Unsteady flow mechanism of spike emergence in a subsonic axial flow compressor rotor

Pre-stall behavior in a subsonic axial compressor rotor has been investigated by multi-passage three-dimensional time-accurate numerical simulations. The propagation speed of high pressure spot on flow annulus ahead of the rotor and its associated circumferential and radial influencing area agree well with the available experimental observations. This demonstrates that the current method captured the flow mechanism of spike emergence reasonably. The dominant feature of tip flow fields during the transition process is the formation of tip separation vortex and its propagation at different tip-passages. Should the strength of tip separation vortex in some passage exceed some critical value, its rear end will stand on the suction side of the blade. The tip clearance backflow from the adjacent passage spilled into the next passage as the rear end of tip separation vortex approached the trailing edge. This phenomenon can be considered as the threshold event of spike emergence for the test rotor.