Development of Microchannel Heat Exchanger for Refrigeration

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An experimental study on two-phase flow, nitrogen gas and water (or glycerin solution), in microchannels has been conducted to investigate characteristics of fluid flow and heat transfer. An array of 25 parallel microchannels was fabricated on a stainless steel plate by electric discharge machining process. Dimensions of the microchannel are 209 μm high, 212μm wide and 9.96 mm long. Pressure drop across the channels was measured and flow behavior was simultaneously recorded with a high speed video camera. Temperatures of the wall and fluid were measured to determine heat transfer coefficients at the channel surface. The results show that the Chisholm's parameter C for two-phase flow in microchannels is smaller than that in conventional-sized channels and the parameter was well correlated with the flow patterns in case of nitrogen gas and water. Nusselt numbers obtained in single-phase experiments agree approximately with Peng and Peterson's experimental results. In two-phase experiments, heat transfer was enhanced with the stirring effect by gas flow and Nusselt number reaches twice as large value as that in liquid single-flow.Flow and thermal characteristics of a new type of microchannel heat exchanger were examined experimentally concerning the both cases of single and two phase flow. The heat exchanger is composed by laminating thin stainless-steel plate of 200μm thickness with microchannels of 250μm width, and other plate with manifolds (i.e. header & footer) alternately. The microchannel heat exchanger was shown to have higher overall heat transfer coefficient than that of conventional heat exchangers. The performance of the microchannel heat exchanger in liquid phase region was shown to be predicted by the characteristics of laminar flow. In case of boiling of working fluid, large pressure drop appeared for a few bubbles but rapid increase of heat transfer rate was not observed until hard boiling.