Numerical Simulation On Critical Heat Flux Of Flow Boiling Heat Transfer In Microchannel With V-typed Grooves

Critical heat flux(CHF)is the key parameter of flow boiling heat transfer,which is related to the safe and reliable transport of heat transfer equipment.With the miniaturization of the size of the heat exchanger,the study on the CHF of the flow boiling at micro-scale is of great concern,especially how to improve the CHF of the flow boiling applied to the micro-scale has become the focus of micro-scale flow heat transfer.In this paper,V-typed grooves are formed in the horizontal rectangular microchannel heating wall to study the effects of the groove structure and quantity on CHF of flow boiling in microchannel.The effects of flow boiling parameters(working fluid velocity,inlet subcooled temperature,working pressure,etc.)and channel height on CHF were discussed.To this end,based on the VOF model in CFD software,the mass and heat transfer rate of the working fluid are obtained by using the user-defined function,and the model of vapor liquid two phase flow in microchannel flow boiling is established.By means of numerical simulation on micro channel flow boiling process,the variations of the wall superheat,pressure drop and the average heat transfer coefficient with the heat flux,the vapor phase contours,the curves between flow boiling parameters,the channel height and CHF of flow boiling in microchanel were achieved.The main work and conclusions are as follows:(1)Compared the effects of groove structure on the CHF of flow boiling in microchannel.The results show that under the condition of Tin=369K,vin=0.2m·s-1,p=101kPa,compared with the smooth microchannel,V-typed grooves are helpful to improve the CHF of flow boiling;When the ratios of groove depth to width(h/R,R=0.02mm)are 1,2,3,4 respectively,the corresponding channel CHF are 350kW·m-2,400kW·m-2,375kW·m-2,500kW·m-2and 450kW·m-2.h/R=3 corresponds to the highest CHF(500kW·m-2),which can be regarded as the ideal groove structure under the condition.(2)The effects of the groove number on the CHF of flow boiling in microchannel(h/R=3)were investigated.It can be found that the groove number is taken values of 30,90,150,210,the critical heat fluxes of flow boiling in microchannel are500kW·m-2,525kW·m-2,575kW·m-2,and 525kW·m-2,respectively.Increasing groovenumber can improve the critical heat flux(n=30,90,150),however,when groove number increases to a certain value(n=210),the critical heat flux decreases.Increasing groove number could not continuously increase the critical heat flux.In this paper,150 V-typed grooves are recommended.(3)The effects of working fluid velocity,inlet subcooled temperature and working pressure on the CHF of flow boiling in microchannels were compared and analyzed.The CHF is increased with the increase of working fluid velocity and operating pressure.Meanwhile,the critical heat flux increase is more obvious when vin=0.2m·s-1~0.3m·s-1(p=47kPa).With the increase of working fluid velocity,the maximum value of the average wall heat transfer coefficient is also higher when the critical state is reached.The inlet subcooled temperature has little effect on the critical heat flux of flow boiling in microchannels,and the vapor volume is not affected by the inlet subcooled temperature.(4)The influences of channel height on the CHF of flow boiling in microchannels were compared and analyzed.When increasing the channel height(0.1mm,0.2mm,0.3mm),the corresponding CHFs of flow boiling in microchannels are promoted.However,before the critical state occurs,decreasing the channel height will improve the average convective heat transfer coefficient of flow boiling in microchannels.