Bubble Dynamics Effects on the Rotordynamic Forces in Cavitating Inducers

Abstract

The present work investigates the dynamics of idealized bubbly and cavitating flows in whirling helical inducers, with the purpose of understanding the impact of the bubble response on the rotordynamic forces exerted by the fluid on the turbomachine under cavitating conditions. Inertial, damping, and compressibility effects in the dynamics of the bubbles are included. The effect of the whirl excitation on the two-phase flow is dependent on the wave propagation speed and the bubble resonance behavior in the bubbly mixture. These, in turn, lead to rotordynamic forces which are complicated functions of the whirl frequency and depend on the void fraction of the bubbles and on the mean flow properties. Under cavitating conditions the dynamic response of the bubbles induces major deviations from the non-cavitating flow solutions. The quadratic dependence of rotordynamic fluid forces on the whirl speed, which is typical of cavitation-free operation is significantly modified. Results are presented to illustrate the influence of the various flow parameters.