Tinitial = 25 °C and pinitial = 1.5 bara in a 76 mm wide tube so long that a pressure rise of the unreacted gas in front of the piston given by the expanding reaction gases would be due to the inertia of the gas only and not be influenced by the finite length of the tube. Fig. 168 shows the experimental setup, the results for the four investigated mixtures are attached to the appropriate mixture composition in the explosion diagram of propene/O2/N2 at pinitial = 1.5 bara and Tinitial = 25 °C (Fig. 169). Both mixtures that lie in the detonative range as determined in the 20 dm3 sphere have predetonation distances less than 58% of the radius of the sphere. Therefore it is obvious that these mixtures gave rise to detonations in the sphere. The interesting question which shall be answered by future experiments is, whether mixtures showing predetonation distances in the tube slightly longer than the radius of the 20 dm3 sphere will still produce a DDT in the sphere or not.

P1

7 m, PN160, DN80, φo = 88.9 mm, φi = 76.3 mm, s = 6.3 mmlocation ofthermal ignition source

0.1 m

2 m 2 m 2 m 1 m

P2 P3 P4 P5 P6 P7 P8 P9

0.2 m 0.3 m

0.4 m

Enlarged section of pipe close to ignition source

P1,..P4: piezoelectric pressure transducers

Fig. 168: Experimental setup to determine the predetonation distances in quiescent Propene/O2

mixtures at pinitial = 1.5 bar and Tinitial = 25 °C. For the tests only pressure sensors P1 to P4 were installed.


0 10 20 30 40 50 60 70 80 90 100Propene C3H6 [Vol.-%]

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O2 [Vol.-%

]

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]


Fig. 169: Results of the experiments to

determine the predetonation distance Lpredet of Propene/O2 mixtures in long tubes at pinitial = 1.5 bara with the experimental setup as displayed by fig. 24.

Test no.

Propene content [vol.-%]

O2 content [vol.-%]

1 18.18 81.82 2 13 87 3 8 92 4 4 96

r e of defl ative explosion,

bar abs, 25 °C

angagr

1

range of detonative

explosion

stoichiometric

C 3H 6+ 1.5 O 2 -

> 3 CO + 3 H 2

stoich

iometr

ic C 3H

6 + 3

O 2 -> 3

CO +

3 H2O

stoich

iometr

ic C 3H

6 + 4.

5 O2 -

> 3 C

O 2 + 3

H 2O


Results in the

lpredetonation ≤ 10 cmlpredetonation ≤ 10 cm

40 cm ≤ lpredetonation ≤ 7 monly deflagrative


4 Conclusions 4.1 Hydrogen The explosion indices of hydrogen/air mixtures were determined for initial pressures up to 30 bara and initial temperatures of 250 °C. Thereby the volume dependence, pressure dependence and temperature dependence on explosion limits, explosion pressure ratios and rates of explosion pressure rise respectively KG-values were obtained. It was presented that the explosion indices of hydrogen often show a contrary behaviour compared to that of hydrocarbons. Here to mention is the pressure anomaly on the upper explosion limit. Such pressure anomaly seems also to be present when looking to the normalized KG-values. Instead of a steady decrease with increasing pressure as it was determined for methane no clear tendency is obvious for the examined conditions. The explosion indices of hydrogen/oxygen were also determined. Due to the heaviness of the explosion reactions it was only possible to carry out experiments at 1 bara and 20 °C. Nevertheless the analysis of these experiments is critical, especially in determining the rates of pressure rise for mixtures close to the stoichiometric composition. The combustion is more or less detonation-like. After a modification of the ignition vessel and the data acquisition system it might be possible to carry out further experiments at higher conditions. 4.2 Methane Through the systematic examinations for methane/air mixtures the volume dependence, pressure dependence and the temperature dependence on the determined explosion indices were obtained over a wide range of initial conditions. All three parameters influence the explosion indices in that way that an increase of one or more parameter leads to an increase of the explosion indices. Special attention has to be given to the fact that the KG-value determined at elevated conditions in a 2000-dm3 sphere is much higher than in smaller volumes. Occurring turbulences which were seen in a 2.0-dm3 vessel by high speed video frames influence especially the KG-value determination. This will lead to important discussions when a standard for the determination of explosion indices at non-atmospheric conditions will be developed. 4.3 Propene The explosion ranges for propene/O2/N2 and the explosion characteristics like pex, KG and Tcombustion were determined at different initial pressures and temperatures. The oxidation reaction proceeds fastest for propene/O2 mixtures stoichiometric with respect to CO and H2O formation. The explosion pressure ratio for propene/O2 is about 24 at 25 °C. For the combustion in vessel-like apparatuses three different combustion domains were found. Whereas the deflagrative and the detonative domain can clearly be deduced from the recorded pressure-time diagrams, the exact course of the combustion in the region in between is not perfectly clear, but might be interpreted as a domain with thermal explosions. Due to precompression effects the maximum mechanical load on the walls of the vessel is not produced by detonations of stoichiometric combustible/O2 mixtures but by mixtures lying on the boundary between the detonative and the thermal explosion domain, where pressures can be larger by factors of the order of 5 to 10. First experiments suggest that these domains do not change too much with vessel volume.


