Characterization of Cavitation Instabilities in a Four-Bladed Turbopump Inducer

Abstract

Rocket engine turbopump inducers can suffer from cavitation instabilities, dramatically limiting performance and life of the turbomachinery components. An unrestricted inducer geometry, representative of the Space Shuttle main engine low-pressure oxidizer pump inducer, is defined and serves as a platform for characterization of the cavitation dynamics. Experiments demonstrate rotating cavitation, alternate blade cavitation, and cavitation surge. Borrowing ideas from the modeling and assessment of rotating stall in aeroengine compressors, a previously established traveling wave energy analysis is applied. At flow coefficients higher than design, a single cell cavity occurs rotating at supersynchronous frequencies between 1.2 and 1.5 times rotor frequency. Optical measurements corroborate the findings and underlying mechanisms; the onset of rotating cavitation is governed by the interaction of the blade cavity with the leading edge of the adjacent blade. This leads to a change of incidence, which causes sheet cavity breakoff with periodic growth and collapse of cavities, yielding the apparent supersynchronous rotation of the cavities around the annulus. The paper breaks new ground in diagnostics of inducer dynamic behavior during rotating cavitation, alternate blade cavitation, and cavitation surge. The details of the unrestricted inducer geometry are provided for advancement of international research on rocket engine cavitation dynamics.