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A Three-Dimensional Analysis of Rotordynamic Forces on Whirling and Cavitating Helical Inducers

d'Agostino, Luca and d'Auria, Fabrizio and Brennen, Christopher E. (1998) A Three-Dimensional Analysis of Rotordynamic Forces on Whirling and Cavitating Helical Inducers. Journal of Fluids Engineering, 120 (6). pp. 698-704. ISSN 0098-2202. http://resolver.caltech.edu/CaltechAUTHORS:DAGjfe98

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Abstract

This paper investigates the linearized dynamics of three-dimensional bubbly cavitating flows in helical inducers. The purpose is to understand the impact of the bubble response on the radial and tangential rotordynamic forces exerted by the fluid on the rotor and stator stages of whirling turbomachines under cavitating conditions. The flow in the inducer annulus is modeled as a homogeneous inviscid mixture, containing vapor bubbles with a small amount of noncondensable gas. The effects of several contributions to the damping of the bubbly dynamics are included in the model. The governing equations of the inducer flow are written in "body-fitted" orthonormal helical Lagrangian coordinates, linearized for small-amplitude perturbations about the mean flow, and solved by modal decomposition. The whirl excitation generates finite-speed propagation and resonance phenomena in the two-phase flow within the inducer. These, in turn, lead to a complex dependence of the lateral rotordynamic fluid forces on the excitation frequency, the void fraction, the average size of the cavitation bubbles, and the turbopump operating conditions (including, rotational speed, geometry, flow coefficient and cavitation number). Under cavitating conditions the dynamic response of the bubbles induces major deviations from the noncavitating flow solutions, especially when the noncondensable gas content of the bubbles is small and thermal effects on the bubble dynamics are negligible. Then, the quadratic dependence of rotordynamic fluid forces on the whirl speed, typical of cavitation-free operation, is replaced by a more complex behavior characterized by the presence of different regimes where, depending on the whirl frequency, the fluid forces have either a stabilizing or a destabilizing effect on the inducer motion. Results are presented to illustrate the influence of the relevant flow parameters.


Item Type:Article
Additional Information:Contributed by the Fluids Engineering Division for publication in the Journal of Fluids Engineering. Manuscript received by the Fluids Engineering Division January 21, 1998; revised manuscript received May 5, 1998. This research project was carried out on internal funds by CentroSpazio, Ospedaletto (Pisa) Italy, as well as under a European Space Agency External Post-Doctola1 Fellowship. Financial support was also provided by a grant from the Foundation Blanceflor Boncompagni-Ludovisi, fodd Bildt. The authors would like to express their gratitude to Mr. Riccardo Marsili, undergraduate student at Pisa university, for carrying out most of the computations, to Prof. Mariano Andrenucci, Director of Centrospazio, and Prof. Renzo Lazzeretti of the Aerospace Engineering Department, University of Pisa, Pisa, Italy, for their friendly encouragement in the completion of the present work.
Record Number:CaltechAUTHORS:DAGjfe98
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:DAGjfe98
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:89
Collection:CaltechAUTHORS
Deposited By: Christopher Brennen
Deposited On:21 Sep 2004
Last Modified:26 Dec 2012 08:39

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