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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]

Boiling instability and periodic flow reversal are commonly seen in microchannel heat exchangers, but have been studied in air conditioning systems only recently. The amount and frequency of reverse flow largely depends on the geometry of the channels. This paper presents results obtained in a R134a automotive air conditioning system operated under flash gas bypass mode with venting of reversed vapor (to provide better control of the flow). Results for multiple microchannel evaporators with different channel diameters and lengths are presented. The mass flow rate and frequency of reversed vapor are measured for each geometry. Meanwhile, a mechanistic model is developed to simulate the bubble dynamics inside of a single microchannel tube. This model is validated against experimental data and employed to explain the effect of channel length and diameter on flow reversal. Both experimental and simulation results show that larger channel diameter and longer channel length result in less flow reversal with a lower frequency. [ABSTRACT FROM AUTHOR]

Two leading edge channels of a microchannel evaporator are made transparent to visualize the flow regimes inside. The evaporator is operated in an automotive air conditioning system under both two-phase refrigerant feeding and liquid refrigerant feeding modes. The flow regimes in the microchannels are different in these two operation modes. In two-phase feeding mode: churn, bubbly/slug, and annular flows occur alternatively, but the period of each flow regime is not constant. Flow reversal is only witnessed occasionally in bubbly/slug flow regime. In liquid feeding mode, only liquid and bubbly/slug flows are observed. Reverse flow occurs periodically. The duration of flow reversal is shorter in two-phase feeding mode than in liquid feeding mode. Most likely it is due to higher upstream resistance caused by two-phase refrigerant feeding. [ABSTRACT FROM AUTHOR]

A newly developed mechanistic model of bubble dynamics in single microchannel is presented in this paper. The comparison between high-speed visualization and simulation results shows that this model is capable of capturing the transient flow regime and slug velocity inside of single microchannel and predicting flow reversal. The model quantitatively demonstrates the mechanism of flow reversal. Within one periodic cycle, the evolvement of flow regime, pressure distribution along channel length and mass flux at tube inlet are well correlated with each other. The local pressure peak caused by the build-up of downstream flow resistance can cause negative pressure gradient, which induces flow reversal. [ABSTRACT FROM AUTHOR]