Model for Ullage Flammability, Ignition and Explosion – BlazeTank by N. Albert Moussa and Venkat Devarakonda BlazeTech 24, Throndike St, Cambridge, MA (617) Fax. (617) and Gregory Czarnecki US Air Force, 46th Test Wing, WPAFB, OH 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 , 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 Also, supported in part by Contract F C-0167

Overall Model Architecture Fuel Conditions: type, amount & temperature Tank Geometry anddimensions Ignition Characterization: Source location, type and strength Flight Profile: Altitude versus time, Fuel extraction rate to engine, and Fuel and tank wall temperatures BlazeTank Model Inputs Temp. and concentration vs. height and time Flammable volume inside fuel tank Ignition and Propagation If explosion occurs, Temp., burn rate and Overpressure vs. time Output

1. Ullage Flammability BlazeTank TM 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

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 Flammable Not Flammable

2. Ignition BlazeTank TM 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 T b (t) Hot Spot, T w

Ignition Kinetics Source: Mullins, B.P., Fuel, London, 23, pp , 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

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

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 BlazeTank TM 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 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 = m3 Driver Tank Volume = m3

Test Conditions

Sample BlazeTank Output Unburned Gas Temperature T unburned increases by a maximum of a few hundred degrees due to compression by burnt gases: consistent with Metghalchi and Keck (1982)

Sample BlazeTank Output Burned Gas Temperature –Temperature of burned gases o initially = AFT at constant pressure o decreases rapidly initially due to large flame specific area o increases later as specific area decreases –Maximum temp. of burned gases << AFT at constant volume

BlazeTank Output Density of Burned and Unburned Gases

BlazeTank Output Burning Velocity and Flame Speed

Comparison of BlazeTank Predictions with Tests in Quarter Scale CWT for TWA 800 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 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 BlazeTank TM 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% O 2 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