Thermal Effects on Cavitation Instabilities in Helical Inducers

The present paper illusn·ates the main results of an experimental campaign conducted in the CPRTF (Cavitating Pump Rotordynamic Test Facility) at Alta S.p.A. The experiments have been carried out on two really different axial inducers (a 3-bladed aluminium-made inducer of extremely simple helical geometry and a prototype of the axial inducer of the Vulcain MK1 engine LOX turbopump) in order to characterize the instabilities affecting the pumps in 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 at the middle of the axial chord of the blades. For each axial station at least two transducers were mounted at a given angular spacing, in order to cross-correlate their signals for coherence and phase analysis. The most interesting instabilities have been detected on the 3-bladed inducer, including a 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 band, very few oscillating phenomena have been detected on the MKl inducer, with a practically flat frequency spectrum at flow coefficients near to its nominal operating point. A natural frequency of the facility has also been detected, having a value in good agreement with that evaluated using a simple linear lD model