5 Perspectives The results presented in this report contain the explosion indices of three different fuels. According to the Detailed Project Plan [1] six more fuel gases shall be examined at various initial conditions, namely ethane, ethylene, n-butane, ammonia and carbon monoxide. The raw data will be analysed and the explosion indices of tests with these fuels presented in Deliverable Report No. 9 “Temperature and pressure dependence of EL, Pex and dp/dt of ethane, ethylene, n-butane, ammonia and carbon monoxide”. Explosion limits and other indexes are of great significance for practice because they serve as basic design information for maintaining safety. Since in this project several unexpected results have been found which need explanation and also since the test criterion of 5% pressure increase for risk assessment purposes seems to be too simple, in a separate report designated Deliverable No. 10, further thoughts will be developed. This has also to do with the requirement to propose a model for the explosion limits, so that mixture properties could be predicted by calculation and no additional testing would be necessary, given the properties of the pure components are known. Such model, or at least an attempt to that effect, will be presented in Deliverable Report No. 19 “Model, software for calculation of flammability limits and its validation”. On the basis of the systematic determination of the explosion indices for several fuel gases where some interesting effects and dependencies were found a limited number of tests will be defined. Especially the industrial partners in the project will announce what fuel and/or conditions are of greatest interest to them. The results will be reported in Deliverable No. 13 “Report on determination of non-standardised explosion parameters or report provided by industrial end-users”.


6 References [1] STEEN M. (2000), Handbuch des Explosionsschutzes, Wiley-VCH, New-York

[2] Project SAFEKINEX, Contract No. EVG1-CT-2002-00072, Detailed Project Plan, June 2003

[3] SAFEKINEX, Deliverable No.2: “Report on experimental influences on the determination of explosion indices”

[4] EN 1127, Explosive atmospheres – Explosion prevention and protection, Part 1: Basic concepts and methodology

[5] CHEMSAFE, Database of evaluated safety characteristics, Update 2002, DECHEMA, BAM und PTB, Frankfurt/M., Germany

[6] prEN 14522: Determination of the minimum ignition temperature of gases and vapours, document for enquiry (2002)

[7] SAFEKINEX, Standard Operating Procedures for the determination of auto ignition delay times, SOP/IDT

[8] K. Holtappels, V. Schröder: Explosion characteristics of hydrogen-oxygen and hydrogen-air mixtures at elevated temperatures and pressures, 2nd European Hydrogen Energy Conference, November 22-25, 2005, Zaragoza, Spain, PG 664 – PG666

[9] K. Holtappels, V. Schröder: Explosion characteristics of hydrogen-air and hydrogen-oxygen mixtures at elevated pressures, International Conference on Hydrogen Safety, September 8-10, 2005, Pisa, Italy, PG 35 – 47

