Influence of Rarefaction Wave-flame Interaction on .../media/Documents/Subject...

IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

INFLUENCE OF RAREFACTION WAVE-FLAME INTERACTION ON PREMIXEDPROPANE-AIR FLAME STRUCTURE AND PROPAGATION BEHAVIOR

Chen Xianfeng Sun Jinhua and Yao Liyin1State Key Laboratory of Fire Science, University of Science and Technology of China, Hefei, Anhui 230026, China;

e-mail: [email protected]

To explore the influence of rarefaction wave on flame structure and propagation behavior, the

premixed propane/air flame was experimented in a rectangle combustion pipe. The high speed

camera and Schlieren images methods were used to record the processes of interaction between

rarefaction wave and flame. Meanwhile, the pressure sensor was fixed up to catch the pressure vari-

ation in the process of flame propagation. Two opposite direction rarefaction waves were induced

by different ignition scheme to intervene in the flame. The experiment result shows that the inter-

ference of rarefaction wave on flame causes the flame front structure change, which leads to the

flame transition from laminar to turbulent quickly. The coflow rarefaction wave intervenes in the

flame and making the flame front surface turn into dentiform structure, where violent turbulent

combustion begins to appear in part of the flame front firstly; subsequently the turbulent combustion

spreads to the whole flame front surface. On the other hand, the counterflow rarefaction wave

induces the whole flame fronts to be turbulent since the beginning and subsequently intensifies tur-

bulence. In addition, the coflow rarefaction leads the flame propagation speed to decrease. On the

contrary, the counterflow rarefaction wave accelerates the flame speed on the whole, accompanied

with sharp vibration.

KEYWORDS: gas explosion, rarefaction wave, flame structure, laminar flame, turbulent combustion

INTRODUCTIONFlammable gas explosion is one of the most destructiveaccidents in industry and living field. Incident statisticsdata of fires and explosions shows that flammable gasinvolved in 90% of the total accidents [1]. Therefore, it isimportant to discern the gas explosion mechanism, andthus to predict and suppress the explosion disaster with cor-responding methods.

In order to prevent gas explosion, it is indispensableto control the fuel-air combustion process. Generally, inthe case of gas explosion, the transition from laminarflame to turbulent combustion takes place firstly and follow-ing accelerates the flame propagation speed, which increasesthe combustion surface area and heat release rate, and finallyleads to serious explosion disaster due to the rapid overpres-sure rise [2,3]. Accordingly, the flame propagation andpressure rise are closely related to turbulent combustionoccurrence. Then it is necessary to discern the micro-dynamic process and the intrinsic mechanism during thelaminar-turbulent transition.

Gas combustion process coexists with flow in almostall the practical combustion cases. The flame-flow inter-actions have been one of the most challenging areas in com-bustion science field, which related to the fundamentalelements of combustion process, i.e., flame structure,flame propagation, flame instabilities, et al [4,5,6,7]. Adetailed physical understanding the laminar-turbulent tran-sition and turbulence-flame interactions are still lackingdue to the complexity of the problem itself. Many elementsinfluence the gas combustion process, of which the pressurewave-flame interaction is one of the most typical reasons to

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induce turbulence combustion [8]. In most industrydisasters, the pressure wave always influences the gaspropagation speed. At the same time, the pressure-flameinteraction can induce the flame unstable and further elicitturbulence, even result in the transition from deflagrationto detonation. Accordingly, the pressure–flame interactionmay change the whole combustion process and character-istic, which has attracted many researchers’ attention.Markstain [9] firstly recorded the photos of flame instabilityinduced by shock wave, describing the flame variationcourse when affected by incidence and reflection shockwave respectively. Fairweather [10] carried out methane-air explosion in a semi-vented pipe, and found that theflame propagation along the pipe axis was almost laminar,only at the late stage of explosion did the fast turbulencecombustion appear and obvious overpressure occurred.Thomas et al experimenting on flame–shock wave inter-action showed the shock wave can obviously acceleratethe flame propagation speed, which was testified byGamezo [11] et al. Many numerical simulations [12,13]and theoretic analysis on pressure-flame interaction arealso achieved, most of which focus on the variation offlow field and flame behavior after the pressure waveswept flame front. But the instant interaction at the initialstage of flame propagation is rarely studied, especially themicrostructure variation during the interaction between rar-efaction wave and flame is few available [14].

