COMMENTS ON A NEWLY-IDENTIFIED DESTABILIZING ROTORDYNAMIC MECHANISM ARISING IN VERTICAL HYDRAULIC TURBINES AND THE BACK SHROUDS OF CENTRIFUGAL IMPELLERS
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In three 2010 papers, Tsujimoto, Ma, Song, and Horiguchi [13] 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. [4] 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 disk suspended vertically 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 disks displacements and pitch and yaw degrees of freedom. 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-Songs 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 Tsujimotos experience and predictions. A comparison of the predicted reaction moments of Model A and Tsujimotos 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.
