세미나

ACCURATE PREDICTION OF THERMOPHYSICAL PROPERTIES OF SUPERCRITICAL HYDROCARBON AVIATION FUELS AND ITS APPLICATION

Hyung Ju Lee, Ph.D.

Assistant Professor, Department of Mechanical Engineering, Pukyong National University

ABSTRACT

Scramjet engines have been studied worldwide for decades for applications in hypersonic flight over the Mach 5 within the atmosphere, as they offer high specific impulse and operational flexibility from the supersonicto the hypersonic ranges. There remain, however, many technical barriers to realization of operational air-breathing hypersonic systems; two of the most serious challenges are aero-thermodynamic heating and efficient supersonic combustion. In order to overcome these issues effectively, it will be essential to develop regenerative heat sink cooling systems using hydrocarbon aviation fuels. Hydrocarbon fuels are sufficiently dense and usually in a liquid state at ambient conditions, and also have excellent cooling characteristics, absorbing heat through chemical reactions such as thermal and catalytic cracking. When the liquid fuel is used as coolant, active cooling systems can resolve the aero-thermodynamic heating on the fuselage and the combustor effectively in a volume- (and weight-) limited hypersonic vehicle. Furthermore, the heating of the fuel and cracking into light hydrocarbons and hydrogen during the regenerative cooling process may increase combustion efficiency considerably, helping to overcome the short residence time and complicated air flow environment in a scramjet combustor.

In such a system, however, the fuel at the injector exit reaches a very high temperature, depending on overall heat load and heat sink capacities, and the heated fuel must be highly pressurized for injection into a scramjet combustor. At these high temperatures and pressures, the recirculating fuel may not only reach supercritical conditions but also be cracked through endothermic reactions, producing a variety of low molecular-weight hydrocarbons, including H2, CH4, C2H4, and C2H6. When the hydrocarbon fuel is pressurized and heated to above its critical temperature and pyrolyzed into multiple components in a regenerative cooling system, its thermophysical properties change greatly, which influences the overall fuel flow and convective heat transfer characteristics inside the channels of the cooling system. Furthermore, when the fuel heated to high-temperature, supercritical and/or cracking conditions is supplied to a supersonic combustor (through injectors with various shapes and sizes), its injection and atomization characteristics will differ from those of a fuel in the liquid state, and this will substantially affect its mixing, ignition, and combustion performance of a scramjet engine. In order to make precise experimental and numerical analyses of the performance characteristics of a regenerative cooling system or a scramjet combustor, therefore, it is essential to acquire the thermodynamic and transport properties of the hydrocarbon fuels and their decomposed mixtures accurately over a wide range of temperature and pressure conditions.

The first half of this presentation will discuss several methodologies that have been developed to improve the prediction accuracy of thermophysical properties, and compares their prediction performances for a set of hydrocarbon fuels and their decomposed products after pyrolysis in the regenerative cooling system of a hypersonic flight vehicle. The materials used for the prediction of the thermophysical properties are 27 pure substances and two types of mixture, including both low and high molecular-weight hydrocarbons as well as hydrogen. Theprediction of the thermodynamic and transport properties has been carried out in the range of temperatures between 300 and 1000 K and pressures between 0.1 and 5.0 MPa, which corresponds to typical operating conditions of a hydrocarbon aviation fuel circulating as coolant in a regenerative cooling system as well as includes the critical temperatures and pressures of most of the hydrocarbon fuels of interest. The values of density, constant pressure specific heat, viscosity, and thermal conductivity estimated using various methods are compared in terms of relative deviation from those obtained from the NIST database. Presented in the second half will be some results on the modeling and simulation of a practical orifice nozzle issuing pressurized high-temperature hydrocarbon fuel, simulating injection of aviation fuel after use as coolant in the active cooling system of a hypersonic flight vehicle. The fuel temperature is raised up to 700 K (427 °C), the thermodynamic and transport properties of the fuel corresponding to the operating temperature and pressure are implemented in the simulation, and the simulation results are compared with experimental data at identical injection conditions for the same injector, confirming the validity and capability of prescribed thermophysical property prediction methodologies.