Analysis And Optimization Of A Microchannel Heat Sink With Nanofluids

In recent years,The heat dissipation powers of electronic chips increase rapidly with the miniaturization and integration of equipment in microelectronic industry.The heat flow density can exceed hundreds of watts per square centimeter.Therefore,various heat exchangers with high efficiency are pressed for to ensure the safety of electronic equipment.In this dissertation,two kinds of microchannel heat sinks by using nanofluids as heat transfer medium are designed and optimized to clarify the mechanism of heat transfer enhancement.Firstly,it is found that chaotic convection in the channels and the destruction of the thermal boundary layer have a great impact on the heat transfer performance after reviewing the research status of heat transfer by nanofluids in microchannels.In addition,the heat transfer of microchannels can be enhanced by increasing the heat dissipation area and using additives.Considering the heat exchange and friction resistance,the heat transfer performance was evaluated by the Nusselt number at same pump power.Secondly,it is introduced that the problems caused by micro-flow on the basis of the microfluidic theory and the control equations of computational fluid dynamics.The numerical simulation model,the non-uniform two-phase flow model named Buongiorno model,are employed.In addition,it is explained that the boundary of conditions,processes and methods of numerical simulation which are lay for the subsequent research.Thirdly,a model with staggered trapezoidal ribbed array was designed,and the temperature and velocity distribution at different Reynolds Numbers are obtained by the numerical simulations.The results show that the non-uniform two-phase flow model is closer to the real situation than the single-phase flow model.The proposed trapezoidal rib microchannel heat transfer is better than triangular rib microchannel,because the large opposite vortices will produce the top area of trapezoidal rib,which can drive the thermal fluid upward from the bottom to enhance the heat transfer.When other conditions are fixed,the higher the concentration of nanofluid is,the more particles is,the more likely to cause convection enhancement.The optimization results of ribbed parameters were as follows: rib width 1.0 < b/c < 1.25,angle =45°,rib height 1.0 < 2hr/ H < 1.25,intercostal spacing d/a =0;The mechanism ofenhancing heat transfer is chaotic convection in ribs,interruption of thermal boundary layer and increase of cooling area.Finally,a microchannel heat sink with single rib channel(SRC)and the double rib channel(DRC)was designed.and the temperature and velocity distribution at different Reynolds numbers were obtained by the numerical simulation.The results show that each v-shaped rib will generate vortexes on both sides of the rib in the single rib channel,and the heat can be carried away with the motion of the vortexes.The double rib channel carried away more heat than SRC.Vortexes at the bottom transfer heat to the medium then the vortexes,which caused by upper ribs,continue to transfer heat.The optimization results of ribbed parameters were as follows: angle ? = 60 °,rib height Hr/h = 0.3,intercostal distance d/W = 6:6,rib length b/a = 0.75.For the bilateral costal channels,the angle ? = 75 °,height Hr/h =0.3,intercostal distance d/W=7:6,length b/a=0.75.