Comparison of the mixed flow and heat transfer characteristics in the evaporator of a vapor compression heat pump in normal gravity and microgravity

Abstract

As a thermal control system for space applications, the vapor compression heat pump is crucial for the future development of aerospace technology. The evaporator in the heat pump is significantly affected by the space environment. Lubricant is present in the system during the operation, and the gas-liquid separation is difficult to achieve in a microgravity environment. The oil-containing refrigerant may form an oil film on the evaporator wall, and deposition may occur. A mixed flow and heat transfer model of the refrigerant and lubricating oil in the evaporator of the vapor compression heat pump was established to investigate the gas-liquid separation and lubricating oil deposition of the vapor compression heat pump in microgravity. The heat flux effects on the flow and heat transfer characteristics were simulated in normal gravity and microgravity using FLUENT. The simulation and experimental results of the evaporator outlet temperature in normal gravity were compared, and the simulation results of the heat transfer coefficient of the refrigerant R134a were compared with experimental data obtained from the literature. The results demonstrate that the velocity field exhibits a large gradient and an asymmetric distribution in normal gravity and an asymmetric distribution with lower values in the lower and upper right positions in microgravity. The heat transfer performance of the evaporator is lower in microgravity than in normal gravity for the same heat flux. For the same heat flux (100000 W/m2), the vapor phase volume ratio of the evaporator outlet is 0.95-1 in normal gravity and 0.6-0.7 in microgravity. After the phase change of the refrigerant and lubricant mixture in the evaporator, the lubricant deposition is not affected by the heat flux in normal gravity and microgravity, and normal oil return occurs. A maximum deviation of 14.9% is observed between the simulated and experimental values of the evaporator outlet temperature in normal gravity. The heat transfer coefficient of the R134a obtained from the simulation is within the range of the experimental values in the literature. This work expands our understanding of microgravity two-phase flow heat transfer and provides theoretical guidance for the development of the evaporator in vapor compression heat pumps for aerospace applications.