A Theoretical Study of Fluid Forces on a Centrifugal Impeller Rotating and Whirling in a Vaned Diffuser

Abstract

The title problem is treated on the assumption that the numbers of impeller and diffuser vanes are so large that the flows are perfectly guided by the vanes. It is shown that the interaction with the vaned diffuser may cause destabilizing fluid forces for whirling motion even at the design flow rate. Discussions are made to elucidate the cause of the destabilizing fluid forces. The losses in the impeller and the diffuser are assumed to be given by the quasi-steady application of steady incidence and hydraulic losses. Based on this assumption, the effects of rotating stalls due to the impeller and the diffuser on the impeller whirling forces are discussed. The impeller forces are divided into components due to impeller inlet/outlet pressure distributions and momentum transfers and to the rate of momentum change of the fluid in the impeller. It is shown that the pressure forces around the impeller outlet dominate in a range of small positive to negative whirl velocity ratios.