Impact of Inlet Conditions on Performance of a Supercritical CO2 Centrifugal Compressor

The centrifugal compressor is one of the core components in a supercritical carbon dioxide (SCO2) Brayton cycle, in which the inlet state is near the critical point of CO2 to reduce the compression work. However, the thermodynamic properties of CO2 change dramatically near the critical point. Therefore, the performance and flow in a SCO2 compressor will be sensitive to the variation of inlet conditions. In this paper, a centrifugal compressor stage designed by the in-house code for a 20MW supercritical CO2 Brayton cycle is numerically investigated with the variation of considered inlet state point. Firstly, to take account of the nonlinear variation of the properties near the critical point, the accuracy of look-up table of CO2 properties is evaluated and implemented into the commercial CFD software. Then, the effect of small change in inlet temperature or pressure near the critical point on the compression process and performance is investigated, and the flow fields in compressor with different inlet states are compared. It is shown that the CFD-predicted compressor performance is highly sensitive to the resolution of look-up table as well as the inlet conditions. Slight increase in inlet temperature or inlet pressure will cause a significant decrease in pressure ratio. The impact of small change of inlet state on the flow structures are mainly concentrated on the first half of impeller blade passage.