扁材多孔管内单相流体换热

对水力直径分别为0.72 mm的矩形扁材多孔管和0.86 mm的圆形扁材多孔管中单相流体换热特性以及流动压降进行了实验研究.流体Re小于500时,随着热通量的增大,Nu减小;随着Re的增大,热通量对Nu的影响减小,Nu趋于一致.实验结果显示,这些变化是由于入口段效应和共轭效应相互制约的结果.圆形扁材多孔管的Nu高于矩形扁材多孔管的Nu.对于乙醇溶液,低热阻时,入口温度越高,压降随热阻的增大降低的斜率越小,随着热阻值的增大,压降降低速率减小趋于平缓.对水、酒精和丙酮进行了单相对流换热的研究.通过比较发现,3种流体的换热特性并不相同,而且差别很大.在同一热通量和入口温度条件下,相同Re,乙醇的Nu最大,丙酮次之,水的Nu最小.同时测试了不同乙醇浓度水溶液的换热特性,发现随着乙醇浓度的增大,溶液的Nu由基本保持不变到随着Re的增大而增大,并且乙醇浓度越大相邻浓度间的Nu差别越大,出现以上现象的理论原因还有待进一步的研究.

Heat transfer performance of liquid flow in multi-port micro-channel tubes

An experiment was performed to study the heat transfer characteristics of liquid flow through two multi-port extruded (MPE) micro-tubes with water, ethanol, acetone and ethanol/water mixtures. The cross-section geometries of two tubes were rectangular (approximate square) and circular, and with hydraulic diameters of 0.72 and 0.86 mm respectively. The results showed that at Reynolds number (Re) below 500, Nusselt number (Nu) decreased with increasing heat flux but Nu approached a minimum for increasing heat flux at Re > 500. Experimental results showed that such change was caused by the effect of entrance function and conjugate heat transfer. The experimental Nu of the circular multi-tube was higher than that of rectangular multi-tube. For ethanol, pressure drop decreased dramatically with increasing thermal resistance beginning at a low thermal resistance. Additionally, the higher the inlet temperature, the smaller the slope of pressure drop decrease against thermal resistanc. Further investigations of water, ethanol and acetone were conducted. Nu of ethanol were highest and those of water were the lowest among three solutions with the same heat flux and inlet temperature. Experiments of rectangular multi-tube, with different mixing ratios of ethanol/water showed that the trend of Nu against Re remained unchanged at low ethanol concentration. Furthermore, the higher the ethanol concentration, the faster the growth rate of Nu against Re and the greater the Nu difference between adjacent concentrations. Theoretical reasons for this remains to be further studied.