Model for Local Cavitation Instabilities in Two-Bladed Turbopump Inducers

Abstract

Various local cavitation instabilities in turbopump inducers (e.g., alternate blade cavitation, supersynchronous rotating cavitation) have been previously experimentally identified. However, a model that is capable of predicting such local instabilities is still lacking. Therefore, a new model, which models an N-bladed inducer as a system of N-coupled mass-spring-damper systems, is presented to predict postonset behavior of alternate blade cavitation instabilities. The model predicts that alternate blade cavitation will occur. As the flow coefficient is decreased, the model predicts that the magnitude of alternate blade cavitation first increases and then decreases and then finally vanishes. If the flow coefficient is further decreased beyond the range of alternate blade cavitation, a sudden increase in equilibrium incidence is predicted. All of these predictions agree with the previous experimental findings. Furthermore, simultaneous in-phase oscillations of the blade cavities are predicted during alternate blade cavitation. These in-phase oscillations of the cavities correspond to the surge mode oscillation, which had been previously mistakenly categorized as a global (system) instability. Finally, it is proposed that the aforementioned postonset behaviors of alternate blade cavitation are general in two-bladed inducers, as long as the change in incidence on the following blade has a local minimum when it is expressed as a function of leading blade's incidence.