Development of a high power density combustion system for a silicon micro gas turbine engine
Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics.
Ian A. Waitz.
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As part of an effort to develop a microfabricated gas turbine engine capable of providing 10-50 Watts of electrical power in a package less than one cubic centimeter in volume, this thesis presents the design, fabrication, packaging and testing of the first combustion system for a silicon micro heat engine. The design and operation of a microcombustor is fundamentally limited by the chemical reaction times of the fuel, by silicon material and fabrication constraints, and by the inherently non-adiabatic nature of the operating space. This differs from the design of a modern macro combustion system that is typically driven by emissions, stability, durability and pattern factor requirements. The combustor developed herein is shown to operate at a power density level that is at least an order of magnitude higher than that of any other power-MEMS device (2000 MW/m 3), and establishes the viability of using high power density, silicon-based combustion systems for heat engine applications at the micro-scale. This thesis presents the development of two specific devices - the first device is a 3-wafer level microcombustor that established the viability of non-premixed hydrogen-air combustion in a volume as small as 0.066 cm 3, and within the structural constraints of silicon; the second device is known as the engine "static-structure", and integrated the 3-stack microcombustor with the other non-rotating components of the engine. Fabricated by aligned fusion bonding of 6 silicon wafers, the static structure measures 2.1 cm x 2.1 cm x 0.38 cm, and was largely fabricated by deep reactive ion etching through a total thickness of 3,800 pm. Packaged with a set of fuel plenums, pressure ports, fuel injectors, igniters, fluidic interconnects, and compressor and turbine static airfoils, this structure is the first demonstration of the complete hot flow path of a multi-level microengine. The 0.195 cm 3 combustion chamber has been tested for several tens of hours and is shown to sustain stable hydrogen combustion with exit gas temperatures above 1600K and combustor efficiencies as high as 95%. The structure also serves as the first experimental demonstration of hydrocarbon microcombustion with power density levels of 500 MW/m 3 and 140 MW/m 3 for ethylene-air and propane-air combustion, respectively. In addition to the development of the two combustion devices, this thesis also presents simple analytical models to identify and explain the primary drivers of combustion phenomena at the micro-scale. The chemical efficiency of the combustor is shown to have a strong correlation with the Damkohler number in the chamber, and asymptotes to unity for sufficiently large values of Da. The maximum power density of the combustor is also shown to be primarily limited by the structural and fabrication constraints of the material. Overall, this thesis synthesizes experimental and computational results to propose a simple design methodology for microcombustion devices, and to present design recommendations for future microcombustor development. Combined with parallel efforts to develop thin-film igniters and temperature sensors for the engine, it serves as the first experimental demonstration of the design, fabrication, packaging and operation of a silicon-based combustion system for power generation applications at the micro-scale.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 2000."February 2000."Includes bibliographical references (p. 203-211).
DepartmentMassachusetts Institute of Technology. Dept. of Aeronautics and Astronautics.
Massachusetts Institute of Technology
Aeronautics and Astronautics.