Analysis of Cavitation Surge Considering the Acoustic Effect of the Inlet Line in a Rocket Engine Turbopump

Abstract

In a liquid rocket engine, cavitation in an inducer of a turbopump sometimes causes instability phenomena when the inducer is operated at low inlet pressure. Cavitation surge (auto-oscillation), one such instability phenomenon, has been discussed mainly based on an inertia model assuming incompressible flow. By using this model, the frequency of the cavitation surge decreases as the inlet pressure decreases. However, we obtained interesting experimental results in which the cavitation surge frequency varied disconnectedly. Therefore, we considered the factor of fluid compression employed one-dimensional analysis applying an acoustic model, combining the inlet pipe with the sonic velocity of liquid oxygen. Consequently, the analytical results qualitatively corresponded with the experimental results. In addition, an actual liquid rocket propulsion system is usually equipped with a Pogo suppression device (PSD), which is a kind of accumulator with a hydraulic compliance, upstream of a liquid oxidizer turbopump. We modified the analytical model to include the effect of this PSD and compared the analytical results with the experimental results. It was found that the frequency of cavitation surge basically became the Helmholtz frequency, defined by the cavitation compliance and the length of pipe between the PSD and the turbopump. And when the frequency of cavitation surge coincided with one of the acoustic resonance frequencies of the inlet pipe, the cavitation surge was strongly excited.