Rotordynamic Instabilities Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner

This paper addresses the rotordynamic instability of an overhung rotor caused by a hydrodynamic moment due to whirling motion through the structural coupling between whirl and precession modes. First, the possibility of instability is discussed based on a vibration model in which the hydrodynamic forces and moments are assumed to be smaller than structural forces with the structural coupling being represented by a structural influence factor. Then, the fundamental characteristics of rotordynamic moment on the backshroud of a Francis turbine runner under whirling motion were studied using model tests and numerical calculations. The runner is modeled by a disk positioned close to a casing with a small radial clearance at the outer periphery. The moment is caused by an inward leakage flow that is produced by an external pump in the model test. The experiments were designed to measure the rotordynamic fluid force moments under various leakage flow rates with various preswirl velocities and various axial clearances between the backshroud and casing. The computation was carried out based on a bulk flow model. It was found that the fluid force moment is generally destabilizing, except for a small region of positive whirling speed ratios.

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Comments on a Newly-Identified Destabilizing Rotordynamic Mechanism Arising in Vertical Hydraulic Turbines and the Back Shrouds of Centrifugal Impellers

Childs

Muhammed

2013

Journal of Engineering for Gas Turbines and Power

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In three 2010 papers, Tsujimoto et al. (2010, “Moment Whirl Due to Leakage Flow in the Back Shroud Clearance of a Rotor,” Int. J. Fluid Mach. Syst., 3(3), pp. 235–244), Song et al. (2010, “Rotordynamic Instability Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner,” Int. J. Fluid Mach. Syst., 3(1), pp. 76–79), and Song et al. (2010, “Rotordynamic Moment on the Backshroud of a Francis Turbine Runner Under Whirling Motion,” ASME J. Fluids Eng., 132, p. 071102) discussed and explained a novel destabilizing mechanism arising in both hydraulic turbines and the back surface of vertical pump impellers. The destabilizing mechanism can be explained via a reaction force-moment model that includes both the customary radial displacement vector of an impeller plus the pitch and yaw degrees of freedom. This coupling between radial displacements and tilt plus the coupling of the shaft support structure can create negative damping. In 1993, Verhoeven et al. (1993, “Rotor Instability of a Single Stage Centrifugal Pump, Supersynchronous Whirling at Almost Twice the Operating Speed, A Case History,” Proceedings of the 1st International Symposium on Pump Noise and Vibration, pp. 457–468) identified negative damping arising from U-shaped wearing-ring seals as causing a super-synchronous instability in a horizontal coke-crusher pump. However, several case studies have been presented of super-synchronously unstable pumps for which (until now) no explanation could be provided. Tsujimoto–Song started with a 2DOF model for a vertically suspended disk via a cantilevered shaft. They used an f = ma model for the lateral displacements of the disk and used flexibility coefficients to account for reaction forces and moments from the back shroud of the impeller. The present work starts with a 4DOF model that includes the disk's displacements and pitch and yaw degrees of freedom. The Guyan reduction is used to create two reduced 2DOF models: model A that retains the displacements and discards the rotations and model B that retains the rotations and discards the displacements. Model A produces a requirement for instability that is inconsistent with Tsujimoto–Song's experience and predictions. However, it is useful in predicting the reaction moments produced by a nominally planar precession of the impeller. The instability requirement of Model B is consistent with Tsujimoto's experience and predictions. A comparison of the predicted reaction moments of model A and Tsujimoto's reaction-moment data supports the instability predictions of model B (and Tsujimoto–Song) that the instability arises due to coupling between the displacement and rotation degrees of freedom in the 4 × 4 damping matrix.

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Rotordynamic Instabilities Caused by the Fluid Force Moments on the Backshroud of a Francis Turbine Runner

Cited by 6 publications

References 11 publications

Rotordynamics of Turbopumps and Hydroturbines

Rotordynamics of Turbopumps and Hydroturbines

Rotordynamic Moment on the Backshroud of a Francis Turbine Runner Under Whirling Motion

Comments on a Newly-Identified Destabilizing Rotordynamic Mechanism Arising in Vertical Hydraulic Turbines and the Back Shrouds of Centrifugal Impellers

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