Stall Mode Transformation in the Wide Vaneless Diffuser of Centrifugal Compressors

A review on the rotating stall in the vaneless diffuser of centrifugal compressors is presented showing that different stall modes characterized by different numbers of cells can be detected within the diffuser even if the operating condition remains unchanged. The interaction between the inlet perturbation and the stall cells near the diffuser outlet is supposed to be the trigger of the stall mode transformation. In order to determine if the inlet perturbation will interact with the downstream stall cells, a characteristic time analysis is proposed to estimate the characteristic time of the perturbation in radial and tangential directions. An additional theoretical model which focused on the development of the vaneless diffuser rotating stall is presented to determine the propagation velocity of the cells. The comparison between the characteristic time in two directions shows that one stall mode is able to evolve into another stall mode if a critical condition is met, and the stall mode transformation is more likely to start from a mode with a higher number of cells and is more likely to occur in the diffuser with a large radius ratio. Experimental results are also employed to validate the proposed critical condition, and good agreements are obtained.