Effect of oil on R134a distribution in the microchannel heat exchanger with vertical header

•R134a and oil distribution in a microchannel HX with vertical header was explored.•Distribution improves as quality decreases and/or mass flow rate increases.•As OCR increases, distribution is firstly worse, then better than pure R134a.•As OCR increases, oil creates more bubbles with smaller size that unifies the flow.•As OCR increases, local churn flow region is extended by examining visualization.

This paper presents refrigerant R134a and oil (PAG 46) mixture upward flow in the vertical header of microchannel heat exchanger (compared to pure R134a and R410A), and its effect on distribution. The mixture enters into the header through the five tubes in the bottom pass and exits through the five tubes in the top pass representing the flow in the heat pump mode of outdoor heat exchangers in reversible systems. The quality was varied from 0.2 to 0.8 and the mass flow rate from 1.5 to 4.5 kg h−1 per tube. The oil circulation rate (OCR) was varied from 0% to 4.7%. It was found that at low OCR, e.g. 0.5%, the distribution was worse than pure R134a, whereas at high OCR, the distribution was improved and better than pure R134a in some cases. This was due to sufficient oil would create a lot of foams and enlarge the liquid region in the header based on the visualization.