Flow-Induced Instabilities and Rotordynamic Forces in Rocket Engine Inducers and Turbopumps

After a brief review of flow-induced instabilities in turbopumps for liquid propellant feed systems of modern rocket engines, some recent results of the work carried out by the author and his collaborators to elucidate the role of cavitation on the development of unsteady flow phenomena and rotordynamic impeller forces in these machines are illustrated. Specifically, the results are presented of a series of tests conducted in water in the Cavitating Pump Rotordynamic Test Facility (CPRTF) under similarity conditions on a three-bladed, tapered-hub, high-head inducer. The flow instabilities detected in these machines include axial cavitation surge oscillations, backflow oscillations, non-synchronous and synchronous (alternating blade) rotating cavitation, higher-order surge and higher-order rotating cavitation, depending on the operational conditions of the machine. The occurrence of these instabilities under the relevant operational conditions is critically discussed. The characterization of the rotordynamic forces acting on the whirling impellers of the above machines under different load and cavitation conditions is also illustrated and discussed. The results have been obtained in the CPRTF by means of a novel experimental technique that allows for the continuous measurement of the rotordynamic force spectra as functions of the whirl ratio. Comparison with simultaneous high-speed movies of the inducer inlet flow highlights the relationship between the cavitation dynamics in the inducer backflow and the spectral behavior of the rotordynamic force as functions of the whirl ratio.