Experimental investigation of evolution characteristics tip flow instability in a centrifugal compressor with a vaneless diffuser

Flow stability confines the stable operating range of compressors, which is an essential issue in developing advanced turbochargers with high-pressure ratios. The present study investigated the spatial-temporal evolution characteristics of the tip flow instability in a centrifugal compressor with a volute and a vaneless diffuser. The signatures of the tip flow disturbances in the time and frequency domains and the instantaneous discrete flow disturbances at various streamwise positions during the evolution process were experimentally analyzed. Three tip flow instabilities of various time and spatial scales are identified, including rotating instability, impeller stall, and diffuser stall. The investigated speed lines are divided into two parts by the instability evolution characteristics. At low speeds, the first onset of RI occurred at the impeller leading edge and moves upstream with decreasing flow rate, which eventually induces the impeller stall. Moreover, the simultaneous coexistence of RI near the main blade leading edge and impeller stall near the splitter blade leading edge is observed before the whole impeller stall. RI maintains its local pattern in the front impeller passage while the whole impeller stall appears. At high speeds, the compressor operates at a relatively narrow stable flow range and successively experiences diffuser stall, impeller rotating instability, and surge. The experimental investigations reveal that flow instabilities with various length scales can coexist at the near-surge points.