Development of Microchannel Heat Exchanger for Refrigeration

对微通道内氮气和水(或甘油溶液)两相流动进行了实验研究，研究了微通道内流体的流动和换热特性。采用电火花加工技术在不锈钢平板上制作了25个平行微通道阵列。微通道的尺寸为高209μm，宽212μm，长9.96 mm。通过高速摄像机测量了流道间的压降，并同时记录了流动行为。测量了壁面和流体的温度，以确定通道表面的换热系数。结果表明，微通道中两相流的Chisholm参数C小于常规尺寸通道中的Chisholm参数，且该参数与氮气和水两相流流型有很好的相关性。单相实验得到的Nusselt数与Peng和Peterson的实验结果基本一致。在两相实验中，气流的搅拌强化了换热，努塞尔数达到液体单流的两倍。对一种新型微通道换热器的流动和热工特性进行了单相和两相流动的实验研究。换热器由厚度为200μm的不锈钢薄板和宽度为250μm的微通道与其他具有歧管(即集管和集管)的板交替组成。结果表明，微通道换热器具有比常规换热器更高的总换热系数。结果表明，利用层流特性可以预测微通道换热器在液相区的性能。在工质沸腾的情况下，少数气泡出现较大的压降，但直到完全沸腾时才观察到换热速率的快速增加。

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.