Unsteady Flow due to Cavitation in Mixed Flow Pumps Second Report: The Effect of Tip Vortex Cavitation on Hydraulic Force

Abstract

Large vertical mixed flow pump is often used as the sea water supply pump in MSF (Multi-stage Flash) seawater desalination plant. From the point of view of cost reduction, the smaller pump size is strongly required for this type of pump. To satisfy this requirement, higher suction performance should be achieved but there are several issues such as excessive noise, vibration and erosion caused by cavitation in high velocity flow field [1]. Therefore, the present work is aimed at establishing the design technique for suppressing the cavitation which occurs in high velocity field in mixed flow pump. As reported in the previous paper [2], we experienced irregular excessive vibration due to cavitation in vertical mixed flow pump. Although this issue had been solved by applying forward swept impeller [3], the relation between the vibration and tip vortex cavitation was still unclear. This paper describes the investigation results about the relation between unsteady hydraulic exciting force and tip leakage vortex cavitation by CFD (Computational Fluid Dynamics). In the CFD analysis, the tip clearance was considered and full cavitation model was used to evaluate the effect of tip vortex cavitation on hydraulic exciting force [4]. From the CFD analysis, the vortex core induced from leading edge of impeller blade was detected and the static pressure and vorticity distributions on vortex center were obtained. From these results, there was no significant difference of these values between no-swept and forward swept impellers. Therefore, it implies that sweep technology does not affect to the strength of vortex. On the other hand, the unsteady behavior of pressure and cavitation area on the impeller blade was observed. Accordingly it is supposed that this fluctuation induced excessive noise, vibration and erosion.