Investigation On Heat Transfer Enhancement Of Microchannels With Porous Structures

With the development of electronic devices toward integration and miniaturization,their power density is getting higher and higher,and a series of thermal problems have seriously affected the stability of electronic devices.As an efficient heat dissipation technology,microchannel heat sink has great potential in the heat dissipation of electronic equipment with high heat flux.There are many methods to improve the heat transfer performance of microchannels,but these methods have an inevitable problem that improving the heat transfer capacity of microchannels will bring about a pressure drop increase,and the pump power consumption would also increase.In order to alleviate this problem,this thesis combines the common ribs and porous media,and the comprehensive performance of microchannels is improved from three aspects:the arrangement of ribs,the shape of ribs and the structure of the microchannels,which explore new schemes for the cooling technology of electronic equipment.Firstly,the heat transfer and flow characteristics of microchannels with solid and porous ribs are numerically investigated and the influence of different rib arrangements on the heat transfer and flow in microchannels is studied.The results show that the heat transfer performance of the microchannel with solid and porous ribs is much better than those without any ribs,but the pressure drop also increases greatly.In addition,the pressure drops and friction factors of microchannels with solid ribs are larger than those of the corresponding porous ribs microchannel under the same rib arrangement.When the solid ribs are replaced by the porous media,the pressure drop of the microchannel with the ribs in the middle,symmetrical and staggered on both sides of the microchannel is reduced by about 67%,57%and 12%respectively.From the velocity and streamline distributions,it can be observed that vortices and secondary circulation flow are generated behind the solid fins,and there is a non-uniform velocity distribution in the porous region,which intensifies the fluid mixing,enhances the disturbance and improves the convective heat transfer performance.In addition,the field synergy principle is adopted to analyze the synergy effect between velocity field and temperature gradient field for different microchannels,and the microchannel with porous rib shows better synergy effect.In the comprehensive performance evaluation,the microchannel with porous ribs in symmetrical and staggered arrangement shows the best comprehensive thermal and hydraulic performance.Then,this work sets up a series of ribs with different cross-section shapes in microchannels,including circle,square and regular hexagon,and investigate the effect of cross-section shapes on the heat transfer and flow performance in microchannels.Meanwhile,this work adopts fractal structure based on the hexagonal cross-section ribs in the microchannel,and compares the heat transfer and flow characteristics between hexagonal cross-section ribs and fractal structure ribs.In terms of heat transfer characteristics,the results show that the Nusselt number of the microchannel with solid circular ribs is the smallest,and its heat transfer performance is the worst;the microchannel with porous fractal ribs has the largest Nusselt number,and its heat transfer performance is the best.Under the same rib shape,the porous structure ribs can increase the Nusselt number of microchannels,but the effect is not significant.The fractal structure has a significant improvement in heat transfer of microchannels.In terms of flow characteristics,among all the microchannels with solid ribs,the microchannel with solid fractal structure ribs displays the best heat transfer performance,but its pressure drop is the largest,followed by microchannels with ribs of circular cross-section,square cross-section and hexagonal cross-section.Although the fractal structure can enhance heat transfer of microchannels,the pressure drop of microchannels with solid fractal structure increases by 108.4%～140.8%compared with the hexagonal cross-section ribs.After replacing the solid ribs with porous media,the pressure drops of microchannels with circular cross-section ribs,square cross-section ribs,hexagonal cross-section ribs and fractal structure ribs can be reduced by 56.5%～59.7%,66.5%～67.7%60.5%～61.5%and 81.1%～81.7%,respectively.In terms of comprehensive performance,the j/f factors of the microchannel with porous fractal structure maintain the largest,showing the best comprehensive performance.Finally,in view of the heat transfer enhancement in double-layer microchannels,the effects of rib size variation along flow direction,porous ribs with variable porosity along flow direction,and variation of rib size in the upper layer on the heat transfer and flow characteristics of double-layer microchannels are investigated.In terms of the effect of rib size variation along flow direction,when the size of the ribs gradually increases along the flow direction,the change of the rib size has little effect on the heat transfer and flow performance of the microchannel,and the comprehensive performance of the microchannel with double layer cannot be improved.For the microchannels with porous ribs,when the porosity of ribs changes along flow direction,compared to the microchannels with constant porosity,the heat transfer and pressure drop are basically maintained at the same level,and their comprehensive performances are similar to each other.Changing the porosity along flow direction cannot effectively improve thecomprehensive performance of the microchannel with double-layer.Besides,compared with the microchannel with the constant rib size in both layers,the heat transfer performance of the microchannel with smaller rib size in upper layer is slightly deteriorated,but the pressure drop is reduced effectively,and the j/f factor has been improved by 8.2%～10.8%,showing that the overall performance has been effectively improved.Therefore,adding ribs in the double-layer microchannel can improve the heat transfer performance,and appropriately reducing the rib size of the upper layer is effective to improve the comprehensive performance of the double-layer microchannel.