Effects of Variable-Speed Operation on the Performance and Loss Characteristics of an Industrial Centrifugal Compressor

Abstract

In this study, the effects of variable-speed operation on the performance and internal flow of an industrial centrifugal compressor were investigated using computational fluid dynamics. Simulations were conducted at 100%, 90%, and 80% of the design rotational speed under identical inlet total conditions. As the rotational speed decreased, the pressure ratio and maximum flow rate decreased, and the best efficiency point shifted toward a lower flow rate. The non-dimensional head coefficient and efficiency curves did not fully collapse at different rotational speeds. At an identical inlet flow coefficient, the radial velocity at the impeller exit remained similar, whereas the circumferential velocity and blade speed decreased. Consequently, the absolute flow angle and the magnitude of the relative flow angle decreased, increasing the diffuser incidence. The increased incidence enlarged the low-velocity and high-loss regions near the diffuser suction surface and hub. The main flow was redistributed toward the upper-span region, causing a local reversal of the spanwise loss distribution. The impeller loss coefficient varied only slightly, whereas the diffuser loss coefficient increased significantly. The discrepancy between the non-dimensional performance curves was therefore mainly attributed to increased diffuser losses induced by changes in the impeller exit flow.