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Multifrequency Instability of Cavitating Inducers

Brennen, Christopher E. (2007) Multifrequency Instability of Cavitating Inducers. Journal of Fluids Engineering, 129 (6). pp. 731-736. ISSN 0098-2202. http://resolver.caltech.edu/CaltechAUTHORS:20091019-152528509

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Abstract

Recent testing of high-speed cavitating turbopump inducers has revealed the existence of more complex instabilities than the previously recognized cavitating surge and rotating cavitation. This paper explores one such instability that 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 that 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 rotating as well as surge components at all these frequencies. In addition, these interactions between the frequencies and the surge and rotating 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 surge mode with a frequency around 0.9 Ω and a low-frequency rotating mode about 0.1 Ω. 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.


Item Type:Article
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URLURL TypeDescription
http://dx.doi.org/10.1115/1.2734238 DOIUNSPECIFIED
Additional Information:© 2007 American Society of Mechanical Engineers. Contributed by the Fluids Engineering Division of ASME for publication in the Journal of Fluids Engineering. Manuscript received August 28, 2006; final manuscript received December 21, 2006. Review conducted by Akira Goto. The author wishes to acknowledge the comments of Sheldon Rubin [9], 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 No. NAG8-1934.
Funders:
Funding AgencyGrant Number
NASANAG8-1934
Subject Keywords: cavitation, pumps, turbomachinery, rotational flow, flow instability
Classification Code:# 89.20.Kk Engineering # 47.55.dp Cavitation and boiling # 47.20.-k Hydrodynamic stability # 47.32.Ef Rotating and swirling flows
Record Number:CaltechAUTHORS:20091019-152528509
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20091019-152528509
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:16388
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:21 Oct 2009 15:40
Last Modified:26 Dec 2012 11:29

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