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Although refrigerant maldistribution among parallel microchannel tubes is mainly caused by phase separation of vapor and liquid in the header, it is also affected by pressure drop in the header. This study experimentally investigates the pressure drop of single-phase and two-phase R134a flow in the vertical header of a multi-pass microchannel heat exchanger. R134a is circulated into the transparent 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. The pressure drop in the vertical header causes the top tube has lower mass flow rate than the lower tubes for both single-phase and two-phase flow. The overall pressure drop in the header includes four components: acceleration, gravitation, friction, and minor pressure drop due to microchannel tube protrusion.

Lubricant in compressor usually flows out with refrigerant. Thus, it is evitable for lubricant to be present in the heat exchanger, which significantly affects the heat exchanger performance. This paper is to investigate the effects of PAG oil on R134a distribution in the microchannel heat exchanger (MCHX) with vertical headers and to provide a tool to model R134a (with oil) distribution and its effects on MCHX capacity. The flow configuration in MCHX under the heat pump mode of the reversible system is mimicked in the experimental facility: refrigerant-oil mixture is fed into the test header from the bottom pass and exits through the top pass. It is found that a small amount of oil (OCR=0.5%) worsen the distribution. But further increasing OCR to 2.5% and 4.7%, the distribution becomes better.

Distribution of R134a in four different vertical headers of microchannel heat exchanger was investigated experimentally. R134a was provided into the header by the microchannel tubes (5 or 10 tubes) in the bottom pass. It left the header through the microchannel tubes (5 or 10 tubes) in the top pass representing the upward flow in the heat pump mode of the reversible systems. The inlet quality was varied from 0.2 to 0.8, and the inlet mass flow rate was from 1.5 to 4.5 kg/h per microchannel tube. Among the test conditions, the aluminum and transparent headers show similar results: refrigerant distribution is better when reducing quality at the same mass flow rate and when increasing mass flow rate at the same quality. Increasing the tubes protrusion and the number of the microchannel tubes usually improve the distribution due to the increase in mass flux. Based on the visualization, churn and separated flow regimes are identified.