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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]

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]