Investigation of Hydrodynamic Forces on Rotating and Whirling Centrifugal Pump Impellers

A two-dimensional fluid model is developed to investigate the hydrodynamic forces exerted on a rotating impeller caused by the impeller-fluid-volute interaction in a centrifugal pump. In this model, the impeller periphery and the volute contour are replaced by a distribution of unsteady vortices. The impeller center is assumed to execute a whirling motion about the rotor center. This is an improvement of the earlier quasi-steady flow model of Colding-Jo̸rengsen (1980) where the impeller was taken as a single vortex source point. The forces can be presented as a sum of a steady and an unsteady part. The rotordynamic coefficients are deduced from the unsteady forces decomposed into radial and tangential components relative to the orbit described by the impeller center. In comparison to most of the theoretical and experimental results found in the literature, the model seems to give good prediction. It appears clearly from this analysis that, under certain operating conditions, the fluid forces on the impeller have a destabilizing effect on the pump rotor.