Jet Fuel Vaporization and Condensation:

Modeling and Validation

C.E. PolymeropoulosRobert OchsRutgers, The StateUniversity of NewJersey

International Aircraft

Systems Fire Protection

Working Group Meeting

Part I: Physical Considerations and Modeling

Motivation

• Combustible mixtures can be generated in the ullage of aircraft fuel tanks

• Need for estimating temporal dependence of F/A on:– Fuel Loading– Temperature of the liquid fuel and tank walls– Ambient pressure and temperature

Physical Considerations• 3D natural convection heat

and mass transfer– Liquid vaporization– Vapor condensation

• Variable Pa and Ta

• Multicomponent vaporization and condensation

• Well mixed liquid and gas phases– Rayleigh number of liquid

~o(106)– Rayleigh number of ullage

~o(109)

Principal Assumptions• Well mixed gas and liquid phases

– Uniformity of temperatures and species concentrations in the ullage and in the evaporating liquid fuel pool

• 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 from samples with similar flash points as those tested

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

• Natural convection enclosure heat transfer correlations• Heat and mass transfer analogy for the mass transfer

coefficients

Liquid Jet A Composition• Liquid Jet A composition depends on origin and

weathering• Jet A samples with different flash points were

characterized by Woodrow (2003):– Results in terms of C5-C20 Alkanes– Computed vapor pressures in agreement with measured data

• JP8 used with FAA testing in the range of 115-125 Deg. F.

• Present results use compositions corresponding to samples with F.P.=115 Deg. F. and 120 Deg. F. from the Woodrow (2003) data

Composition of the Fuels Usedfrom Woodrow (2003)

0

5

10

15

20

25

30

0 5 10 15 20 25C atom

Vo

lum

e F

rac

tio

n,

%

115 Deg. Flashpoint

120 Deg. Flashpoint

Part II: Experimentation

Requirements for Experimental Setup

• Ability to vary fuel tank floor temperature with uniform floor heating

• Setup with capability of changing ambient temperature and pressure with controlled profiles

• Measurement of temporal changes in liquid, surface, ullage, and ambient temperatures

• Ability to asses the concentration of fuel in the ullage at a point in time

Measuring Input Parameters for the Model

Heat Transfer

•Thermocouples on tank surface, ullage, and liquid fuel.

Mass Transfer Fuel Properties

•Fuel tested in lab for flashpoint

•Used fuel composition from published data of fuels with similar flashpoints

•FID Hydrocarbon analyzer used to measure the concentration of evolved gasses in the ullage

•Pressure measurement for vaporization calculations

Experimental Setup• Fuel tank – 36”x36”x24”, ¼” aluminum• Sample ports • Heated hydrocarbon sample line• Pressurization of the sample for sub-atmospheric pressure

experiments by means of a heated head sample pump• Intermittent (at 10 minute intervals) 30 sec long sampling• FID hydrocarbon analyzer, cal. w/2% propane• 12 K-type thermocouples• Blanket heater for uniform floor heating• Unheated tank walls and ceiling• JP-8 jet fuel

Experimental Setup

• Fuel tank inside environmental chamber– Programmable variation of chamber pressure

and temperature• Vacuum pump system

• Air heating and refrigeration

Thermocouple LocationsThermocouple Channel:

1. Left Fuel

2. Center Fuel

3. Right Fuel

4. Left Ullage

5. Center Ullage

6. Right Ullage

7. Rear Surface

8. Left Surface

9. Top Surface

10. Ambient

11. Heater

12. Heater Temperature Controller

12

3

4

5

6

78

910

1112

Experimental Procedure• Fill tank with specified quantity of fuel• Adjust chamber pressure and temperature to desired

values, let equilibrate for 1-2 hours• Begin to record data with DAS• Take initial hydrocarbon reading to get initial quasi-

equilibrium fuel vapor concentration• Set tank pressure and temperature as well as the

temperature variation• Experiment concludes when hydrocarbon

concentration levels off and quasi-equilibrium is attained

•5 gallon fuel load for every test

•Temperature, pressure profiles created to simulate in-flight conditions

Test Matrix

Test Type: 0 10,000 20,000 30,000

Const. P X X X X

Vary T & P N/A X X XIsooctane X N/A N/A N/ADry Tank X N/A N/A X

Altitude

Dry Tank Ullage Temperature

40

50

60

70

80

90

100

110

0 1000 2000 3000 4000 5000 6000

Time, seconds

Te

mp

era

ture

, D

eg

. F

.Input TempMeasured Ullage Temp

Calculated Ullage Temp

Comparison of measured vs. calculated ullage temperatureShows validity of well-mixed ullage assumption:

