Effect of inlet temperature on flow behavior and performance characteristics of supercritical carbon dioxide compressor

Centrifugal compressors have been widely applied in supercritical carbon dioxide (SCO₂) Brayton cycle because of its compactness and low power consumption. However, the dramatic change in supercritical carbon dioxide near the critical point (304.13 K, 7.38 MPa) under different inlet temperatures brings challenges to compressor operation, especially with asymmetric boundary conditions. In this study, full annular calculations of a centrifugal compressor with a volute are conducted by imposing different inlet total temperatures. The sensitivity of the performance characteristics of each component to the inlet temperature was obtained. Two-phase region, dominant flow structure in the impeller, and downstream flow field structure distortion caused by inlet temperatures were revealed. The results showed that when the inlet condition was close to the critical point of supercritical carbon dioxide, the size of the two-phase region in the impeller increased and its circumferential nonuniformity was intensified. At 309 K, the two-phase region caused an enthalpy rise fluctuation at the blade tip and a difference in enthalpy at the blade outlet. Moreover, with the decrease in inlet temperature, the size and range of the counterclockwise vortex gradually increased, leading to the transportation of low-momentum fluid from the pressure side to the suction side of the blade. In addition, the wake at the impeller outlet was accumulated and the impeller discharge flow deteriorated, resulting in local flow separation in the vaneless diffuser.