The unsteady flows in three sections of the volute with rectangular section were mapped by using LDV at both best efficiency and part-loading points respectively while pumping water. The pump is a single-stage, cantilevered centrifugal pump, its impeller is in a closed configuration and the blade outlet angle is 44°. The results show that the fluid velocity fluctuates periodically with the impeller angular positions with respect to a measurement point where the flow was investigated. As increase in the distance from impeller outlet, the velocity damps rapidly and its periodical fluctuation is suppressed. The magnitude of the fluctuation is about 0.2 - 0.7 of the averaged velocity and decreases with the increase in the distance. The fluctuation magnitude in the flow angle seems larger than that in velocity by 1 - 2 order. The more the measurement point closes to the tongue of volute, the larger the fluctuation in both velocity and flow angle becomes. The averaged velocity declines with increase in the distance from the impeller outlet. However, the averaged flow angle rises at first, then decreases. The liquid angular moment is not conservational and the flow is not axisymmetrical as well in the volute.
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