Characterization of the Rotordynamic Forces on Tapered Axial Inducers by Means of a Rotating Dynamometer and High-Speed Movies

The paper presents the main results of an experimental activity conducted in the Cavitating Pump Rotordynamic Test Facility at ALTA S.p.A. for the characterization of the rotordynamic forces acting on a whirling four-bladed, tapered-hub, variable-pitch inducer, designated as DAPAMITO4. The results obtained by a novel experimental technique, consisting in measuring the continuous spectra of the forces as functions of the whirl ratio, have been compared to high-speed movies taken under forced whirl conditions, in order to get a deeper insight of the role of the structure and oscillating behavior of the cavitating region on the blades. More in detail, a clear relationship between the cavitation structure and the destabilizing peak of the rotordynamic force observed at a positive value of the whirl ratio has been found: the peak appears to be associated to the reorganization of the cavitating regions into a single well defined structure synchronously rotating with the whirl mo...