Microchannel Heat Exchanger, Vertical Header, Two-phase Flow, Refrigerant Distribution, CFD
Refrigerant maldistribution in the header of microchannel heat exchanger (MCHX) may reduce MCHX capacity by up to 30% because it creates the unwanted superheated region, where the heat transfer is much lower than the two-phase region due to the lower heat transfer coeeficient of superheated vapor and less temperature difference between refrigerant and air. The objective of this study is to gain more insights on the two-phase flow in the header through CFD simulation based on experimental results. In the experiment, the two-phase R134a or R410A 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 system. Visualization reveals that R410A distribution is related to the size of churn flow region in the header. The best distribution is at high mass flux and low quality. When mass flux is lower, the churn flow region is smaller and the distribution is worse due to lack of sufficient momentum to supply liquid to the top tubes. At higher quality (e.g. xin = 0.6 and 0.8), besides lacking sufficient momentum to lift liquid, the churn flow region is much smaller because the semi-annular flow appears at the bottom parts of the header. The liquid film in semi-annular flow bypasses the bottom exit tubes and makes these tubes get less liquid than other tubes in churn flow region. The experimental results are compared with CFD simulation for better understanding. The Eulerian-Eulerian model in the commercial software Fluent is used to conduct simulation. Good accuracy of CFD simulation is achieved by comparing with the experimental results. The CFD simulation shows more details of two-phase flow pattern in the vertical header. The pressure contour shows the pressure increases along the upward flow. It is due to the fluid is exiting and the flow is decelerating. The velocity vectors show that there is a swirling between each two tubes in the header, except for the last two tubes. The swirling creates non-uniform pressure distribution in the same cross-section plane. Therefore, the location where to measure the pressure in the header affecting the understanding of two-phase flow in the header significantly.