# Presentation: 412121

## Vaporization of JP-8 Jet Fuel in a Simulated Aircraft Fuel Tank Under Varying Ambient Conditions

- Robert I Ochs
- Federal Aviation Administration William J. Hughes Technical Center
- International Aircraft Systems Working Group Meeting
- October 26, 2006

## Outline

- PART ONE – INTRODUCTION
- Motivation
- Review of Literature
- Objectives
- PART TWO – MODEL DESCRIPTION
- Description of Model
- Discussion of JP-8 and Jet A fuel characterization
- PART THREE – EXPERIMENTAL
- Description of Experimental Setup and procedures
- Typical fuel vaporization results

## PART ONE:  INTRODUCTION

## Introduction

- Focus of this work is the study of jet fuel vaporization within a fuel tank
- Primary motivation resulted from the TWA Flight 800 disaster in 1996
- NTSB-led accident investigation determined the cause of the crash was an explosion in a nearly empty center wing fuel tank caused by an unconfirmed ignition source

## Fuel Vaporization

- Flammable vapors were said to exist due to the combined effects of bottom surface heating and very low fuel quantity within the tank
- Low fuel quantity results in different compositions between the liquid and the vapor
- Lighter low molecular weight components vaporize first
- These components are known to have a significant effect on vapor flammability

- Liquid Fuel Layer

- Heat Source

- Vent

- Q

- Fuel Vapor

- Ullage Space

## Review of Literature

**Fuel Tank Flammability**
- Nestor, 1967: *Investigation of Turbine Fuel Flammability Within Aircraft Fuel Tanks*
- Kosvic, et al., 1971: *Analysis of Aircraft Fuel Tank Fire and Explosion Hazards*
- Summer, 1999, 2000, 2004: *Mass Loading, Cold Ambient effects on Fuel Vapor Concentrations, Limiting Ullage Oxygen Concentrations *

**Jet Fuel Research**
- Shepherd, et al, 1997, 1999: *Jet A composition, flashpoint, and explosion testing*
- Woodrow, 2000: *Characterization of Jet Fuel Vapor and Liquid*

- No fuel vaporization data sets including simultaneously varying ambient temperatures and pressures

## Objectives 

- An experiment was designed to:
- Simulate in-flight environment around a fuel tank
- Fuel tank situated in an environmental chamber that could simultaneously vary the ambient chamber temperature and pressure
- Measure conditions in and around the fuel tank
- Fuel tank instrumented with thermocouples
- Ullage fuel vapor concentration measured with a flame ionization detector
- Comprehensive data sets were generated for model validation
- A pre-existing model was used to compare measured and calculated ullage gas temperature and ullage vapor concentration
- The same model was used to make flammability assessments and to discuss the flammability in terms of the overall transport processes occurring within the fuel tank

## PART TWO: MODEL DESCRIPTION

## Modeling Fuel Vaporization

- Calculations can be performed to determine the amount of fuel vapor existing in the ullage space at a given moment
- The model used in this work (Polymeropoulos 2004) employed the flow field that developed as a consequence of natural convection between the heated tank floor and the unheated ceiling and sidewalls
- Combined with flammability limit correlations, the model can give estimates of the duration of time in which the fuel tank can be considered flammable

## Physical Considerations

- 3D natural convection heat and mass transfer
- Liquid vaporization
- Vapor condensation
- Variable Pa and Ta
- Multicomponent vaporization and condensation
- Well mixed gas and liquid phases
- Raullage~o(109)
- Raliquid~o(106)

## Principal Assumptions

- Well mixed gas and liquid phases
- Uniformity of temperatures and species concentrations in the ullage gas and in the evaporating liquid fuel pool
- Based on the magnitude of the gas and liquid phase Rayleigh numbers (109 and 105, respectively)
- Use of available experimental liquid fuel and tank wall temperatures
- Quasi-steady transport using heat transfer correlations and the analogy between heat and mass transfer for estimating film coefficients for heat and mass transfer
- Liquid Jet A composition from published data of samples with similar flash points as those tested (Woodrow 2000)

## Heat and Mass Transport

- Liquid Surfaces (species evaporation/condensation)
- Fuel species mass balance
- Henry’s law (liquid/vapor equilibrium)
- Wagner’s equation (species vapor pressures)
- Ullage Control Volume (variable pressure and temperature)
- Fuel species mass balance
- Overall mass balance (outflow/inflow)
- Overall energy balance
- Heat transfer correlations from natural convection in enclosures
- Heat and mass transfer analogy for the mass transfer coefficients

