Effects of fluid properties on two-phase flow and refrigerant distribution in the vertical header of a reversible microchannel heat exchanger – Comparing R245fa, R134a, R410A, and R32

This paper explores the effects of fluid properties on refrigerant distribution in the vertical header of a reversible microchannel heat exchanger under heat pump mode, by comparing the visualization and distribution results of R245fa, R134a, R410A, and R32. Each refrigerant enters into the header through five microchannel tubes in the bottom pass and exits through five microchannel tubes in the top pass. The distribution of R245fa is best, and then R134a, R410A, and R32, because the churn flow region of R245fa in the header is largest, so more tubes have opportunities to receive liquid. For R245fa and R134a, the main reason for maldistribution is because of lacking sufficient momentum to lift liquid to the top. Hence, the distribution may be improved by increasing mass flux in the header at both low and high qualities. For R410A and R32, it is difficult to improve distribution by increasing mass flux at high qualities because it is very easy to form semi-annular flow in the bottom part of the header where the liquid film bypasses the bottom exit tubes and these tubes receive less liquid.