Performance Improvement of a Centrifugal Compressor Stage by Increasing Degree of Reaction and Optimizing Blade Loading of a 3D-Impeller

Performance improvement of 3D impellers in a high specific speed range was investigated using computational fluid dynamics (CFD) analyses and experimental tests. In order to reduce the loss production within the stator passages, the backsweep angle of the impellers was increased. At the same time, the inlet-to-exit relative velocity diffusion ratio was also increased by increasing impeller exit width to prevent the reduction in pressure ratio. Moreover, the blade loading distribution at the impeller shroud side was optimized to suppress the surge margin reduction caused by the increased relative velocity diffusion ratio. Five types of unshrouded impellers were designed, manufactured and tested to evaluate the effects of blade loading, backsweep angle and relative velocity diffusion ratio on compressor performance. The design suction flow coefficient was 0.125 and the machine Mach number was 0.87. Test results showed that compressor stage efficiency was increased by 5% compared to the base design without reducing the pressure coefficient and surge margin. It was concluded that an increased relative velocity diffusion ratio coupled with large backsweep angle was a very effective way to improve compressor stage efficiency. An appropriate blade loading distribution was also important in order to achieve a wide operating range as well as high efficiency.