Experimental Investigation on Flow Pattern and Bubble Behavior during Subcooled Flow Boiling of R1233zd(E) in Parallel Channels

In this study, the flow pattern and bubble behavior of R1233zd(E) during subcooled flow boiling in parallel channels are experimentally investigated with visualization and thermal measurement. The test section is composed of 21 rectangular mini channels with the hydraulic diameter of 1.5 mm and the length of 140 mm. Bubbly flow, slug flow, churn flow and wavy-annular flow occur in sequence with the increase of vapor quality, while transient flow pattern transition process involving multiple flow patterns are also captured. The distribution of flow pattern is non-synchronized and axial-asymmetric, with earlier flow pattern transitions observed in peripheral channels away from the center axis. The initial nucleate site in each channel also show a random and axial-asymmetric distribution, while faster bubble growth can be noted in some channel under the comprehensive effects of liquid evaporation and bubble coalescence. The variation of heat transfer coefficient is correspondence to the flow pattern transition, showing different trends along the flow direction. The increase of mass flux can lead to delayed flow pattern transition and variation of heat transfer coefficient. In addition, higher heat transfer coefficient can be noted in channels away from the center axis.