Development of a liquid-fueled micro-combustor

Abstract

Advances in Micro-Electro-Mechanical Systems (MEMS) have made possible the development of shirtbutton-sized gas turbine engines for use as portable power sources. As part of an effort to develop a microscale gas turbine engine, this thesis presents the modeling, design, fabrication, and experimental characterization of a microcombustor that catalytically burns JP8 fuel. Due to high energy densities stored in hydrocarbon fuels, microscale heat engines based on them are estimated to have specific energies about one order of magnitude higher than those of current battery systems. In addition, utilizing a commonly available logistics fuel would provide advantages for military applications. Thus, a microengine burning JP8 fuel is attractive as a portable power source and potential replacement for batteries. The thesis first presents a number of models developed to design the fuel vaporizer, the fuel-air mixing chamber, and the combustion chamber. Among these is a reduced-order mass transfer model that simulates catalytic combustion of a slow diffusing hydrocarbon fuel. A two-phase heat transfer model was also developed to design an on-board fuel vaporizer with an array of micro-channels. Using the model results, a liquid-fueled micro-combustor test rig with a combustion chamber volume of 1.hcc and an overall die size of 36.4 mm x 36.4 mm x 6.5 mm was built. This device is a hybrid structure composed of silicon, sapphire, and glass. Deep reactive ion etching was mainly used to fabricate the silicon parts. The sapphire and glass parts were built by ultrasonic machining. The liquid-fueled micro-combustor was then experimentally characterized. Two configurations were tested and compared; one with the whole combustion chamber filled with a catalyst, and the other with a catalyst filling the chamber only partially.

(cont.) In the fully-loaded configuration, JP8 combustion was stably sustained at mass flow rates up to 0.1 g/sec, and an exit gas temperature of 780 K, an overall combustor efficiency of 19%, and a power density of 43 MW/m" were achieved. The primary limitation on increasing the mass flow rates and temperatures further was structural failure of the device due to thermal stresses. With the partially-loaded configuration, a mass flow rate of 0.2 g/sec, and a corresponding power density of 54 MW/mrn were obtained. The exit gas temperature for the partially-loaded configuration was as high as 720 K, and the maximum overall efficiency was over 22%. Although the reduced amount of catalyst led to incomplete combustion, smaller thermal losses resulted in an increase in the overall combustor efficiencies and power densities. The overall efficiency and the exit gas temperature were lower than the operational requirement of the microengine in both of the device configurations. A non-dimensional operating map was constructed based on the experiment, and suggestions for future liquid fueled micro-combustors were made; to achieve maximum efficiency for a volume as small as possible, improving the thermal efficiency would be necessary. Thesis keywords: Power-MEMS, microengine, micro-combustor, catalytic combustion, JP8 combustor, micro fuel vaporizer, micro-fabrication, deep reactive ion etching