Flow Boiling Heat Transfer Of Low GWP Refrigerants In Mini Channels

Energy saving and environment protection are two principle themes ofrefrigeration and air conditioning industry. HCFCs refrigerants which can causeozone depletion and global warming are being substituted. Zero ozone depletionpotential and low global warming potential refrigerants such as HFO1234yf,HFO1234ze, natural refrigerants and some mixtures are new potential alternativerefrigerants. HFO1234yf and HFO1234ze are a type of possible alternativerefrigerants witch are being concerned and investigated for their advantages ofenvironment protection, similar thermodynamic behavior to R134a and safety.Efficient and compact micro-channel heat exchanger is considered as a solution ofefficiency, energy saving and environment protection. The usage of micro-channelheat exchanger in refrigeration equipments will play an important role during thealternative period of HCFCs and HFCs.The flow boiling heat transfer characteristics of refrigerants in mini-channels areanalyzed. As the decreasing of channel hydraulic diameter, the prominent forceduring the two phase flow is different from that in conventional channels. Surfacetension is the prominent force of two phase flow in mini-channels and no stratifiedflow will be observed. Slug flow and annular flow occupy a much larger range ofvapor qualities when compared with large diameter channels. The vapor quality whichis the onset of dry-out is lower in mini-channels. Flow pattern maps proposed forconventional channels can not properly predict the flow pattern inmicro/mini-channels and some flow pattern maps proposed for micro-channels issuggested for use in limited conditions. Some heat transfer and pressure droppredictive models were compared with experimental data. A new superposition heattransfer model was proposed for predicting the heat transfer coefficient before dry outin mini channels. The proposed model is outperformance to other correlations. Themean absolute deviation of prediction with the new model against experimental datais20.9%and it can catch77.0%of the data within a deviation30%. Homogeneousmodel, Muller-Steinlagen and Heck model and Zhang and Webb model are the bestthree pressure drop models compared with the experimental data. An experimental facility for flow boiling heat transfer in horizontal stainless steeltube with inner diameter of2mm was described. Experiment method, conditions anddata processing method were designed in detail. Experimental study of flow boilingheat transfer of R32in the designed test section was conducted. As the results shownthe heat transfer coefficients increase with the increasing mass flux and heat flux. Butvariation of heat transfer coefficients with the vapor quality was inconsistent indifferent experiment conditions. More investigation should be conducted to evaluatethe accuracy of the results in this study.