Numerical research on refrigerant injection for motor cooling of screw compressor for heat pump with large temperature lift

Abstract

In the screw compressor for heat pump with large temperature lift, the large load on the motor makes the motor heat up severely. The stability and safety of the motor directly affects the operation of the compressor. High motor temperatures or strong impact on winding will destroy the insulating coating on the copper wires of the motor winding, which could cause the motor to burn out. Refrigerant injection for motor cooling is an effective way of motor cooling. There is a lack of research on this method. In this paper, a computational fluid dynamics model containing evaporation–condensation and conjugate heat transfer is developed. In order to reasonably simulate the evaporation–condensation process, the physical parameters varying with temperature of the refrigerant are taken into account. The temperature field of the screw compressor by refrigerant injection for motor cooling is predicted, and the accuracy of the model is verified through experiments of an R134a heat pump with large temperature lift. The maximum discrepancy between the simulation results and the experimental results under four working conditions does not exceed 3.0%. Next, the effects of the injection pressure and the injection position on motor cooling, impact caused by refrigerant injection and temperature uniformity are investigated. The results show that by adjusting the injection pressure to make the refrigerant two-phase mixture, the impact caused by refrigerant injection is reduced by 57.0%, while the cooling effect is reduced by only 9.0%. By choosing the injection position appropriately, the cooling effect is increased by 12.0% and the impact caused by refrigerant injection is reduced by 21.2%. In addition, the temperature in the middle of the motor is the highest before refrigerant injection, so the motor temperature sensor should be arranged in the middle of the motor winding.