Calculated vs. Measured Ullage Gas Temperature

Fuel Vaporization:Constant Ambient Conditions at Atmospheric Pressure

0

20

40

60

80

100

120

140

0 500 1000 1500 2000 2500 3000

Time, seconds

Te

mp

era

ture

, De

g. F

.

0

0.5

1

1.5

2

2.5

% P

rop

ane

Liquid TempMeasuredCalculated, 120 FPCalculated, 115 FP

Calculated vs. Measured Ullage Vapor Concentration

Sea Level Vaporization:

-0.005

0

0.005

0.01

0.015

0.02

0.025

0 500 1000 1500 2000 2500 3000

Time, seconds

Mas

s, k

g

0

20

40

60

80

100

120

140

Deg. F.

EVAPORATED

CONDENSED

STORED

VENTED

LiquidTemperature

Calculated Temporal Mass Transport Occurring within the Tank

-As fuel temperature increases, mass of liquid evaporated, and hence stored in the ullage, increases

-As gas concentration in ullage increases, condensation is seen to occur

-As condensation increases, mass of fuel stored in the ullage decreases due to fuel condensing

Sea Level Vaporization:Flammability Assessment

0

0.01

0.02

0.03

0.04

0.05

0 500 1000 1500 2000 2500 3000

Time, seconds

FA

R

Measured Fuel to AirMass Ratio

Lower Flammability Limit

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 500 1000 1500 2000 2500 3000

Time, seconds

LeC

hate

lier's

Rat

io

LeChatelier's Ratio, 115 FP

LeChatelier's Ratio, 120 FP

LeChatelier's FlammabilityLimit

Flammability Assessment using the FAR rule, 0.033<LFL<0.045

Flammability Assessment using LeChatelier’s Rule, Flammable if LC>=1

0

20

40

60

80

100

0 2000 4000 6000 8000 10000 12000

Time, secondsT

empe

ratu

re,

Deg

. F

.

0

5

10

15

20

Pre

ssur

e, p

sia

Liquid Fuel TempAverage Ullage TempAverage Surface TempAmbient Pressure

0

0.5

1

1.5

2

2.5

3

0 2000 4000 6000 8000 10000 12000

Time, Seconds

% P

ropa

ne

MeasuredCalculated, 115 FPCalculated, 120 FP

Simulated Flight Profile up to 30,000’:

Fuel Tank Temperatures and Ambient Pressure

Calculated vs. Measured Ullage Vapor Concentration

Varying T & P:Modeled Transport Processes

-0.015

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

0 2000 4000 6000 8000 10000 12000

Time, seconds

Mas

s, k

g

MassEvaporated

MassCondensed

MassStored inUllageMassVented

Varying T & P:Flammability Assessment

0

0.2

0.4

0.6

0.8

1

1.2

0 2000 4000 6000 8000 10000 12000 14000

LeChatelier's Ratio,115 FPLeChatelier's Ratio,120 FPLeChatelier'sFlammability Limit

0

0.01

0.02

0.03

0.04

0.05

0 2000 4000 6000 8000 10000 12000 14000

Time, seconds

FA

R

Measured Fuel to AirMass Ratio

Lower FlammabilityLimit Flammability

Assessment using the FAR rule, 0.033<LFL<0.045

Flammability Assessment using LeChatelier’s Rule, Flammable if LC>=1

Summary of Results• Experiment was well designed to provide usable model

validation data• Model calculations of ullage gas temperature and ullage

vapor concentration agree well with measured values• Model calculations of mass transport within the tank give a

good explanation of the processes occurring in a fuel tank• Model can be used to determine the level of flammability

using either the FAR rule or LeChatelier’s Flammability Rule

• The calculations show that flammability is dependent on the composition of the ullage gas.