Numerical Study On Cavitating Flow And Heat Transfer Characteristics Of Micro-Orifice Channels

The strong disturbance effect caused by cavitating flow has been proved to be a promising route to enhance heat transfer process.However,limited by the micro-size,the characteristics of microscale cavitation are quite different from those of conventional scale.The flow pattern of microscale cavitation and the sensitive factors of the flow characteristics have not been clearly explained yet.In addition,the mechanism effect of cavitatimg flow on microchannel heat transfer is not completely clear.Therefore,the micro-orifice channel is selected as the research object,and the mumerical simulation on cavitating flow and heat transfer characteristics of refrigerant in microscale are investigated.In this work,the effects of working fluid properties and channel structure parameters(micro-orifice structure and wall characteristics)on cavitating flow characteristics are evaluated.Besides,the cavitatiing flow characteristics of the comprehensive heat transfer performance in microchannel with different flow parameters(including inlet Reynolds number,cavitation number and heat flux density)are furtherly analyzed.The main results are as follows:(1)Based on Rayleigh-Plesset bubble dynamics equation,a microscale cavitation model is developed by considering scale effect and thermodynamic effect.Compared with the existing experimental results,the microscale cavitation model is proved accurately.In addition,it is found that the thermodynamic effect of cavitation and surface tension are unfavorable to the cavitation in microscale.(2)The cavitating flow characteristics of common refrigerants(water,R123 and liquid nitrogen)are compared,and the transformation mechanism,flow pattern,thermal effect and pressure drop characteristics of different refrigerants in each cavitation stage are carried out.The results show that compared with R123 and liquid nitrogen,the cavitation inception of water shows obvious "lag" with a larger surface tension,the transformation of each stage of cavitation of R123 and liquid nitrogen is faster.The thermodynamic effect of R123 and liquid nitrogen is more intense,and the temperature drop in the cavitation area is obvious.In addition,due to the high sensitivity of critical vaporization pressure to temperature,the cavitation intensity of R123 and liquid nitrogen is significantly affected by the working fluids temperature.(3)The effects of micro-orifice aspect ratio,micro-orifice structure and wall roughness on cavitating flow are investigated.The results show that the mechanism parameters of microorifices channel have obvious influence on the cavitating flow intensity,and there is an optimum micro-orifice aspect ratio of 3.6 in the range investigated in this work.In which,the cavitation rate is higher and the flow resistance loss is reduced.Besides,the cavitation rate of vertical structure microchannel is significantly higher than that of scale structure and circular structure channel with the same mass flow rate,while the pressure drop of vertical structure channel is remarkable increased.The roughening treatment of the bottom wall will significantly increase the flow resistance,restrain the cavitating flow intensity,and furtherly contribute to a high-frequency pressure fluctuation.(4)The influence of cavitatiing flow on heat transfer performance under the conditions of different Reynolds number,cavitation number and wall heat flux is analyzed.It is found that compared with the non-cavitating flow,the cavitating flow can not only improve the heat transfer performance of the microchannel,but also reduce the flow resistance loss.A higher inlet Reynolds number could contribute a better comprehensive heat transfer performance,and the best heat transfer enhancement effect occurs in the middle of the cavity.Based on the same mass flow,the resistance loss could be significantly reduced with a decreasing cavitation number,and furtherly contributes to an optimized comprehensive heat transfer performance.In addition,by considering a low heat flux,the heat transfer of the cavitating flow in microchannel could be obviously enhanced.