Enhanced Flow Boiling Heat Transfer Of Micro/Nano-scale Surfaces In Different Geometric Microchannel

With the development of science and technology,more and more equipment and components in the fields of energy,chemical industry and electronics require efficient thermal management.Due to the advantages of high heat transfer performance,compact structure and low pump power,flow boiling in microchannel is one of the most efficient heat transfer technologies worthy of further research.As ones of main methods of flow boiling heat transfer enhancement in microchannel,micro-/nano-scale structured surface and special geometry of microchannel have the advantages of exceptional heat transfer performance and low pressure drop.They have broad research prospects and need to be further researched.In this paper,with the purpose of enhancing the heat transfer of subcooled flow boiling,the micro-/nano-scale structured surface and the structured microchannel are taken as the research objects.And the enhanced heat transfer performance of different micro-/nano-scale structured surfaces in the different structured microchannel is experimental investigated.The micro-/nano-scale structured surface in this paper includes nano-scale nanowire surface,micro-scale pores surface and porous copper surface,micro-nanoscale surface,involving different scaled structure and different wettability.The structured microchannel includes straight rectangular channel,straight rectangular channel with bubble generator,and stairstep channel.By setting different mass flux,heat flux and inlet subcooled degree,the heat transfer coefficients,boiling curves,maximum heat flux before dryout,pressure drop fluctuation and other parameters are analyzed.Also,two-phase flow patterns are observed and analyzed.It aims to explore the mechanism of heat transfer enhancement of flow boiling on the micro-/nano-scale structured surface and the structured microchannel.In this study,the research about heat transfer enhancement of micro-/nano-scale structured surface and structured microchannel is reviewed,followed by the introduction of the microchannel flow boiling experiment system.And the correlation analysis is conducted about the subcooled flow boiling heat transfer coefficients on smooth silicon surface in rectangular channel.The reliability of the experiment system is demonstrated through the agreement between the experimental measurement values and the correlation results.In the experiment on nanowire surfaces of different heights,it is founded that heat transfer performance on 12?m height nanowire surface is worse than that on 4?m height nanowire surface,because the stack by higher nanowires prevents bubbles generated in the nucleation cavity to leave.Thus the thermal resistance of the near-wall area is increased.Periodic two-phase flow pattern of elongated bubble generation,bubble expansion,local dryout and re-wetting is showed on the 4?m height nanowire surface.As the heat flux increases,the proportion of local dryout in the periodic flow pattern becomes larger,resulting in heat transfer deterioration.However,the flow pattern on the 4?m height nanowire surface does not develop so rapidly.Under high heat flux,the main flow pattern is still constrained bubble flow couple with expanding to downstream,not forming elongated bubble.That is why heat transfer deterioration does not happen.The wall superheat required for the onset of boiling(ONB)on the micro-scale pores surface at different mass flux conditions is basically the same,which is much lower than the untreated surface investigated in other studies.The hydrophobicity and porous structure is the main reason causing the earlier onb.At the conditions of low mass flux,elongated bubble flow gradually appeares at the downstream position as the heat flux increases.The thin liquid film beneath the elongated bubble has a better heat transfer efficiency during the evaporation process.Therefore,the downstream local heat transfer coefficients are better than the upstream ones at high heat flux conditions.The heat transfer performance of the micro-scale pores surface is better than that of a smooth copper surface,with a strengthened heat transfer coefficient of up to about 36%.Because of the lower wall superheat with the micro-scale pores surface,it is possible to conduct flow boiling experiments for a larger range of heat fluxes.Aiming at the advantages of multiple nucleation sites and large heat transfer area on the super-hydrophobic porous copper surface,and the disadvantage that the forming vapor film is difficult to detach from the wall,the bubble generator is designed to force the ONB point to be advanced,which leads to periodic flush of elongated bubble flow from the generator to prevent vapor film attachment.The elongated bubbles with thin film evaporation is a highly effective heat transfer mechanism to enhance the heat transfer coefficient.On the basis of the enhanced heat transfer effect of the super-hydrophobic porous copper surface,the heat transfer performance at low mass flow is further improved.The heat transfer coefficient at conditions of 200 kg/(m~2·s)and 300kg/(m~2·s)are enhanced by 53.1%and 33.2%respectively,but the enhancing effect of the bubble generator at higher mass flux conditions are negligible.The enhanced boiling heat transfer method that combines the micro-nanoscale surface and the stairstep channel can effectively improve the boiling heat transfer performance under high heat flux conditions which inhibit the occurrence of local dryout phenomenon.The heat flux of dryout increases from 126 k W/m~2 to 302 k W/m~2at low mass flux.It is found that the boiling number can be used to judge the turning point from the sweeping flow to the churn flow at different conditions.Due to the different heat transfer mechanisms at different flow patterns,the local heat transfer coefficient is strongly correlated with the flow patterns.The local heat transfer of the sweeping flow is relatively uniform compared to the ones with other flow patterns,while the downstream heat transfer of the churn flow is significantly better than that of the upstream.