UNSTEADY FLOW MECHANISM OF IMPELLER STALL INCEPTION IN A HIGH-PRESSURE RATIO CENTRIFUGAL COMPRESSOR WITH BLEED SLOTS

The impeller stall inception in a high-pressure ratio centrifugal compressor with bleed slots was investigated by the experiments and unsteady numerical analysis. This study reported that the flow instability of the internal flow was first initiated around the leading edge of the splitter blades and finally resulted in tip-leakage flow instability at the leading edge of the main blades. The impeller stall consisted of a stall cell and rotated about 70% of the impeller rotational speed. Furthermore, a pressure disturbance occurs at the splitter blade leading edge before the main blade leading edge. The blockage within the passage of the suction side at the leading edge of the splitter blade was first increased because the leading-edge separation on the suction surface of the splitter blade was enlarged. The blockage within the passage of the pressure side was increased after that of the suction side stopped increasing. Therefore, the blockage development of both pressure and suction passage at the leading edge of the splitter blade was the main cause of the stall inception. Then, a longitudinal vortex was formed near the bleed slots and blocked the mainstream near tip side. Subsequently, the reverse flow region in the inducer was developed and extended to the impeller inlet. Finally, the flow angle to the main blade increased, the spillage occurred at a leading edge of a main blade, and the rotating stall was generated. The cause of impeller instability was the separation of the suction surface of splitter blade and the formation of a stagnation region on the pressure surface at the splitter leading edge.