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JET A VAPORIZATION IN A SIMULATED AIRCRAFT FUEL TANK (INCLUDING SUB-ATMOSPHERIC PRESSURES AND LOW TEMPERATURES) PowerPoint PPT Presentation


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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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JET A VAPORIZATION IN A SIMULATED AIRCRAFT FUEL TANK (INCLUDING SUB-ATMOSPHERIC PRESSURES AND LOW TEMPERATURES)

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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: [email protected]


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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 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


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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 < 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.


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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 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


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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 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


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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


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