JET A VAPORIZATION IN A SIMULATED AIRCRAFT FUEL TANK INCLUDING SUBATMOSPHERIC PRESSURES AND LOW TEMP

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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
Rd Piscataway, New Jersey, 08854-8058, USA Tel
732 445 3650, email poly_at_jove.rutgers.edu
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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

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Outline

• Brief background discussion
• Description of the model
• Comparisons with experimental data
• Discussion of model results
• Conclusions

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Mass Transfer Considerations

• Natural convection heat and mass transfer
• Liquid vaporization
• Vapor condensation
• Variable Pa and Ta
• Vented tank
• Multicomponent fuel

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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

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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

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Heat and Mass Conservation Relations
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Heat and Mass Transfer Correlations
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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

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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

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Data for Jet A Characterizationwas based on
Woodrows (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 Woodrows data having
  flash points similar to the fuel samples used
  with the experimentation

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Jet A Compositions used for Comparisons with
Experimental Data
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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 lt F.P.lt 325.2 K
• Data on ullage temperature, and total hydrocarbon
  concentration with test tank in altitude chamber
  (Ochs, 2004)
• Samples with 322.3 K lt F.P. lt 319.5 K
• Data data from aircraft fuel tank (Summer, 2004)
• Samples with various F.P.

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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 0C F/A mass
  ratio
• at the LFL is 0.0350.05 (Kuchta,1985)
• Le Chateliers rule at the LFL LR
  1 where,
• Note Use of Le Chatelierss rule with the
  present equivalent
• n alcane species Jet A characterization
  needs further examination

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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

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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

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Conclusions (continued)

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

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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