[10] EN 13673 part 1 and 2: Determination of the maximum explosion pressure and maximum rate of pressure rise of gases and vapours (European Standard)

[11] SAFEKINEX, Deliverable No.29: “Report on intermediate species concentration during the ignition process”

[12] Lewis, van Elbe, Combustion, flames and explosions of gases, Academic Press, New York-London 1961

[13] SAFEKINEX, Deliverable No.16: “Explosion pressure” – The program for calculation of maximum pressure of explosion for chemical equilibrium conditions

[14] Pekalski, A.A.: “Theoretical and experimental study on explosion safety of hydrocarbons oxidation at elevated conditions”, PhD dissertation, Delft University of Technology, 18 November 2004, Delft, The Netherlands.

[15] Cashdollar K.L.; Zlochower I.A.; Green G.M.; Thomas R.A.; Hertzberg M.; Journal of Loss Prevention in Process Industries, 13 (2000) 327

[16] Mashuga Ch.V.; Crowl D.A.; Journal of Loss Prevention in Process Industries, 13 (2000) 369

[17] Takahashi A.; Urano Y.; Tokuhashi K.; Kondo S.; Journal of Hazardous Materials 105 (2003) 27

[18] http://www.gaseq.co.uk

[19] DIN 51649-1: Bestimmung der Explosionsgrenzen von Gasen und Gasgemischen in Luft (German Standard)

[20] EN 1839: Determination of explosion limits of gases and vapours, 2003 (European Standard)

http://www.gaseq.co.uk/


[21] E. Brandes and W. Möller: Sicherheitstechnische Kenngrößen - Band 1: Brennbare Flüssigkeiten und Gase, Wirtschaftsverlag NW, Bremerhaven, 2003 (ISBN: 3-89701-745-8)

[22] CHEMSAFE – A database of evaluated safety characteristics, edited by DECHEMA, BAM and PTB, Frankfurt/Main, Germany, Update 2004

[23] M. Molnarne, T. Schendler and V. Schröder: Sicherheitstechnische Kenngrößen - Band 2: Explosionsbereiche von Gasgemischen, Wirtschaftsverlag NW, Bremerhaven, 2003 (ISBN: 3-89701-746-6)

[24] National Institute of Standards and Technology (NIST) Standard Reference Database Number 69, June 2005 Release, http://webbook.nist/gov/chemistry/

[25] F. R. Russel and R. H. Mueller, Chem. Eng. News 30, 1239 (1952)

[26] A. A. Pekalski, H. P. Schildberg, P. S. D.Smallegange, S. M. Lemkowitz, J. Z. Zevenbergen, M. Braithwaite, H. J. Pasman, Loss Prevention and Safety Promotion in the Process Industries, 11th International Symposium, Praha, PG2118 – PG2138 (2004)

[27] E. Brandes, W. Moeller, W. Hirsch, T. Stolz, Physikalisch-Technische Bundesanstalt, Braunschweig (private communication)

[28] M. Gödde: Zündtemperaturen organischer Verbindungen in Abhängigkeit von chemischer Struktur und Druck, PHD Thesis, ISBN 3-89701-261-8 (1998)

[29] Mc GEEHIN P., WEINBERG F., CARLETON F., BOTHE H., PROUST Ch., TORTOISHELL G. (1994), Optical techniques in industrial measurement : safety in hazardous environments, Final report of E.U contract ref 3365/1/0/165/90/8-BCR-UK(30)

[30] CARLETON F., BOTHE H., PROUST Ch., HAWKSWORTH S. (2000), Prenormative Research on the use of Optics in Potentially Explosive Atmospheres, Final report of E.U contract ref SMT4-CT96/2104

[31] HAWKSWORTH S., BEYER M., ROGERS R., PROUST Ch., (2006), Mechanical ignition hazards in potentially explosive gas and dust atmospheres, Final report of E.U contract ref G6RD-2001-00553

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