# Presentation: 814824

## Modeling Wing Tank Flammability

- Dhaval D. Dadia

- Dr. Tobias Rossmann
- Rutgers, The State University of New Jersey
- Piscataway, New Jersey

- Steven Summer
- Federal Aviation Administration
- Atlantic City Airport, New Jersey

## Motivation

- Numerous accounts of  wing tank explosions  across the world
- Current flammability models are for center wing tanks
- The proposed regulation for wing tank safety are mostly based on center wing tank models
- Models will predict ullage concentrations existing during typical ground and flight operations

## Current Work

- Flammable mixtures can be achieved in the wing tank
- Experiments are being conducted to build flammability models for wing tanks
- Current work involves
- Predicting the influence of the surrounding temperatures on the characteristic fuel surface temperature
- Creating a model that will predict flammability in wing tanks using heat transfer correlations

## Overview

- Single Thermocouple Method (STM)
- Difference between Center Wing Tank and Wing Tank
- Center Wing Tank Flammability Model
- Heat and Mass Transfer Correlations
- Experimental Results
- Computational Results

## Distillation Curve

- Jet fuel is a mixture of many different hydrocarbons
- Fuel composition is characterized by the number of alkane reference hydrocarbons
- The approach reduces the number of components from over 300 down to 16 species (C5-C20 alkanes)
- Liquid compositions of different JP-8 samples with varying flashpoints are presented in terms of the mole fractions of C5-C20 alkanes

## Single Thermocouple Method

- Uses Fuel Air Ratio (FAR) calculator by Ivor Thomas
- Calculates fuel air ratio over a range of altitudes and temperatures
- All compounds with same carbon number were assigned together
- Fuel is segregated based on boiling points of alkane species respective of their carbon number

## Single Thermocouple Method

- At constant temperature the THC increases as the pressure decreases
- Polynomial correlation between Scaled THC and temperature
- Film temperature is calculated at a given pressure and THC

## Center Wing Tank (CWT)

- The CWT has thin layer of fuel at the bottom of the tank
- 30% Mass Loading
- The tank is heated from  the bottom due to  heat released from  underneath the tank

## Wing Tank (WT)

- The WT is mostly filled with fuel
- 80 % Mass Loading
- The tank is heated from the top from an ambient heat source such as the Sun

## Sorting Data

- The data is sorted because of the difference in the driving force
- The data is sorted into
- Ascending Profile
- The top surface is hotter than the fuel surface
- The ullage temperature governs the film temperature
- Descending Profile
- The fuel is hotter than the top surface
- The fuel temperature governs the film temperature

## Correlations

- Ascending profile
- Correlations between ullage temperature and fuel temperature
- Ullage temperature is greater than liquid fuel temperature
- Correlation coefficient 0.89

## Correlations

- Ascending profile
- Correlating ullage temperature with film temperature
- Liquid fuel temperature is greater than ullage temperature
- Correlation coefficient 0.976

## Correlations

- Descending profile
- Correlations between ullage temperature and fuel temperature
- Ullage temperature is greater than liquid fuel temperature
- Correlation coefficient 0.41

## Correlations

- Descending profile
- Correlations between ullage temperature and fuel temperature
- Liquid fuel temperature is greater than ullage temperature
- Correlation coefficient 0.93

## Summary of STM

- Correlation works best when:
- Liquid fuel temperature is larger than the ullage temperature
- Fuel temperature is the driving force
- During Ascending pressure profile
- Vapor pressure remains constant

## Base Model

- Current CWT Model (Polymeropoulos 2004)*
- Natural convection flow field between the heated floor and the unheated ceiling and sidewalls
- Ullage gases are well mixed due to natural convection and mass transfer
- Liquid vaporization
- Vapor Condensation
- Natural convection flow is in the turbulent regime

- * JET A VAPORIZATION IN A SIMULATED AIRCRAFT FUEL TANK, Polymeropoulos  and Ochs 2004

