Behavior of Vaneless Diffuser Stall in a Centrifugal Compressor

The rotating stall in a centrifugal compressor with a vaneless diffuser was investigated both experimentally and numerically with focus on the effect of the internal flow field within the impeller on the diffuser stall. Through numerical analysis, the boundary layer separation at the impeller outlet was found to play an important role in the expansion and rotation processes of the diffuser stall. In particular, the expanded boundary layer separation near the hub side at the outlet of the main blade (M.B.) suction surface passage was considered to be the main cause of the expansion and rotation processes. A longitudinal vortex existed at the throat of the M.B. passage, and the mass flow rate in the M.B. passage was significantly reduced by the blockage effect. In addition, the longitudinal vortex induced the rolling up flow near the hub side at the impeller exit. Thus, the boundary layer separation expanded.