Thermodynamic Effect on Rotating Cavitation in an Inducer

Abstract

Rotating cavitation in inducers is known as one type of cavitation instability, in which an uneven cavity pattern propagates in the same direction as the rotor with a propagating speed ratio of 1.0–1.2. This rotating cavitation causes shaft vibration due to the increase of the unsteady lateral load on the inducer. On the other hand, cavitation in cryogenic fluids has a thermodynamic effect because of the thermal imbalance around the cavity. It improves cavitation performances due to the delay of cavity growth. However, the relationship between the thermodynamic effect and cavitation instabilities is still unknown. To investigate the influence of the thermodynamic effect on rotating cavitation, we conducted experiments in which liquid nitrogen was set at different temperatures (74 K, 78 K and 83 K) with a focus on the cavity length. At higher cavitation numbers, super-synchronous rotating cavitation (Super-SRC) occurred at the critical cavity length of Lc/h ≅ 0.5 with a weak thermodynamic effect in terms of the fluctuation of cavity length. In contrast, synchronous rotating cavitation (SRC) occurred at the critical cavity length of Lc/h ≅ 0.9–1.0 at lower cavitation numbers. The critical cavitation number shifted to a lower level due to the suppression of cavity growth by the thermodynamic effect, which appeared significantly with rising liquid temperature. The unevenness of cavity length under synchronous rotating cavitation was decreased by the thermodynamic effect. Furthermore, we confirmed that the fluid force acting on the inducer notably increased under conditions of rotating cavitation, but that the amplitude of the shaft vibration depended on the degree of the unevenness of the cavity length through the thermodynamic effect.