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Effect of Structural Heat Conduction on the Performance of Micro-combustors and Micro-thrusters

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Technical Report

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University of Maryland, College Park College Park United States

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This thesis investigates the effect of gas-structure interaction on the design and performance of miniaturized combustors with characteristic dimensions less than a few millimeters. These are termed micro-combustors and are intended for use in devices ranging from micro-scale rocket motors for micro, nano, and pico-satellite propulsion, to micro-scale engines for micro-Unmanned Air Vehicle UAV propulsion and compact power generation. Analytical models for the propagation of a premixed laminar flame in a micro-channel are developed. The models predictions are compared to the results of more detailed numerical simulations that incorporate multi-step chemistry, distributed heat transfer between the reacting gas and the combustor structure, heat transfer between the combustor and the environment, and heat transfer within the combustor structure. The results of the modeling and simulation efforts are found to be in good qualitative agreement and demonstrate that the behavior of premixed laminar flames in micro-channels is governed by heat transfer within the combustor structure and heat loss to the environment. The key findings of this work are as follows First, heat transfer through the micro-combustors structure tends to increase the flame speed and flame thickness. The increase in flame thickness with decreasing passage height suggests that microscale combustors will need to be longer than their conventional-scale counterparts. However, the increase in flame speed more than compensates for this effect and the net effect is that miniaturizing a combustor can increase its power density substantially. Second, miniaturizing chemical rocket thrusters can substantially increase thrustweight ratio but comes at the price of reduced specific impulse i.e. overall efficiency. Third, heat transfer through the combustors structure increases steady-state and transient flame stability.

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