Distribution function for reversible microchannel heat exchanger with vertical headers - Considering the effects of inlet conditions, geometries and fluid properties.

Refrigerant maldistribution deteriorates microchannel evaporator performance because it creates unwanted superheated region. The objective of this study is to explore what affects distribution in the vertical header and how to model distribution. Two-phase R245fa, R134a, R410A or R32 is circulated into the transparent vertical header through multi-parallel microchannel tubes in the bottom pass and exits through multi-parallel microchannel tubes in the top pass representing the flow in the heat pump mode of a reversible microchannel heat exchanger. Based on the visualization, the best distribution is when the momentum in the upper region is high enough to lift liquid to the top, while the momentum in the lower region is not too high to create semi-annular flow. An empirical function is derived to model refrigerant distribution by relating liquid take-off ratio with inlet quality, vapour phase Reynolds number and Froude number in the header immediately upstream of the microchannel tube.