This paper presents the refrigerant adiabatic upward flow in the vertical header of microchannel heat exchanger and its effect on distribution. R410A is circulated into the header through the microchannel tubes (5 or 10 tubes) in the bottom pass and exits through tubes (5 or 10 tubes) in the top pass representing flow in the heat pump mode of reversible systems. Three circular headers were explored, each with the microchannel tubes inserted to half depth. The quality was typically varied from 0.2 to 0.8. Mass flow rate was from 1.5 to 4.5 kg/h per microchannel. The best distribution is found at high flow rate and low quality. Distribution is improved by doubling the number of microchannel tubes although elongation of the header has negative effect. Visualization reveals the effects of flow patterns in terms of homogeneity and liquid momentum. Refrigerant in the churn flow has better distribution than in the separated flow since the two-phase mixture is more homogeneous. The distribution is better at high mass flux in the header because the higher momentum liquid can be supplied to the top exit tubes
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