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The amount and frequency of reverse flow in microchannel evaporators largely depends on the thermophysical properties of the working fluid. In this study, four refrigerants (R134a, R1234yf, R245fa and R32) are examined in the same air conditioning system operated under flash gas bypass mode with venting of reversed vapor (to provide better control of the flow). The flow rate and frequency of reversed vapor are measured for each refrigerant. It has been found that fluids with lower heat of vaporization and higher specific volume difference between vapor and liquid phase) tend to generate more reversed vapor flow. Experimental results validate qualitatively and in most cases quantitatively the mechanistic model presented in Li and Hrnjak (2017a). The validated model is used to demonstrate the effects of thermophysical properties on flow reversal for nine widely used refrigerants. [ABSTRACT FROM AUTHOR]

Periodic flow reversal are commonly present in microchannel heat exchangers, but have been studied in air conditioning systems only recently. This paper presents the effect of periodic reverse flow on the heat transfer performance of a heat exchanger. Two heat exchangers with identical geometries in the heat transfer areas are employed and an artificial upstream flow resistance is added for one of them. The heat exchanger without artificial flow resistance is subject to more severe boiling instabilities and consequently generates four times more reverse vapor flow than the other one. The comparison of capacities under identical operating conditions reveals that higher intensity of reverse flow helps to improve cooling capacity by up to 13.3%. Meanwhile, numerical simulations of bubble dynamics coupled with heat transfer are carried out for both heat exchangers. Results show that in the heat exchanger with more reverse flow, the refrigerant side heat transfer coefficients are enhanced, especially in the upstream part of a channel where the flow velocity is relatively low. [ABSTRACT FROM AUTHOR]

This paper presents the further development of Li and Hrnjak's (2013) microchannel heat exchanger model which includes the thermodynamic and transport properties of refrigerant–oil mixture. Effect of lubricant is accounted in boiling heat transfer, pressure drop and refrigerant distribution. A newly proposed infrared thermography based method is implemented in the model to describe the liquid refrigerant distribution in the inlet header of the microchannel heat exchanger. The new model is validated against experimental results (R134a-PAG 46 oil) at various oil circulation ratios (0.1%–8.3%). Simulation results also indicate that lubricant addition improves refrigerant distribution which is in agreement with experiments and the infrared thermography based method enables the model capture lubricant effect on capacity more comprehensively. [ABSTRACT FROM AUTHOR]