Study On Fluid Flow And Heat Transfer Performance In Microchannels

Microchannel heat sinks has high heat dissipation efficiency due to its compact structures and large specific surface area.And it can meet the increasing demand of heat dissipation and has been widely used to cooling electronic chips in Micro-Electro-Mechanical System(MEMS).As the foundation of the flow medium transport in the heat sinks,it is of great significance to study the flow and heat transfer performance of the microchannel heat sinks for the structural design and the improvement of the heat dissipation efficiency.Surface quality can be neglected in the study of macro-size channel,however it has a great influence on microchannels which must be taken into account because of the micro-size of microchannel.In this paper,the microflow and heat transfer performance of microchannel are studied in three aspects:surface roughness,surface microstructures and flow medium.First,surface roughness model of rectangular microchannel based on fractal geometry method is established.Micro-injection molding method is used to fabricate various sizes of microchannel and confocal microscope is used to measure the bottom roughness of microchannel.It is found that the size of roughness is in the same order of magnitude as microchannel,that is,micrometer.Therefore,the effect of roughness on the flow and heat transfer performance in microchannel cannot be ignored.The roughness model of microchannel is established by fractal geometry method and compared with the measured results of roughness in which the error is less than 10%.Second,the effect of bottom roughness on flow and heat transfer performance in microchannel is studied and the relationship between surface roughness and flow,heat transfer parameters in microchannel is revealed.Models of microchannel with different surface roughness are established by changing fractal parameters.Effects of channel size,surface fractal dimension and surface roughness on friction factor f,the mean temperature on the bottom surface Tave,Nusselt number Nuave are studied by numerical simulation and experiment method respectively.The results show that the greater the bottom surface roughness of the microchannel,the worse the flow performance and the better the heat transfer performance.Roughness can promote the heat transfer performance in the microchannels.Then,the effect of microstructures on flow and heat transfer performance is studied and the relationship between microstructures and flow,heat transfer parameters in microchannel is revealed.The 3D model of microchannel with microstructures is established,and the effects of types(cubic shape,cylindrical and conical shape)and heights(4-6?m)on friction factor f,Tave and Nuave in the microchannel is studied.The results show that with the increase of structure height,the flow performance is weakened and the heat transfer performance is enhanced.Cubical,cylindrical and conical microstructures all can promote the heat transfer performance in the microchannel,among which the cylindrical microstructure has highest heat transfer performance.Under the studied working conditions,the microchannel with 4 ?m high cylinder microstructure has the highest hydro-thermal performance.Finally,two-steps method is used to prepare nanofluids,the effect of nanofluids on the flow and heat transfer performance is studied and the mechanism of the effect of nanofluids and microstructure on the flow and heat transfer performance in microchannel is revealed.Homogeneous and stable Al2O3 nanofluids is prepared by two-step method,and.the thermal physical properties of the nanofluids is analyzed.The thermal conductivity of nanofluids increased with the increase of volume fraction.The flow and heat transfer properties of Al2O3 nanofluids with different volume fraction flow through microchannel with microstructures are studied by numerical simulation and experiment.And the effects of volume fraction on.f and Nuave are analyzed.The results show that the effect of nanofluids on heat transfer performance is much higher than that of its resistance on flow performance.Compared with the flow of pure water through smooth microchannels,the heat transfer performance in the microchannel is greatly improved under the action of nano-particles and microstructure.