Pressure Drop And Visualization Of Flow Boiling Of R22in Rectangular Microchannls

In recent years,the study on heat transfer and pressure drop characterisitics withmicrochannel is becoming an important subject,however,the mechanisms of heat transfer andpressure drop on two-phase flow in microchannels need to be further studied.A newrefrigeration system experimental platform using microchannel heat exchanger as theevaporator has been constructed in this study,and the microchannel experimental sectionwhich can be used for visualization also has been designed. On this basis,the flow boilingexperiment using R22as the working fluids for studying two-phase pressure dropcharacteristics was carried out through the rectangular micochannels with differentcross-sectional area of0.6mm×2mm,1mm×2mm,2mm×2mm respectively.Besides, thetwo-phase flow pattern and pressure fluctuations was studied through visualization.The characteristics of two-phase flow boiling pressure drop in microchannels with R22were studied.The effect of mass flux,saturation temperature,heat flux, exit steam quality andhydraulic diameter on two-phase frictional pressure drop were investigated.After comparedthe experimental data and nanofluid data with other existing pressure drop predictive model,anew two-phase frictional pressure drop predictive model was established based on the supportvector machine (SVM) model.The calculated and experimental results domenstrated that theacceleration and gravity pressure drop accounted for20%~25%of the total pressure;thetwo-phase frictional pressure drop increased with the increasing of mass flux and thedecreasing of saturation temperature; with the increasing of heat flux or exit steam quality, thetwo-phase frictional pressure drop increased, when heat flux exceeded a certain value, thepressure drop decreased;when the hydraulic diameter was smaller,the higher frictionalpressure drop would be obtained, especially in the micochannels width of0.6mm,it washigher nearly2～3times than in the micochannels width of2mm.The existing two-phasepressure drop predictive model can not predict the two-phase frictional pressure drop underthis experimental condition accurately. According to the characteristics of the two-phasefriction pressure drop data, a two-phase frictional pressure drop predictive model wasdeveloped with the introduction of support vector machine (SVM),which showed goodprediction effect less than7%of the average relative error. The visualization study on the microchannel two-phase flow of R22was conducted usinghigh speed video system.The visualization study indicated that four main flow regimes, slug,bundle of annular, annular and mist flow appeared in the0.6mm x2mm microchannels,andthe bundle of annular flow appeared in most time; in the2mm x2mm microchannels,whenin high heat flux,it appeared liquid film backflow phenomenon during annular flow.With thehelp of visualization, pressure drop and temperature fluctuations in different heat flux werestudied,and found that with the increasing of heat flux, pressure drop and temperaturefluctuations were increased.