Single-phase Flow Pressure Drop of R134a Vapor and Liquid in the Vertical Header of Multi-Pass Microchannel Heat Exchanger

Refrigerant maldistribution in the microchannel heat exchanger (MCHX), though mainly due to phase separation in the header in two-phase flow, is also affected by the pressure drop in the header. This paper investigates the single-phase flow pressure drop of nitrogen, R134a vapor, and R134a liquid in the vertical header of a multi-pass microchannel heat exchanger. The objective is to develop a model for the single-phase pressure drop in the header and provide the basis for two-phase flow pressure drop model. The fluid enters into the vertical header through five microchannel tubes in the bottom pass and exits through the five microchannel tubes in the top pass representing the flow in the outdoor MCHX of a reversible system under heat pump mode. The local pressure drop across each exit microchannel tube in the header is measured at various inlet mass flow rates. For nitrogen and R134a vapor, the local pressure drop usually reduces along the upward flow in the header. For R134a liquid, the local pressure drop is highest across the second exit tube, then it decreases along the upward flow. The measured overall local pressure drop includes the acceleration, gravitation, friction, and minor pressure drop due to the protruded microchannel tube. The minor pressure drop coefficient based on the experimental results is compared with the existed empirical correlation. It is found that the empirical correlation works well for nitrogen and R134a vapor but not for R134a liquid. A new correlation for R134a liquid is proposed with the same format, but the coefficients are derived based on the experimental results of this study using the least square curve-fit method