UNSTEADY PRE-STALL BEHAVIOR IN A CENTRIFUGAL COMPRESSOR WITH VANED DIFFUSER

Unsteady pre-stall behavior in a centrifugal compressor with a vaned diffuser was investigated by experimental and numerical analysis. The pre-stall disturbances occurred at a slightly higher flow coefficient at the stall point in the diffuser region. Five disturbances occurred in the circumferential direction, and each rotated at approximately 1.7%N at this flow coefficient. Numerical analysis showed that five stall cells rotated at approximately 2.0%N within the diffuser passage. To understand this pre-stall phenomenon, we focused on the rotation mechanism and initiation process of the five-cell rotating stalls. Each of the five-cell stalls was found to rotate by the following mechanism. When the preceding low-velocity region moved to an adjacent passage, the high-velocity region was circumferentially pushed by the low-velocity area and reached the following passage. The incoming flow collided with the back flow around the throat area, and the flow bent at the diffuser inlet of the passage. Consequently, the incidence angle toward the adjacent passage increased, and a separation was induced at the leading edge of the succeeding diffuser vane. Subsequently, the mass flow rate of the succeeding passage started to decrease. These phenomena occurred sequentially, causing the five-cell stalls to rotate. Five stationary low-velocity regions that did not rotate were observed before the initiation of the five-cell rotating stalls. When the outlet mass flow rate decreased, a one-cell rotating stall appeared within the diffuser passage. It provided a low-energy fluid to the diffuser passages where the low-velocity regions existed. Subsequently, five low-velocity regions were clearly formed, which started rotating according to the rotating mechanism explained above.