Numerical study on the effect of intermediate pressure pulsation on COP in the vapor injection cycle

Abstract

The vapor injection cycle with flash tank and two stage compressor is known as an efficient cycle. It is known that the intermediate pressure is a key factor for COP improvement and pressure change during the injection process could reduce the COP. However, the effect of the pulsating intermediate pressure of a two-stage compressor on COP is not clear. In a previous study, the authors experimentally showed that COP varied with injection pipe length. There seemed to be a correlation between COP and the resonance phenomenon caused by pressure pulsation in the injection pipe. However, the mechanism of COP improvement is not well understood. In this study, a one-dimensional simulation model was developed to clarify the mechanism and was verified with the experimental results. The simulation results show that resonance in the injection pipe does not increase the injection mass flow rate into the suction pipe. On the contrary, backflow occurs in the injection pipe and the backflow ratio is minimum in the resonant condition. Due to the high temperature of the backflow coming from the discharge pipe, the saturated vapor from the flash tank is easily heated, causing heat loss and lower COP. It was found that COP can be improved by using resonance to suppress the backflow.