Effect of flow direction on evaporation flow regime of R32/R1234ze(E) mixture inside multiple rectangular minichannels

Abstract

This study visualized the evaporation flow regime of a non-azeotropic mixture R32/R1234ze(E) inside multiple vertical-upward and horizontal rectangular minichannels with a hydraulic diameter of 2 mm. The effects of the mass flux, vapor quality, heat flux, flow direction, and test refrigerant on the flow regime characteristics were explored. The flow regimes of the mixture were compared to those of R1234ze(E). The effect of flow direction on the flow regime was examined under low mass flux conditions. In the vertical-upward flow, three flow regimes: slug, slug-annular, and annular flow regimes were observed. The flow regime transition boundary of R1234ze(E) shifted to lower mass flux and vapor quality conditions compared to the mixture because of the higher vapor velocity of R1234ze(E). Boiling bubbles were generated in the liquid slug and channel corners around the vapor plug, and the number of boiling bubbles increased with an increasing heat flux. The size of the boiling bubble of the mixture was smaller than that of R1234ze(E), which is attributed to the mass transfer resistance of the non-azeotropic mixture. Only in the vertical-upward flow, the reverse flow was observed in some channels, where the slug flow was observed, because of the effect of gravity. The number of channels with reverse flow decreased with increasing vapor velocity. The annular flow did not appear in the horizontal flow, and the flow regime was observed only in two regimes: slug and slug-annular flow regimes. No difference in flow regime transition boundary between R1234ze(E) and R32/R1234ze(E) was observed.