Hwanil Huh

Effect of Scale on the Performance of Heat-Recirculating Reactors

Extinction limits and combustion temperatures in heat-recirculating excess enthalpy reactors employing both gas-phase and catalytic reaction have been examined previously, with an emphasis Reynolds number (Re) effects and possible application to microscale combustion devices. However, Re is not the only parameter needed to characterize reactor operation. In particular, the use of a fixed reactor size implies that residence time (thus Damkohler (Da), the ratio of residence to chemical time scales) and Re cannot be adjusted independently. To remedy this situation, in this work geometrically similar reactors of different physical sizes were tested with the aim of independently determining the effects of Re and Da. It is found that the difference between catalytic and non-catalytic combustion limits narrow as scale decreases. Moreover, to assess the importance of wall thermal conductivity, reactors of varying wall thickness were studied; results were consistent with theoretical predictions. From these results the effect of scale on microscale reactor performance and implications for practical microcombustion devices are discussed.

Micro-Nozzle Performance and Thermal Transpiration Based Self Pumping in Vacuum Conditions

In this study, we designed cold gas propulsion system with minimum 0.25 mm nozzle and micro-thrust measurement system to analyze flow characteristic of micro propulsion system in ambient and vacuum condition. Argon and Nitrogen are used for propellant and the result of experiments is compared with CFD analysis and theory. But there is a point where reduced scale versions of conventional propulsion systems will no longer be practical. Therefore, a fundamentally different approach to propulsion systems was taken. That is thermal transpiration based micro propulsion system. It has no moving parts such as lubricants, pressurizing system and can pump the gaseous propellant by temperature gradient only (cold to hot). We are advancing basic research of propulsion system based on thermal transpiration in vacuum conditions and had tried experiment process and theoretical access in advance. To characterize membrane of Knudsen pump, we select Polyimide material that has low thermal conductivity (0.29 W/mK) and can stand high temperature (300℃) for long time. And we fabricated hole diameter 1, 0.5, 0.2, 0.1 mm using precision manufacturing. Experimental results show that pressure gradient efficiency of Knudsen pump is increased to maximum 82% according to Knudsen number and thick membranes are more effective than thin membranes in transition flow regime