This paper describes in detailed flow field in a centrifugal compressor with a vaned diffuser at off design point. Especially, we conducted both the experimental and numerical analysis in order to investigate the evolution process of a diffuser stall. At the stall point, the diffuser stall was initiated and rotated near the shroud side in the vaneless space. Furthermore, the diffuser stall was developed to a stage stall cell, as the mass flow was decreased. The developed stall cell was rotated within both the impeller and diffuser passages. The evolution process of the diffuser stall had three stall forms. First, the diffuser stall was rotating near the shroud side. Then, the diffuser stall shifted to the hub side and moved into the impeller passages. Finally, a stage stall was generated. From computational fluid dynamics (CFD) analysis, a tornado-type vortex was generated first, near the hub side of the diffuser leading edge, when the diffuser stall was shifted to the hub side. Next, a throat area blockage was formed near the hub side because of the boundary layer separation in the vaneless space. Finally, the blockage within the diffuser passages expanded to the impeller passages and developed into a stage stall. From the pressure measurements along the impeller and diffuser passages, the magnitude of pressure fluctuation on the casing wall of the diffuser throat area also suddenly increased when the diffuser stall shifted to the hub side. Therefore, the evolution area of the diffuser stall was caused by the evolution of the blockage near the throat area of the diffuser passage.
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