JET A VAPORIZATION IN A SIMULATED AIRCRAFT FUEL

TANK (INCLUDING SUB-ATMOSPHERIC PRESSURES AND

LOW TEMPERATURES)

C. E. Polymeropoulos, and Robert Ochs

Department of Mechanical and Aerospace Engineering Rutgers University

98 Bowser RdPiscataway, New Jersey, 08854-8058, USA

Tel: 732 445 3650, email: poly@jove.rutgers.edu

Motivation

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

• Current effort in minimizing explosion hazard

• Present objective of the present work is:– prediction of the influence of different parameters involved

in the evolution and composition of combustible vapors• The tank ambient pressure and temperature• The fuel and tank wall temperatures• The composition and the amount of fuel in the tank

– assessing the flammability of the resulting air-fuel mixtures

Outline

• Brief background discussion • Description of the model

• Comparisons with experimental data

• Discussion of model results

• Conclusions

Mass Transfer Considerations

• Natural convection heat and mass transfer – Liquid vaporization– Vapor condensation

• Variable Pa and Ta

• Vented tank• Multicomponent fuel

Tl

Tg mg Pa

Ta

Vent

mo

me

mc

Gas Control Volume

Pa

ml

Vg

Jet A

Assumptions used for Estimating

Ullage Vapor composition • Well mixed gas and liquid phases

– Spatially uniform and time varying temperature and species concentrations in the ullage and in the evaporating liquid fuel pool

• Quasi-steady transport using heat transfer correlations, and the

analogy between heat and mass transfer for estimating film coefficients for heat and mass transfer

• Low evaporating species concentrations

• The time dependent liquid fuel, and tank wall temperatures, and the tank pressure are assumed known

Additional Assumptions

• Gases/vapors follow ideal gas behavior

• Tank pressure is equal to the ambient pressure

• Condensate layer forms on the tank walls

• Condensate at the tank wall temperature

• No out-gassing from the liquid fuel, no liquid droplets in the ullage, no liquid pool sloshing

• Fuel consumption neglected

Heat and Mass Conservation Relations

)()(

= ˚˚˚˚˚˚˚

=-

/

sTgTshsAf

TgTf

hf

A=in

q

aTpac

gTpgcomgTpgccm

lTpvcem

inq

dtgTpgcgmd

BalanceEnergyOverall

dtgdT

gTi

m

dtdp

pi

m

oim

iN1

eim

cim

Vpi

MgTR

dti

dxBalanceMassSpeciesVapor

Mom

outflowfor

dtgdT

gTi

Mi

m

dtdp

pi

Mi

m

iN1

eim

cim

iM1

iM1

ominflowfor

airN 1.....Ni Balance, Mass Ullage Overall

equationssThodos’KalkwarfFrostorsWagner’UsingPressureporSpecies

pi

pli

x

fixLawsHenry

fN1i

giy

fiy

LiDiShlA

limdtdonCondensati nEvaporatio SpeciesFuel

-)(

--

’

.....)(

δ

δ

Va

Heat and Mass Transfer Correlations

Heat Transfer Correlations

Horizontal Surfaces Nu = hLk

= 0.14(GrPr)1/3 (Hollands et al, 1975)

Vertical Surfaces Nu =hHk

= = 0.664 Re0.5Pr1/3 (Vertical Flat Plate)

Mass Transfer Correlations

Horizontal Surfaces Shi = hiLDi

= 0.14(GrSci)1/3

with Gr = g [(g -

f)]L3

2

- for upper surface

+ for lower surface

Gr=0 if Gr<0

Vertical Surfaces Shi =hiHDi

= 0.664 Re0.5Sci1/3

with Re =[g abs(

g -

f)H]0.5H

Computational Method

• Given:– The tank geometry– The fuel loading – A liquid fuel composition– The tank pressure, and the liquid fuel and the tank wall

temperatures as functions of time (experimental data)

• The previous relations allow computation of the temporal variation of ullage gas composition and temperature

Jet A Characterization

• Jet A is a complex multi-component fuel– Components are mostly paraffin, and to a lesser extend

cycloparaffin, aromatic, olefin, and other hydrocarbons

• Jet A specifications are expressed in terms of allowable ranges of properties reflecting the physical, chemical and combustion behavior of the fuel

• The composition of a Jet A sample therefore depends on its source, on weathering, etc

