Model for Ullage Flammability, Ignition and Explosion – BlazeTank

by

N. Albert Moussa and Venkat Devarakonda

BlazeTech 24, Throndike St, Cambridge, MA 02141

(617) 661 0700 Fax. (617) 661 9242 www.blazetech.com

and

Gregory Czarnecki

US Air Force, 46th Test Wing, WPAFB, OH  45433-7605

Presented at

The Fourth Triennial International Fire and Cabin Safety Research Conference, Sponsored by the FAA and CAA

15 – 18 November, 2004

Lisbon, Portugal

Background

• Recent accidents involving center wing fuel tank explosions– May 1990, Philippine Airlines 737, 8 fatalities – July 17, 1996 TWA 747, 230 fatalities – March 3, 2001,  Thai Airlines 737-300, 1

fatality

• Consequent concern about ullage tank flammability and explosion

When is Ullage Flammable?

• Ullage is flammable (ARAC) during:– 30% of the operational time for heated CWT– 4 to 6% of operational time for unheated CWT– 2 to 4% of operational time for main wing tanks

• Factors governing flammability include:– Fuel properties, temperature and tank design– Flight mission profile– Environmental conditions – External factors: heat rejection from surrounding

equipment

BlazeTank* Model Capabilities• Engineering model developed by BlazeTech

over the last 10 years for commercial and military aircraft

• Model is general and can be run for any fuel• Key modules of BlazeTank

1. Ullage flammability

2. Ignition

3. Deflagration •Moussa, N.A. et al, “BlazeTank Model for the Flammability, Ignition and Overpressure in an Aircraft Fuel Tank,” presented at the FAA’s Conf. on Aircraft Fire and Cabin Safety Research, Atlantic City, NJ., Nov. 1998.•Also, supported in part by Contract F08635-02-C-0167

Overall Model Architecture

Fuel Conditions: type, amount &temperature

Tank Geometry and dimensions

Ignition Characterization: Sourcelocation, type and strength

Flight Profile: Altitude versus time,Fuel extraction rate to engine, and Fueland tank wall temperatures

BlazeTank

Model Inputs

Temp. and concentration vs.height and time

Flammable volume inside fueltank

Ignition and Propagation

If explosion occurs, Temp., burn rate and Overpressure vs. time

Output

1. Ullage Flammability

• BlazeTankTM predicts the ullage flammability accounting for the following effects– Flow in and out of the vent – Stratification – Solubility of oxygen and nitrogen – Oxygen evolution during scrub, wash and refueling– Droplets suspended in ullage due to tank slosh and

vibration (enhanced evaporation induced by the ignition source can create a localized flammable zone in an otherwise fuel lean ullage)

Key Processes in Model

Fuel

Ullage

Vent and Wash

Skin Temperatures

Fuel Temperature

Scrub Gas

Fuel Withdrawal

O 2, N 2 andVapor Evolution

m From Ascent, Descent or Fuel Withdrawal

Two Solutions

• Well Mixed Tank• Concentration gradients in fuel tank (1 D)

– Near fuel surface, vapor concentration corresponds to saturated vapor pressure

– Near a vent, vapor concentration is lower

Model Predictions vs. Test Data

• Test data selected for comparison (Sagebiel, J.C., 1997): – Flight tests to emulate TWA 800

– 3 flights conducted successively

– 50 gallons of Jet A from Athens was added to CWT

– Ullage vapors collected in vacuum containers and analyzed for hydrocarbons

• BlazeTank– Used the upper and the lower limits on measured fuel temperatures

before takeoff

– Used the fuel vapor pressure data from Shepherd et al (2000)

Ullage Flammability: BlazeTank Predictions vs. Test Data

0.5

0.7

0.9

1.1

1.3

0 2 4 6 8 10 12 14

altitude (kft)

fuel

vo

lum

e p

erc

en

t

BlazeTank (low) BlazeTank (high) Test Flight 1

Test Flight 2 Test Flight 3

Flammable

Not Flammable

2. Ignition

• BlazeTankTM includes models for the following main modes of ignition– Ignition source

• spark

• high speed fragment

• hot surface ignition

• .........

Hot Surface Ignition

• Minimum Ignition Temperature occurs when heat loss rate at tank wall = heat release rate due to combustion (Vant Hoff criterion)

x

Reaction Rate

Temperature

Tb(t)

Hot Spot, Tw

Ignition Kinetics

0.1

1

10

100

0.00075 0.0008 0.00085 0.0009 0.00095

1/T (1/K)

ign

itio

n d

ela

y (

ms

)

77 micron spray 70 micron spray fuel vapor at 300 C

fuel vapor at 450 C Model

Tk

748,22exp1097.5 9

Source: Mullins, B.P., Fuel, London, 23, pp. 234-252, 1953.

