Experimental Characterization of Cavitation Instabilities in a Two-Bladed Axial Inducer

The present paper illustrates the main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility. The tests were carried out on the FAST2 inducer, a two-bladed axial pump designed and manufactured using the criteria followed for VINCI180 inducer. The transparent inlet section of the facility was instrumented by several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet and at the middle of the axial chord of the blades. For each axial station at least two transducers were mounted at selected angular spacing, in order to cross correlate their signals for coherence and phase analysis. The most interesting detected instabilities were: a cavitation auto-oscillation at about 5-12 Hz, a high-order cavitation surge at a frequency of about 4.4Ω and a rotating stall at about 0.31Q. Some experiments were carried out under forced vibration conditions: it was observed that a whirl frequency of about 1 Hz can provide excitation for violent surge-mode oscillations. A "cavitation surge" instability was also observed at higher whirl frequencies.