As a whole, the pressure-flame interaction is compli-cated; especially the flame structure change induced by rar-efaction wave needs further research. Therefore, in thispaper the premixed propane-air flame was observed as

Figure 1. Sketch of gas explosive experiment system

1. Combustion chamber, 2. Mercury vapor lamp, 3. Concave mirror, 4. Knife edge, 5. Data recorder, 6. Computer, 7. Spark igniter,

8. High speed video camera, 9. Synchronization Controller


object, and the high speed video camera and Schlierenimage technology were used to record the process offlame propagation. The pressure transducer was used toinvestigate the interaction of rarefaction wave and premixedflame structure.

EXPERIMENT SETUP AND PROCEDURE

EXPERIMENTAL APPARATUSThe experimental system is schematically shown inFigure 1, which is composed of a combustion chamber, anignition system, a data recorder, a Schlieren image system,a high speed video camera and a synchronization controller.The Schlieren image system shown in Figure 1 consists of a25 W mercury vapor lamp, a knife edge and two concavemirrors. The combustion chamber is a square pipe, and theinner size is 500 mm � 80 mm � 80 mm. Two spark igni-ters were fixed at each end of the pipe, which can elicit rare-faction wave propagating towards different directionsrespectively relative to flame propagation. To observe

Figure 2. Sketch of gas ex

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flame propagation characteristic and flame structureevolution process, the sides of the chamber were made ofhigh-intensity glass. The cross section of the combustionchamber is shown in Figure 2. When the membranecovered the vent ruptured, a rarefaction was induced toinfluence the flame propagation behavior. A highfrequency-dynamic piezoelectricity transducer was set upon the combustion pipe to detect the pressure-time historyduring the interaction between rarefaction wave and flame.

EXPERIMENTAL CONDITIONS AND PROCEDUREAfter the experiment pipe was vacuumized, the combustionpipe was filled with 4.0% volume percent of propane/airmixture, and the inner pressure was kept to be 0.1 Mpa.

When the premixed mixture was ignited with an elec-tric spark produced by a high voltage igniter, a flame frontpropagates forward in sphere shape. When the pressureexceeds a certain extent, the membrane will rupturequickly. And then a rarefaction wave will be induced to

plosion experimental pipe


propagate leftward in the combustion chamber. When therarefaction wave overtakes flame, the flame front structureand propagation behavior will change due to the interfer-ence between rarefaction wave and flame. The flame beha-vior and pressure-time history will be recorded with highspeed Schlieren system and piezoelectricity transducers.

Finally, two ignition schemes were performed in theexperiment. When ignited with electrode1, both the flameand rarefaction wave propagate leftward. When ignitedwith electrode 2, the rarefaction wave propagates leftward,while the flame propagates reversely.

In the test, the startup time of the high speed videocamera (Photron, Fastcam Ultima APX), the high speeddigital data recorder (HIOKI, 8826 Memory Hicorder) andthe high voltage igniter was controlled by a synchronizationcontroller. The detailed experimental conditions were givenas following:

Ignition voltage: 20,000 V; Discharge period: 0.001 s;Membrane intensity: 5kpa; Recording speed of high speedvideo camera: 10,000 fps; Sampling rate of data recorder:100 KHz.

RESULTS AND DISCUSSION

THE CASE OF FLAME–COFLOW RAREFACTION

WAVE INTERACTIONIn this case, the premixed mixture was ignited by electrode1. The flame front propagated from right to left. When themembrane ruptured, a rarefaction wave was induced tomove leftward in the pipe. When rarefaction wave caughtup with the flame, the interaction of rarefaction wave and

Figure 3. High speed schlieren

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flame was recorded with high speed Schlieren system andpressure transducer.

