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Observations on Instabilities of Cavitating Inducers

Braisted, David and Brennen, Christopher (1978) Observations on Instabilities of Cavitating Inducers. In: Cavitation and Polyphase Flow Forum, 1978 : presented at Joint Symposium on the Design and Operation of Fluid Machinery, June 12-14, 1978, Fort Collins, Colorado [which was] sponsored by the [ASME, IAHR, & ASCE]. American Society of Mechanical Engineers , New York, NY, USA, pp. 19-22.

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Hydraulic systems involving cavitating turbomachines are known to be susceptible to instabilities at certain critical operating conditions. Two distinct classes of cavitating inducer instabilities have been reported in the literature (Refs. 1-6). The purpose of this note is to report on some preliminary observations of these phenomena. The experiments were performed in the Dynamic Pump Test Facility (DPTF) at the California Institute of Technology (Refs. 7, 8). Results will be presented for two different inducers operating at different flow coefficients, [symbol] ([symbol]= mean axial velocity/inducer tip velocity- [equation]) and cavitation numbers, [symbol] ([symbol]=[equation]; where [equation] are the inlet and vapor pressures, and [symbol] is the liquid density). In general, the instabilities occurred just before the head breakdown. After head breakdown, the system tended to become stable again, although there were some indications of a second region of instability at very small cavitation numbers. Impeller IV is a quarter scale model of the Low Pressure Oxidizer Turbo-Pump (LPOTP) of the space shuttle main engine (Refs. 7, 8). The cavitation performance of this impeller is presented in Figure 1. Some of the mean operating states for which large, constant amplitude oscillations occurred in all the pressures and mass flow rates are indicated by stars. The cavitation in each of the blade passages oscillated in unison. This unstable behavior is termed auto-oscillation. The frequency of the auto-oscillations ranged from 28 to 35 Hz. As might be expected, there does exist a marginal region of operation for which the auto-oscillations have a time varying amplitude. These non-steady oscillations occurred as sporadic bursts of auto-oscillation. It was this feature that makes the boundaries of the auto-oscillation region difficult to define. In addition to the auto-oscillation observations on Impeller IV, two instances of "rotating cavitation" were observed and are labeled by boxes in Figure 1. The presence of rotating cavitation was determined by means of a stroboscope slaved to the rotational speed of the inducer. Rotating cavitation appeared as a non-stationary cavitation patterns which rotated with respect to the "fixed" inducer. (More recent testing has also revealed the existence of a stationary form of rotating cavitation sometime referred to as alternate blade cavitation.) The large amplitude disturbances in the upstream pressure and mass flow rates which characterized auto-oscillation were not observed during rotating cavitation. This suggests the rotating cavitation is most intimately associated with the dynamic characteristics of the cavitating inducer itself irrespective of the hydraulic system in which it resides.

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Additional Information:This investigation is part of an ongoing research program funded under NASA Contract NAS 8-29313. The authors are grateful to the George Marshall Space Flight Center, Huntsville, Alabama for this support. The junior member of this partnership is also grateful to NSF which has supported his research activities at Caltech by means of a Graduate Fellowship.
Funding AgencyGrant Number
NASANAS 8-29313
NSF Graduate Research FellowshipUNSPECIFIED
Record Number:CaltechAUTHORS:BRAcpff78
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:181
Deposited By: Christopher Brennen
Deposited On:11 Nov 2004
Last Modified:02 Oct 2019 22:31

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