The rotordynamic fluid forces on a closed type centrifugal impeller in whirling motion were measured with a vaneless diffuser at various flow rates for three types of casing, with the casing/impeller shroud clearance of 2[mm], 5[mm], and 5[mm] with seven radial grooves to break the flow swirl. The rotordynamic fluid forces are destabilizing within small positive whirl speed ratio Ω/ω, where Ω and ω are a whirling speed and a rotational speed of the shaft, respectively. The radial grooves were effective to reduce the fluid forces and the destabilizing region due to the reduction of the circumferential velocity of the clearance flow. At low flow rates, the fluid forces increased at Ω/ω, ≈ 0.7. By the measurement of unsteady pressure on the casing wall and the CFD analysis of flow field in the pump, it was found that the large fluid forces at Ω/ω ≈ 0.7 were caused by the rotating stall of the impeller. The rotating stall was caused by the backflow along the suction surface of the main blade due to the separation vortices near the middle of the chord.

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