Flame structure and propagation behavior based on

Schlieren imagesAs known to all, the density change of flow field shows thedirect influences of temperature, concentration and pressureon flame structure. The Schlieren image was obtained basedon the light refraction of flow, so the Schlieren image can beused to reflect the inner flow structure characteristics.According to turbulent combustion theory, the turbulenceflame is composed of many different scale vortexes, whichnot only disperses and distorts outside the flame front, butengulfs the premixed gas inside the flame front. The reactionintensity and the heat and mass transfer in the flame frontchange with the density grads, which reveal the inner flowfield structure of premixed flame.

Figure 3 is a series of typical high speed Schlierenphotographs, depicting the flame propagation process andflame front structure behavior. As shown in Figure 3, theconvex flame front propagated towards the unburned gasin regular spherical wave shape and the flame was obviouslaminar at the early stage. Meanwhile, the burned andunburned gases were separated by the thin flame front.When time t ¼ 14.5 ms after ignition, the rarefaction waveinduced by membrane rupture caught up with the flamefront, and began to intervene in the flame structure. Here-after, the flame configuration changed gradually. Firstly,the flame front became plane indicating the flame frontreached the minimum surface area. With the flame propa-gation, the flame front stretched and led to thicker flamefront. From time t ¼ 16.5 ms, the regular flame front was

images on flame propagation

0 5 10 15 20 25 30

0

2

4

6

Ove

rpre

ssur

e/ k

pa

Time after ignition / ms

Figure 5. Relation between pipe pressure with time


torn up into dentiform structure, many irregular wrinklesappearing, all which made for greater flame front surfacearea. Here, the flame structure characteristics indicatedthat obvious turbulence came into being. With further inter-action of rarefaction wave and flame, bifurcation phenom-ena appearing on the flame front led to thicker flame frontand greater turbulence intensity. It can be seen from theSchlieren images that, the turbulent flame propagation in awide flame front area, unlike the laminar flame in a verythin front surface. After t ¼ 22 ms, the flame front outlinebecame blurry and propagated ahead with strong turbulenceas a whole. The Schlieren images reveal the micro-processof flame structure changing clearly, which also showedjust the interference of rarefaction wave on flame inducingthe transition from laminar to turbulence immediately.Many experiments on the same test condition showed thesimilar results and phenomena as above.

Discussion on the results of pressure and flame speedFigure 4 shows the relationship between flame propagationspeed with time. At the initial stage, the laminar flamepropagation speed increased with time increasing. Aftert ¼ 14.85 ms, the rarefaction wave encountered the flameand began to influence the flame propagation behavior.Firstly, the flame speed decreased sharply from 2.1 m/s to27.5 m/s in no more than 1.5 ms. But the flame speedrose quickly again and oscillation appeared on the speedcurve, mainly due to the turbulence effect induced by rare-faction wave-flame interaction.

Figure 5 shows the curve of pressure variation in com-bustion chamber with time. It can be seen that the pressurevalue in combustion chamber increased with time at theinitial stages of flame propagation. At t ¼ 14.8 ms, thepressure dropped sharply, which indicates that rarefactionwave reached the pressure sensor in combustion pipe andbegan to influence the pressure in combustion pipe. Com-bined with Schlieren images, the rarefaction wave-flameinteraction caused the transition from laminar to turbulentquickly, and led to obvious flame speed fluctuation.

0 5 10 15 20 25 30

-6

-3

0

3

Fla

me

spee

d / m

.s-1


Figure 4. Relation between flame speed with time

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Therefore, it can be drawn that just the interference ofcoflow rarefaction wave on flame decreased the flame speedsharply, while increased the turbulence intensity quickly,which caused the flame speed to accelerate again, simul-taneously accompanied with fluctuations.

THE CASE OF FLAME–COFLOW RAREFACTION

WAVE INTERACTIONIn this case, the premixed mixture was ignited by electrode2. After ignition, the flame front propagates rightwards,while the rarefaction wave moved leftward when the mem-brane ruptured. When rarefaction wave encountered flamefront, they may overlap each other. The interaction wouldinfluence the flame propagation configuration and thecharacteristic of flow field.

