Study Of Laminar Flow Drag Reduction And Heat Transfer Enhancement In Microchannel Based On Surface Properties

With the development of the microelectronic industry and micro-machining technology,the thermal power of electronic components is increasing rapidly.Traditional cooling methods will not be able to meet the cooling requirements of high-power electronic devices.Liquid cooling using microchannel has the advantages of high integration,small size,and high efficiency,is the new generation of the cooling method with great application prospects.However,the flow resistance in the microchannel increases sharply,and the operating pressure in the cooling system at a high flow rate has an adverse effect on the sealing and structural stability of the cooling system.Therefore,the theoretical innovation and development of drag reduction and heat transfer enhancement in microchannels under laminar flow is very important,and can provide a solid theoretical basis for guiding the design of high-efficiency and low flow resistance microchannel coolers.Based on the current research situation of micro-channel heat sinks,it is found that the microstructure and properties of the channel surface are important factors that affects the flow resistance and heat transfer enhancement.In this paper,the influence of surface microstructure,surface roughness and surface wettability on the heat transfer under laminar flow at micro-scale and its mechanism were studied.The main research work and conclusions of this paper are as follows:(1)The drag reduction and heat transfer enhancement mechanism of the dimple structure was studied,and the influence of the dimple arrangement,dimple parameters and channel parameters on the flow resistance and heat transfer characteristics were analyzed.The response surface method and genetic algorithm method were used to conduct multi-objective optimization of geometry parameters of channel and dimple.The heat transfer enhancement effect of the dimple structure under complex flow in the dimple-wavy microchannel was studied.The results show that the dimple structure can achieve the dual effects of heat transfer enhancement and drag reduction in the straight channel.The optimal geometry shape of the dimple structure in the straight channel is cover the width of channel,single row,and closely placed.The optimal depth of the dimple structure changes with the channel aspect ratio and pump power.When the aspect ratio increases,the optimal depth of the dimple increases.When the pump power increases,the optimal depth of the dimple decreases.In the special-shaped channel that already has transverse flow,the dimple structure can only enhance the heat transfer on the premise of strengthening the transverse flow.(2)Based on the Fourier series method,a mathematical model of roughness that satisfies randomness,correlation and Gaussian distribution was constructed.The roughness model was applied to simple and complex microchannels includes the flat channel,wavy channel,and dimple channel,a relative roughness of 0.25%～2% was constructed.The effects of roughness on the flow and heat transfer characteristics under micro-scale conditions were studied.The results show that when the triangulate size is less than half of the correlation length,the local flow and heat transfer characteristics could be well captured.The effects of correlation length on flow resistance and maximum temperature are not obvious.Roughness could enhance the heat transfer,but the flow resistance increases.In the flat channel,when relative roughness is small,heat transfer is weakened under the same pump power.In wavy channel and dimple channel,surface roughness could always enhance the heat transfer under the same pump power.(3)A simplified superhydrophobic model was established that considers the viscous shear force of the gas-liquid interface and the coupled heat transfer of the gas-liquid interface.The flow and heat transfer characteristics under different superhydrophobic structures and working conditions were explored,and the relationship between slip length,temperature jump coefficient and wall shear stress was studied.The superhydrophobic model was further simplified by adding the relationship between slip length,temperature jump coefficient and wall shear stress to the boundary conditions.Then the flow and heat transfer characteristics in the flat channel and the dimple channel with superhydrophobic surfaces were numerically analyzed.The results show that the proportion of the shear force and heat flux at the gas-liquid interface in the total wall shear force and total heat flux increases with the increase of the free shear ratio and the decrease of the depth of the superhydrophobic structure.The proportion could reach up to 6%～7% within the scope of this paper.The slip length and temperature jump coefficient of the superhydrophobic surface gradually decrease with the increase of wall shear stress.When wall shear stress is low,the temperature jump coefficient decreases rapidly because of the heat convection in the superhydrophobic structure.Under equal pump power,the maximum temperature decreases when the superhydrophobic surface is employed.