INTERACTION MECHANISM OF IMPELLER AND DIFFUSER STALL IN A CENTRIFUGAL COMPRESSOR

The interaction mechanism of impeller and diffuser stall in a centrifugal compressor with a vaneless diffuser was investigated by experimental and computational analyses. This study focuses on the effect of impeller stall on the diffuser stall behavior. Impeller stall rotated at 58% of the impeller rotational speed was generated inside the impeller. Two-cell diffuser stalls (with each of the cells rotating at 25%-30% of the impeller rotational speed) were generated inside the diffuser. The diffuser stall fluctuations were observed at 180 degrees from the cutoff. The magnitudes of the diffuser stall fluctuations gradually increased near the volute tongue. The diffuser stall fluctuations were generated near both the shroud and hub sides. Finally, the diffuser stall cell vanishes when it passes the cutoff because mass flow recovery occurs. The numerical results revealed that boundary layer separation occurred near the hub side at 45 degrees-90 degrees of the diffuser because of the circumferential adverse pressure gradient. Subsequently, the low-velocity region discharged from the impeller caused by impeller stall merged into the boundary layer separation, which was generated near the hub side at 45 degrees-90 degrees. Diffuser stall was initiated by the hub-side boundary layer separations, which were caused by the impeller stall. The diffuser stall cell was then further developed by the boundary layer separation accumulation and the induced low-velocity area. The boundary separation was further developed by merging the wake from the impeller stall passage.