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In this study, the performance and the internal flow of one stage model centrifugal pump with both vaned and vaneless diffuser were investigated. To measure the internal flow of the diffuser and the impeller easily, air was used in this pump test. As a result of measuring pressure fluctuation, the rotating stall was observed in the vaned and vaneless diffuser. We clarified the generating mechanism and characteristics of the rotating stall in the diffuser and the difference between the unsteady flow fields in both diffusers. In case of the vaned diffuser, the number of rotating stall cells were 4 in the diffuser and the cell propagation speed ratio was about 5 percent of the impeller rotating speed. On the other hand, in case of the vaneless diffuser, the cell number was 2, and the propagation speed ratio was about 10 percent of that. These phenomena in both diffuser pumps were simulated by unsteady 2D and 3D CFD computations. By using these computations, the vortex at the trailing edge of the diffuser vanes blocked the flow and induced separate flow at the leading edge resulted in the rotating stall. This research indicated that these vortices induced the total pressure loss increasing. Also, the rotating stall was found not only in the diffuser but also in the impeller by the flow simulation of the vaneless diffuser. And it was confirmed that the vortices at the impeller trailing edge and leading edge in the stall cells cause the total pressure loss.

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