## Characterization of Multicomponent Jet Fuel

- Samples of Jet-A have been characterized by speciation at and near the fuel flash point (Naegeli and Childress 1998)
- Over 300 hydrocarbon species were found to completely characterize Jet-A and JP-8
- It was found by Woodrow (2000) that the fuel composition could be estimated by characterizing it in terms of a number of *n*-alkane reference hydrocarbons, determined by gas chromatography
- The approach taken by Woodrow effectively reduces the number of components from over 300 down to 16 (C5-C20 alkanes)
- The results from Woodrow’s work present liquid compositions of different JP-8 samples with varying flashpoints in terms of the mole fractions of C5-C20 alkanes
- Since fuels of different composition could be represented by their respective flashpoints, it is evident that the flashpoint is dependent upon the fuel composition

## Characterization of Experimental Fuel

- Fuel used in this experimentation was tested twice for flashpoint
- Both tests resulted in a fuel flashpoint of 117°F
- Characterized fuels from Woodrow’s work with similar flashpoints were sought to represent the experimental fuel

- Compositions of two fuels with flashpoints of 115°F and 120°F were used to essentially “bracket” the experimental fuel with flashpoint of 117°F

## PART THREE: EXPERIMENTAL

- Apparatus, Procedures, and Results

## All experimentation performed at the William J. Hughes Technical Center, Atlantic City Int’l Airport, NJ 

- Facility houses an environmental chamber
- designed to simulate the temporal
- changes in temperature and pressure
- appropriate to an in-flight aircraft
- Can simulate altitudes from sea level to 100,000 feet
- Can simulate temperatures from -100°F to +250°F

- 	Aluminum fuel tank placed inside environmental chamber
- 36”w x 36” d x 24” h, 1⁄4” Al
- 2 access panels on top surface for thermocouple penetration and ullage sampling
- 2” diameter vent hole, 3” diameter fuel fill

- All experimentation performed at the William J. Hughes Technical Center, Atlantic City Int’l Airport, NJ

## Instrumentation

- Omega® K-type thermocouples
- 3 bolt-on surface mount
- 1 adhesive surface mount
- 8 1/16” flexible stainless steel
- Measurement error of ±1°F
- Dia-Vac® dual heated head sample pump
- Technical Heaters® heated sample lines
- J.U.M.® model VE7 total hydrocarbon analyzer flame ionization detector (FID)
- Omega® high sensitivity 0-15 psia pressure transducer
- Brisk-Heat® 2,160 watt silicone rubber heating blanket

## Experimental Procedure

- Initial Conditions
- The initial condition was decided to be at the point of equilibrium, typically achieved about 1-2 hours after fuel was loaded and chamber was sealed
- Initial data indicated that at equilibrium the tank temperatures and ullage vapor concentration varied little with time (quasi-equilibrium)
- This point was critical to the calculations, as the subsequent time-marching calculations initiated with this point
- Quasi-equilibrium was said to exist if the ullage vapor concentration varied by less than 1,000 ppm (0.1%) over a period of ten minutes

- Test Matrix
- A quantity of 5 gallons was used for each test
- An arbitrary fuel temperature setpoint approximately 30°F above the initial temperature was found to create sufficient ullage vapor concentrations within the calibration range
- Dry tank tests
- Isooctane
- Constant ambient pressure
- Varying ambient temperature and pressure
- Repeatability

## Typical Results: Fuel Tank at Sea Level, Constant Ambient Conditions

- Similar liquid heating profiles were used for tests of same type
- Heating and vaporization trends seen here typical of all other tests
- Note the uniformity in the ullage gas temperature (well-mixed)

***Liquid, Heater, Ambient Temperatures***

***Ullage Temperatures***

***Surface Temperatures***

***Ullage Vapor Concentration***

## Validation of the Well Mixed Assumption

- Model assumes uniform, well-mixed ullage gas from the magnitude of the Raleigh number, based on the floor to ceiling temperature difference and the distance between them, typically of order 109
- This assumption is validated by the experimental data from three ullage thermocouples in various spatial locations within the ullage
- One test with no fuel in the tank
- One test with fuel in the tank
- Similar uniformity in ullage gas temperature was found in all other tests as well

*Dry tank*

*Tank with 5 gal. fuel*

## Ullage Gas Temperature Predictions

- The data from the same two tests input into the model to calculate the ullage gas temperature
- Ullage gas temperature predictions were within the thermocouple measurement error
- Ullage gas temperature predictions agree well with measured ullage gas temperature

