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A multifrequency instability of cavitating inducer systems

Brennen, Christopher E. (2006) A multifrequency instability of cavitating inducer systems. In: Sixth International Symposium on Cavitation, CAV2006, 11-15 September 2006, Wageningen, The Netherlands. (Unpublished)

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Recent testing of high speed cavitating turbopump inducers has revealed the existance of more complex instabilities than the previously-recognized cavitating surge and rotating cavitation. This paper explores one such instability which is uncovered by considering the effect of a downstream asymmetry such as a volute on a rotating disturbance similar to (but not identical to) that which occurs in rotating cavitation. The analysis uncovers a new instability which may be of particular concern because it occurs at cavitation numbers well above those at which conventional surge and rotating cavitation occur. This means that it will not necessarily be avoided by the conventional strategy of maintaining a cavitation number well above the performance degradation level. The analysis considers a general surge component at an arbitrary frequency, ω, present in a pump rotating at frequency, Ω, and shows that the existence of a discharge asymmetry gives rise not only to beat components at frequencies, Ω − ω and Ω + ω (as well as higher harmonics) but also to circumferentially-varying components at all these frequencies. In addition, these interactions between the frequencies and the basic and complementary modes lead to “coupling impedances” that effect the dynamics of each of the basic frequencies. We evaluate these coupling impedances and show not only that they can be negative (and thus promote instability) but also are most negative for surge frequencies just a little below Ω. This implies potential for an instability involving the coupling of a basic mode with a frequency around 0.9Ω and a low frequency complementary mode about 0.1Ω. We also examine how such an instability would be manifest in unsteady pressure measurements at the inlet to and discharge from a cavitating pump and establish a “footprint” for the recognition of such an instability.

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Additional Information:The author wishes to acknowledge the comments of Sheldon Rubin who, many years ago, suggested that we should consider the dynamics inherent in the different, parallel flow paths within a pump and volute system. I also wish to thank Tom Zoladz and the NASA George Marshall Space Flight Center for their support under grant NAG8-1934.
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3874
Deposited By: Archive Administrator
Deposited On:17 Jul 2006
Last Modified:02 Oct 2019 23:07

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