Influence of lubricating oil on heat transfer and pressure drop characteristics of R134a flow condensation in mini-channels

Abstract

In space refrigeration and heat pump systems, the presence of lubricating oil in mini-channel condensers introduces problems of reduced heat transfer efficiency and increased pressure drop. In this paper, the flow condensation of refrigerant R134a in a mini-channel under microgravity is investigated by numerical simulation, and the effects of lubricating oil on heat transfer and pressure drop are analyzed for different tube shapes, hydraulic diameters, and mass fluxes. It was found that as the lubricating oil concentration increases, the heat transfer coefficient decreases while the pressure gradient increases for the same vapor quality. This phenomenon is due to the fact that the presence of the lubricating oil increases the fluid viscosity and the viscous shear increases, leading to a reduction in the inertial forces and the relative velocity of the gas-liquid. Define the ratio of the condensation heat transfer coefficient of the oil-containing refrigerant to the condensation heat transfer coefficient of the pure refrigerant at the same operating condition as the "oil impact factor". When holding the oil concentration constant, the oil impact factor increases as the mass flow rate decreases and the channel hydraulic diameter increases. Moreover, as the refrigerant vapor quality decrease, the oil impact factor also exhibits an upward trend. This study provides optimization insights for the design and development of condensers used in space refrigeration/heat pump systems.