Thermal Effects on Cavitation Instabilities in Helical Inducers"

The main results of an experimental campaign conducted in the Cavitating Pump Rotordynamic Test Facility at Alta S.p.A. are presented. The experiments have been carried out on two different axial inducers (a three-bladed aluminum-made inducer of simple helical geometry and a prototype of the axial inducer of the Vulcain MK1 engine liquid oxygen turbopump) to characterize the instabilities affecting the pump on a wide range of flow conditions: some experiments have also been carried out at higher temperatures to investigate the possible influence of the thermal cavitation effects on the observed phenomena. The transparent inlet section of the facility has been instrumented with several piezoelectric pressure transducers located at three axial stations: inducer inlet, outlet, and middle of the axial chord of the blades. For each axial station, at least two transducers were mounted at a given angular spacing to cross-correlate their signals for coherence and phase analysis. The most interesting instabilities have been detected on the three-bladed inducer, including cavitation surge, a rotating stall and an auto-oscillation leading to a violent surge-mode instability. Some of these instabilities have also been found to be slightly affected by temperature. On the other hand, very few oscillating phenomena have been detected on the MK1 Inducer, with a practically flat frequency spectrum at flow coefficients near its nominal operating point.