Analysis on high speed Schlieren imagesFigure 6 shows the high speed Schlieren images of counter-flow rarefaction wave-flame interaction. It can be seen that,at the initial stages of combustion, the laminar flame propa-gated ahead in smooth spherical wave. After t ¼ 16 ms, thereversed V shape flame structure appeared. At aboutt ¼ 20 ms, the flame front and the rarefaction wave encoun-tered and overlapped, which caused the flame front todeform. After then, the flame front configuration becameirregular. With the interaction of rarefaction wave andflame, the flame front stretched to greater flame surfacearea. Here, the whole flame front was turbulent, and theflame profile was clear, indicating that the flame was mid-small scale turbulent combustion. With the interference ofrarefaction wave, the flame front grew blurry quickly,which means that the intense turbulence began to appear.At t ¼ 26 ms, it can be seen clear that the large-scalevortex appeared in the flamelet. In conclusion, the counter-flow rarefaction wave can deform the flame structure andsubsequently led to the transition from laminar to turbulencequickly.

Figure 6. High speed Schlieren images on flame propagation


Discussion on pressure and flame speed resultsThe pressure variation with time is shown in Figure 7. At theearly stage of flame propagation, the pressure value rosewith time increasing. When the membrane ruptured att ¼ 19.8 ms, a rarefaction wave is induced to move left-wards in the combustion pipe, which caused the pressureto decrease rapidly, and obvious pressure fluctuationsappeared on the curve. After time t ¼ 22 ms, the pressurevalue began to rise again with time.

Figure 8 shows the flame speed with time during thecourse of counterflow rarefaction wave-flame interaction.It is clear that laminar flame was not affected by rarefactionwave at the initial stage, and the flame speed increasedalmost linearly. After the flame encountered rarefactionwave at t ¼ 20 ms, the flame speed rose up to morethan 20 m/s quickly due to the flame-rarefaction wave

5 10 15 20 25 30 35 40

0

4

8

12

Ove

rpre

ssur

e / k

pa


Figure 7. Relation between pipe pressure with time

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interference. After then, there appeared sharp vibration onthe speed curve. Comparing with the Schlieren images inFigure 6, it can be seen that, just the interaction of reverserarefaction wave and flame caused the flame speed toincrease on the whole, simultaneous accompanied withgreat fluctuations, all which further accelerated the flametransition from laminar to turbulence.

CONCLUSIONSIn this paper, the interaction between premixed propane-airflame and rarefaction wave was studied. By two ignitionschemes, the rarefaction wave was induced to intervene inthe flame propagating towards different directions. In thetest, high speed Schlieren image and pressure measuring

5 10 15 20 25 30 35 40

0

5

10

15

20

25

Fla

me

spee

d /m

.s-1


Figure 8. Relation between flame speed with time


technology were used to record the flame front structure andpropagation behavior. Based on the result and discussion,the conclusions were drawn as following:

. Rarefaction wave imposed great influence on flamestructure and propagation behavior. When encounteredwith rarefaction wave, the flame was induced tochange from laminar to turbulence quickly due to therarefaction wave-flame interaction. In addition, therarefaction-flame interaction accelerated the laminar-turbulent transition.

. Coflow and counterflow rarefaction wave can makedifferent effect on flame structure deformation andpropagation behavior. The coflow rarefaction wavemade the flame front transform to dentiform configur-ation firstly; here strong turbulence existed in only partof flame front. With the flame propagation, intensive tur-bulence spread to the whole flame front in the end.However, the counter-flow rarefaction led to thelaminar-turbulence transition through the whole flamefront since the beginning, and subsequently intensifiedthe turbulence gradually.

. The cowflow rarefaction wave can decelerate the flamespeed on the whole, even once led to the propagationreversely. After the turbulence became strong enough,the flame speed rose quickly again. While counterflowrarefaction wave can speedup flame propagating, simul-taneously accompanied with sharp fluctuations.

. During the course of flame structure change induced byrarefaction wave, obviously pressure fluctuations alsocomes into being, which further to accelerate laminar-turbulent transition.

ACKNOWLEDGEMENTSThank Prof. Shen Zhaowu and associate Prof. Zhang Li fortheir contributions to pressure measuring in the experiment.This work was supported by the National Natural ScienceFoundation of China (NSFC No. 50576093) and theKey Project of NSFC (No. 50536030).

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