Experimental Study On Flow Boiling In Open Microchannels Enhanced By Micro-nanostructures

With the rapid development of the electronic industry,the heat dissipation technology of the electronic industry has received wide attention duo to the high integration and high heat flux density of the electronic industry.The microchannel flow boiling is one of the effective technologies to solve the efficient heat dissipation of electronic components due to the advantages of compact structure,high heat transfer coefficient,high temperature uniformity and low working medium demand.In a variety of methods to enhance the flow boiling heat transfer in microchannels,the techniques of changing the flow channel structure(such as adding fins,designing open slit spaces,etc.)and channel surface modification,which can improve the heat transfer coefficient and reduce the flow resistance,have received wide attention from academia and industry.In this paper,three types of microchannel heat sinks with micron wire mesh coating structure,nanostructure and micro-nanocomposite structure were designed.The flow boiling experiments were carried out with deionized water / electronic coolant SF-33 as the experimental working fluids.The effect and mechanism of micro/nanostructure enhanced flow boiling heat transfer in open microchannel were investigated by analyzing boiling heat transfer curve,pressure drop curve and visual image.When the experimental working fluid is deionized water,it is found that the single micron wire mesh structure microchannel has a significant effect in reducing wall superheat because of its large heat transfer area and additional vaporization core.The existence of the wire mesh layer makes the unique churning-bubble flow bounded by the wire mesh layer in the microchannel,and the heat transfer is significantly enhanced,but the capillary suction capacity of the wire mesh itself is limited,and the CHF has not been improved.The bubbles on the surface of the single nanostructure microchannel are denser,the bubble detachment diameter is smaller and the detachment frequency is higher,and the nanostructure can accelerate the liquid re-wetting process on the channel surface and enhance the convective evaporation heat transfer of the thin liquid film due to the strong capillary suction,which also can effectively delay local drying.Four kinds of vaporization cores and larger heat transfer area of micro-nanocomposite microchannel are the main reasons for the enhancement of nucleate boiling.Under the combination of the capillary suction performance of wire mesh and the hydrophilicity of nanostructure,the micro-nanocomposite surface microchannel forms the adaptive characteristics of liquid transport and evaporation,and the heat transfer and re-wetting ability are improved obviously,which effectively improves the CHF.The physical model shows that the micro-nanocomposite structure can enhance the flow boiling heat transfer by increasing the heat transfer area,enhancing micro-convection and microliquid layer evaporation,thus,the heat transfer coefficient increases by 149% and the CHF increases by 66% compared with the smooth surface microchannel,which is beneficial to enhance the heat transfer and delay the heat transfer deterioration at the same time.When the experimental working fluid is replaced by electronic coolant SF-33 with low surface tension,the three types of enhanced structures can still significantly enhance the microchannel flow boiling heat transfer.In the single micron mesh structure and micro/nanocomposite structure microchannel covered by wire mesh,the bubbles can be continuously generated from the surface of the wire mesh covered by liquid film,and the nucleate boiling is significantly enhanced,which shows the better heat transfer performance.However,due to the low surface tension and liquid viscosity of SF-33,the surface micro/nanostructure(especially hydrophilic nanostructure)cannot significantly improve the fluid rehydration capacity of microchannel,thus,the effect of liquid film evaporation on enhancing heat transfer is not prominent.On the contrary,the escape of many bubbles increases the flow resistance and reduces the critical heat flux.Thus,changing the hydrophobicity of the micro-nano structure surface is not the best choice to effectively improve heat transfer deterioration for low surface tension working fluid.It is more necessary to innovate and improve the geometric structure of the channel.