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C.E. Polymeropoulos Robert Ochs Rutgers, The State University of New Jersey. International Aircraft Systems Fire Protection Working Group Meeting. Jet Fuel Vaporization and Condensation: Modeling and Validation . Part I: Physical Considerations and Modeling. Motivation.
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C.E. Polymeropoulos Robert Ochs Rutgers, The State University of New Jersey International Aircraft Systems Fire Protection Working Group Meeting Jet Fuel Vaporization and Condensation: Modeling and Validation
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
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 HeatTransfer Mass Transfer Fuel Properties • FID Hydrocarbon analyzer used to measure the concentration of evolved gasses in the ullage • Pressure measurement for vaporization calculations • Fuel tested in lab for flashpoint • Used fuel composition from published data of fuels with similar flashpoints • Thermocouples on tank surface, ullage, and liquid fuel.
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 Locations • Thermocouple Channel: • Left Fuel • Center Fuel • Right Fuel • Left Ullage • Center Ullage • Right Ullage • Rear Surface • Left Surface • Top Surface • Ambient • Heater • Heater Temperature Controller 10 9 6 7 8 5 4 3 2 11 1 12
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
Test Matrix • 5 gallon fuel load for every test • Temperature, pressure profiles created to simulate in-flight conditions
Dry Tank Ullage Temperature 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 Calculated vs. Measured Ullage Vapor Concentration
Sea Level Vaporization: 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 Flammability Assessment using the FAR rule, 0.033<LFL<0.045 Flammability Assessment using LeChatelier’s Rule, Flammable if LC>=1
Simulated Flight Profile up to 30,000’: Fuel Tank Temperatures and Ambient Pressure Calculated vs. Measured Ullage Vapor Concentration
Varying T & P:Flammability Assessment 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.