Experimental Investigation Of Flow And Heat Transfer In Microchannels

In recent years, with the wide use in many fields, the study on microscale heat and mass transfer has been paid great attention of the world. However, the published research results lead to obvious confusions or contradictions each other. The main purpose of the present study is to clarify the mechanisms of microscale heat and mass transfer. Under this background, the experimental study and theoretical analysis on flow and heat transfer in rectangle and circle micro-channel are performed in this paper.Firstly, flow characteristic in rectangle copper microchannels is studied by experimnents. Distilled water used as working fluid flows through ten single cooper rectangle microchannels with hydraulic diameters ranging from 107.76μm to 265.49μm, aspect ratios ranging from 0.2 to 0.9 and the relative roughness of 2-6%. The tests with the Reynolds number range of 50-1400 are to analyse the differences of flow between microscale and macroscale and explain the effect of roughness on flow. At the same time the experimental results tesify that the characteristic of flow in micro-channel is not only influenced by hydraulic diameters, but also by the aspect ratio.Secondly, distilled water, tetrachloromethane and nitrogen are used as working fluid and flow through the smooth quartz micro-tubes with inner diameters 19.60μm, 44.63μm, 91.65μm, 140.86μm. When the working fluid is liquid, the early transition from laminar to turbulent flow is observed in all microtubes, which lead to the values of friction factors larger than those predicted by classical theory. And the results also show that the polar of fluid not have any effect on flow of microtubes, except the tube with inner diameter 19.60μm; When the working fluid is gas, the diameter is smaller, the friction factor is lower as the same Reynolds number. Considering the errors of experiments, the friction factors of tube with inner diameters 44.63μm, 91.65μm, 140.86μm are in rough agreement with the conventional theory when Re number is lowerFinally, to study the forced convective heat transfer in micro-channels, the experiments are carried out to investigate the characteristic of heat transfer and viscous dissipation effect when deionized water flows through the quartz microtubes with inner diameter of 44.63μm, 91.65μm, 140.86μm, 241.98μm, 314.97μm and 520.05μm. The microtubes are heated by directly electrifying a brass wire with diameter 80μm that was coiled evenly up around the microtubes to keep a constant heat flux. The experimental results clearly indicated that the Re number of transition from laminar to turbulent flow is becoming smaller as diameters of microtubes decreasing, and the power of heat transfer is increasing with increasing of diameters at larger Re number. When diameter is smaller and Re number is larger, the influence of viscous dissipation effect on heat transfer becomes stronger, too.