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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

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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: [email protected]

  • 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
  • Brief background discussion
  • Description of the model
  • Comparisons with experimental data
  • Discussion of model results
  • Conclusions
mass transfer considerations
Mass Transfer Considerations
  • Natural convection heat and mass transfer
    • Liquid vaporization
    • Vapor condensation
  • Variable Pa and Ta
  • Vented tank
  • Multicomponent fuel

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
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
computational method
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 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 characterization was based on woodrow s 2002 data
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
comparisons with experimental data
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
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

  • 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
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 continued29
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

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