# Presentation: 526717

## 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](http://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
- Ullage flammability
- Ignition
- 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

## 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 

## 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

- 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)

## Ignition Kinetics

- 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

## 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 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

- 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

## Sample BlazeTank OutputUnburned Gas Temperature

- Tunburned 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
-  initially = AFT at constant pressure
-  decreases rapidly initially due to large flame specific area
-  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

- 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