Model for Local Cavitation Instabilities in 2-Bladed Turbopump Inducers

Abstract Various local cavitation instabilities in turbopump inducers (e.g., alternate blade cavitation, super-synchronous rotating cavitation, etc.) have previously been experimentally identified. However, a model, which 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 post-onset behavior of alternate blade cavitation instabilities. The model predicts alternate blade cavitation to occur. As the flow coefficient is decreased, the model predicts the magnitude of alternate blade cavitation to first increase and then decrease, and then finally vanish. If the flow coefficient is further decreased beyond the range of alternate blade cavitation, 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 above post-onset behaviors of alternate blade cavitations 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.