## Principal Assumptions 

- Well mixed gas and liquid phases
- Buoyancy induced mixing
- Quasi-steady transport using heat transfer correlations
- The analogy between heat and mass transfer for estimating film coefficients for heat and mass transfer
- The liquid fuel and wall temperatures are known from experiments

## Computational Method

- Inputs
- The tank geometry
- Fuel loading
- Liquid fuel composition
- Tank pressure
- Liquid fuel, and tank wall temperatures as functions of time
- Computes
- Equilibrium species concentrations of Jet A in a uniform temperature, constant pressure tank
- Temporal variation of vapor temperature and species concentration

## CWT Model

- Simulation using center wing tank flight test data
- The calculated THC is in good agreement with the measured THC

- *Vaporization of JP-8 Jet Fuel in a Simulated Aircraft Fuel Tank under varying ambient conditions –
- Ochs 2006

## CWT Correlations

- Heat & Mass Transfer Correlations
- Horizontal surface:
- Top surface: Lower surface of cooled plate
- Top of Fuel Layer: Upper Surface of heated plate
- Vertical Surface:
-  Laminar Forced Convection on a flat plate

- Horizontal Surface

- Vertical Surfaces

## Difference in Model Correlations

- The CWT model differs from the WT model in the ascending and cruise conditions due to:
- Percent load
- Ullage height
- Heat and mass transfer correlations
- Heat source
- Surface being heated

## Experiments

- Experiments were conducted
- For 60% and 80% mass loadings
- For equilibrium temperatures from 80°F to 100°F
- For cruising altitudes of 25000 feet and 34000 feet

## Experimental Equipment

- Experiments conducted in an altitude chamber
- Designed to simulate temperature and pressure similar to a flight profile
- Can simulate altitudes from sea level to 100,000 feet
- Can simulate temperatures from  -100 ̊F to +250 ̊F
- NDIR gas analyzer used to measure the total hydrocarbon concentration

## Experimental Setup

- An aluminum fuel tank of dimensions 24”w x 24” d x 36” h was used
- Access panels on top for thermocouple penetration, ullage sampling, vent, and the fill tube
- Thermocouples measured surface, ullage, fuel surface and bulk fuel  temperatures
- The vent was equipped with a mass flow meter

## Flight Profile

- The following flight profile will be used in the altitude chamber
- Cruise at 35000 feet
- Total flight time is 2.5 hours

## Combination of Models

- Simulation using flight test data
- Using wing tank dimensions on a CWT flammability model
- Shows that the CWT model works for wing tanks in descending pressure profiles

## Wing Tank Correlations

- Heat & Mass Transfer correlations
- Horizontal surface: (Ascending)
- Top surface: Laminar Forced Convection on a flat plate
- Top of Fuel Layer: Laminar Forced Convection on a flat plate
- Horizontal Surface (Cruise and Descending)
- Top surface: Turbulent Forced Convection on a flat plate
- Top of Fuel Layer: Laminar Forced Convection on a flat plate
- Vertical Surface:
- Laminar Forced Convection on a flat plate

- Horizontal Surface

- Vertical Surface

## Test Results

- Ascending Top Surface Heat Transfer Correlations

## Test Results

- Cruise and Descending Top Surface Heat Transfer Correlations

## Summary of Experimental Results

- Laminar Forced Convection  during ascent
- Turbulent Forced Convection the rest of the flight
- Shows ullage gases are well mixed in the ullage
- High Reynolds number in the heat transfer cells in the ullage

## Conclusion

- Single Thermocouple method can calculate THC using data from a single thermocouple in the tank
- The differences between the WT model and the CWT model:
- Percent Load
- Ullage Height
- Heat and mass transfer correlations
- Heat Source
- Surface being heated
- The CWT model cannot be applied in the ascending and cruise profiles, but can be applied in the descending profiles
- Experiments will be conducted
- To confirm the state of the ullage
- To compare computed data to experimental data

## Questions?