Hot Surface Ignition Temperature (HSIT) for Jet A

Vapors• 9 ft3 fuel tank• Tank pressure = 1 atm• Time for ignition = 10 – 60 s• Trends consistent with Kuchta et al (1965): HSIT decreases with increasing hot surface area

800

840

880

920

960

1000

1040

1080

0 100 200 300 400

Hot surface area (cm2)

HS

IT (

K)

ignition in 10 s ignition in 60 s Test Measurement

3. Deflagration Analysis

Ignition and flame propagation in premixed fuel vapor and air

Deflagration Model

• Key assumptions– Ullage consists of two zones:

burned and unburned gases– Unburned gases are pressurized by

expanding burnt zone– Burning velocity = f(fuel type,

stoichiometry, T and P) from literature

– Pressure in ullage remains spatially uniform. It equilibrates at acoustic speed >> deflagration speed

• BlazeTank solves for the following coupled equations

– Continuity– Energy – Species mass conservation

Burned GasesP, T b, b, u b

Unburned GasesP, T u, u

u f

Flame front

Fuel Properties Required for Deflagration Calculations

• Molecular formula and molecular weight• Density • Vapor pressure (Antoine coefficients)• Flammability limits• Heat of formation or heat of combustion• Heat capacity as a function of temperature• Burning velocity parameters

Verification and Validation of Deflagration Model

• We compared BlazeTankTM model against test data generated during the TWA 800 accident investigation

• Available data sets:– 2 data sets from single compartment tests in

ARA’s Quarter scale CWT – 3 data sets from tests in JPL’s HYJET test

facility

Quarter Scale Test Facility

78"

48"

242"

Depth of the test chamber = 255”

HYJET Test Facility

• Ignition Source: H2/O2 combustion in driver creates a torch that breaks a diaphragm and enters the receiver tank• Receiver Tank Volume = 1.180 m3• Driver Tank Volume = 0.028 m3

Test Conditions

Quarter Scale Tests HYJET Facility Tests

Fuel Arco Jet ALAX Jet A for 40 and 60 C tests; El Monte Jet A for 50 C test

Tank geometry Trapeziodal CylindricalUllage volume (m3) 0.783 1.18Ullage initial pressure (bar) 0.585 0.585Ullage initial temp (C) 40 and 50 40, 50 and 60Equivalence ratio, phi 0.67 and 0.81 0.61, 0.99 and 1.06Fuel mass loading (kg/m3) 3 3

Ignition source Filament at tank centerTorch produced by H2/O2 reaction in a driver tank

Vents none none

Sample BlazeTank OutputUnburned Gas Temperature

300

350

400

450

500

550

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Time / s

Un

bu

rned

Gas

Tem

per

atu

re /

K

phi = 0.81 phi = 0.67

Tunburned increases by a maximum of a few hundred degrees due to compression by burnt gases: consistent with Metghalchi and Keck (1982)

Sample BlazeTank OutputBurned Gas Temperature

500

1000

1500

2000

2500

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Time / s

Bu

rned

Gas

Tem

per

atu

re /

K phi = 0.81 phi = 0.67

–Temperature of burned gases o initially = AFT at constant pressureo decreases rapidly initially due to large flame specific areao increases later as specific area decreases

–Maximum temp. of burned gases << AFT at constant volume

BlazeTank OutputDensity of Burned and Unburned Gases

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Time / s

Den

sity

/ k

gm

-3

phi = 0.81 phi = 0.67

Burned Gas

Unburned Gas

BlazeTank Output Burning Velocity and Flame Speed

0

100

200

300

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Time / s

Fla

me

Sp

eed

, B

urn

ing

Vel

oci

ty /

cm.s

-1

phi = 0.81 phi = 0.67

flame speed

burning velocity

Comparison of BlazeTank Predictions with Tests in Quarter Scale CWT for TWA 800

0

20

40

60

80

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Time / s

Pre

ssu

re /

psi

a

phi = 0.81, Test phi = 0.67, Test

phi = 0.81, BlazeTank phi = 0.67, BlazeTank

J. E. Shepherd et al, “Results of 1/4-scale experiments, vapor simulant and liquid Jet A tests” Explosion Dynamics Laboratory Report FM 98-6, July 1998

Comparison of BlazeTank Predictions with Tests in Cylindrical Hyjet Tank

0.0E+00

1.0E+05

2.0E+05

3.0E+05

4.0E+05

5.0E+05

6.0E+05

7.0E+05

0 0.5 1 1.5 2 2.5 3

time (s)

pre

ssu

re (

Pa)

phi = 1.06, Test phi = 0.99, Test

phi = 0.61, Test phi = 1.06, BlazeTank

phi = 0.99, BlazeTank phi = 0.66, BlazeTank

J. E. Shepherd et al, “Results of 1/4-scale experiments, vapor simulant and liquid Jet A tests” Explosion Dynamics Laboratory Report FM98-6, July 1998

Conclusions

• BlazeTankTM accounts for most of the key processes associated with fuel tank ullage explosion– Ullage flammability including transient effects– Various types of ignition sources– Explosion overpressures, flame temperatures and burn velocities

• Parametric calculations over a wide range of conditions yield reasonable predictions verifying the model

• Model predictions agree very well with test measurements validating the model

Applications of BlazeTank

• Aircraft related– Investigation of accidents related to fuel tank flammability – Evaluation of various fuel tank protection technologies e.g.,

deviations from 12% O2 requirement in fuel tanks due to • improper mixing • OBIGGS equipment malfunction

– Assessment of the effect of variations in fuel properties – Fuel tank vulnerability to terrorist attacks

• Assessment of deflagration hazards in closed/vented tanks containing flammable liquids/vapors– Tanks in automobiles, trucks, tankers, ships, barges, etc. – Industrial storage tanks– Storage bottles in laboratories