Data for Jet A Characterizationwas based on Woodrow’s (2002) data

• Jet A samples with flash points between 37.5 °C and 59 °C were characterized using chromatographic analysis

• The characterization was in terms of equivalent C5 to C20 normal alcanes

• Equilibrium vapor pressures computed with the resulting compositions were in good agreement with measured data

• For comparisons with test tank results the model used fuel compositions from Woodrow’s data having flash points similar to the fuel samples used with the experimentation

Jet A Compositions used for Comparisons with Experimental Data

Table 1. % Mole Fractions and Flash Points of Three Liquid Jet A Compositions (Woodrow, 2002)

No. Carbon Fuel 1a Fuel 2a Fuel 3a

Atoms FP=322.3 K FP=325.2 K FP=319.5 K

5 0.005 0.032 0.056 0.03 0.22 0.167 0.96 1.08 1.108 5.01 2.85 4.029 11.50 7.77 12.80

10 21.70 15.60 26.2111 23.80 20.00 24.4012 17.30 18.10 16.9013 9.84 15.20 9.0814 5.37 10.50 3.9015 2.95 5.49 1.1516 1.11 2.10 0.2017 0.42 0.82 0.0218 0.012b 0.13 0.0119 0.00 0.112b 0.0020 0.00 0.00 0.00

aIn Woodrow (2002) Fuel 1 is FAA-1, Fuel 2 is FAA-2, and Fuel 3 is FAA-5b added by authors for 100% moles

Comparisons with Experimental Data

• Data on ullage temperature, and total hydrocarbon concentration with test tank at ambient pressure (Summer, 1997)– Samples with: 322.3 K < F.P.< 325.2 K

• Data on ullage temperature, and total hydrocarbon concentration with test tank in altitude chamber (Ochs, 2004)– Samples with: 322.3 K < F.P. < 319.5 K

• Data data from aircraft fuel tank (Summer, 2004)– Samples with various F.P.

Ullage Vapor Lower Flammability Limit

• The lower flammability limit (LFL) of ullage vapor is not well defined.

• Empirical definitions (used by Shepherd 2000)– For most saturated hydrocarbons the 0°C F/A mass ratio at the LFL is 0.035±0.05 (Kuchta,1985) – Le Chatelier’s rule: at the LFL LR =1 where,

Note: Use of Le Chateliers’s rule with the present equivalent

n alcane species Jet A characterization needs further

examination

iLFL

ix]C25T0.000721[1.02LR )¡ -(

Conclusions

• The temporal evolution of Jet A fuel vapor in experimental tanks was estimated using perfectly mixed fluids due to natural convection, and correlations based on the analogy between heat and mass transfer

• Principal required inputs to the model were the tank geometry, the fuel loading, a component characterization of the liquid fuel, the tank pressure, and the temperature history of the liquid fuel and the tank walls.

• Liquid Jet A was characterized using mixtures of C5-C20 n-alcanes with flash points equivalent to those of the samples used with the experimental test tanks

• There was good agreement between measured and computed total Jet A vapor concentrations within a constant pressure test tank, and also within one undergoing pressure and temperature variations similar to those encountered with aircraft flight

Conclusions (continued)

• The model was used for detailed examination of evaporation, condensation and venting in the test tanks, and of the observed variations in total hydrocarbon concentration

• The model was also used for estimating the effect of different parameters on the ullage F/A mass ratio – The temperature of the liquid fuel had a strong influence on the

F/A– The effect of fuel loading was of minor significance, except for

small fuel loadings. Of importance, however, is the potential of increased liquid fuel temperatures at low fuel loading

– Of major significance was the choice of liquid fuel composition, which was based on previous experimental data with samples differentiated by their flash point

Conclusions (continued)

• The flammability of the ullage vapor was assessed – Using as criterion a previously proposed limit range of F/A mass

ratios – Le Chatelier’s ratio with ullage species mole fractions computed

with C5-C20 liquid fuel compositions

• For the cases considered the two approaches yielded comparable LFLs. However, prediction of the LFL of Jet A requires additional consideration, especially with the use of an equivalent fuel composition

• The model needs to be applied to different flight conditions using data from aircraft fuel tanks

Acknowledgment

Support for this work was under the the FAA/Rutgers Fellows Program, provided by the the Fire Safety Division of the FAA William J. Hughes Technical Center, Atlantic City, New Jersey, USA