## Isooctane Fuel Vaporization

- A pure component fuel of known composition was used to remove the ambiguity of fuel composition from the model calculations
- Isooctane is quite volatile at room temperature, so the fuel had was cooled to near 3°F to obtain fuel vapor concentrations within the FID calibration range of 0-4% propane
- Satisfactory agreement between measured and calculated ullage vapor concentrations was obtained, considering the difficulties involved in using isooctane

## Constant Ambient Pressure at Sea Level

- Two fuel compositions (F.P. = 115 and 120 °F) with flashpoints bracketing the experimental fuel’s flashpoint (F.P. = 117°F) were used to calculate the ullage vapor concentrations
- Two tests are shown with similar heating profiles, both with 5 gallons of fuel in a tank at sea level
- Calculated results were in good agreement with measured data

## Intermittent Ullage Vapor Sampling

- The F.I.D.’s built-in sample pump could not maintain the required sample pressures when sampling from reduced ambient pressures
- The dual heated head sample pump was used to supplement the built-in pump to maintain the sample pressure
- However, sampling continuously at a high flow rate had the effect of drawing in air through the tank vents, thus diluting the ullage vapor
- It was decided to sample intermittently in order to maintain sample purity
- Since the F.I.D. had a quick response time, the only sample lag was created by the length of the sample lines
- A sample time of 30 seconds every ten minutes proved to be sufficient for ullage gas sampling
- Intermittent sampling was compared with continuous sampling at sea level for two tests with similar heating profiles

## Simulated Flight Conditions10,000 ft. Cruise

- Simulated Flight Conditions
- One hour of ground time with bottom surface fuel tank heating
- Ascend to cruise at 1,000 ft./min.
- Cruise for one hour
- Descend to ground at -1,000 ft./min.
- Standard atmosphere pressure

## Simulated Flight Conditions20,000 ft. Cruise

## Simulated Flight Conditions 30,000 ft. Cruise

- Good agreement was found between calculated and measured results for varying ambient conditions

## Calculated Mass Transport:Fuel tank at sea level

- The good agreement between calculated and measured values gives confidence in the model
- The temporal variation of ullage gas concentration can be explained by the model’s calculations of temporal mass transport
- The mass of fuel stored in the ullage gas at a given moment can be calculated when considering
- Mass of fuel vaporized
- Mass of fuel condensed on inner surfaces
- Mass of fuel vented out

## Calculated Mass Transport:Simulated Flight at 30,000’

- The variation of ullage gas concentration can be explained by the model’s calculations of temporal mass transport
- The mass of fuel stored in the ullage gas at a given moment can be calculated when considering
- Mass of fuel vaporized
- Mass of fuel condensed on inner surfaces
- Mass of fuel vented out

## Determination of the LFL

- For liquids of known composition, Le Chatelier’s rule can be used to estimate the LFL (Affens and McLaren 1972)
- Empirical formula that correlates flammability limits of multi-component hydrocarbon fuels with the flammability limits of the individual components
- Accounts for both the concentration and composition of the fuel-air mixture
- The mixture is considered flammable if *LC*>1

- An empirical criterion for estimating the FAR at the LFL states that at the LFL the FAR on a dry air basis is (*for most saturated hydrocarbons*) (Kuchta 1985)
*FAR = 0.035**±0.004 at 0°C*

## Flammability Assessment:Fuel tank at sea level

- FAR rule and Le Chatelier’s rule were used to assess the flammability using the model calculations
- Fuel compositions with flashpoints bracketing the experimental fuel flashpoint

## Flammability Assessment:Simulated Flight at 30,000’

- FAR rule and Le Chatelier’s rule were used to assess the flammability using the model calculations
- Fuel compositions with flashpoints bracketing the experimental fuel flashpoint

## Conclusions

- Experimentation was successful in measuring ullage vapor concentration in a simulated fuel tank exposed to varying ambient conditions
- A large data set was generated that can be used for validating fuel vaporization models
- The model used in this work proved to be accurate in it’s predictions of ullage gas temperature and ullage gas vapor concentration
- The model was useful in describing the transport processes occurring within the tank and explaining the ullage vapor concentration with a mass balance
- The model was useful in estimating the level of mixture flammability in the ullage utilizing both FAR and Le Chatelier’s criterion for the lower flammability limit

## Recommendations for Future Research in This Area

- Further detailed experimental data on JP-8 or Jet A flammability limits
- Laboratory testing in scale model partitioned aircraft fuel tanks
- Sampling from a fully instrumented fuel tank on an in-flight aircraft

## Thank You...Questions?