On the auto-oscillation effect of Kaplan rotors during load rejection

Abstract

High vibrations due to auto-oscillation effects can occur at power units with Kaplan type runner during load rejection. These vibrations may be present in the turbine braking mode when the wicket gates are almost closed, and the rotating runner pushes the water towards the draft tube resulting in higher pressure below the runner blades. The upward leakage flow between runner blades and discharge ring is suspected to cause the vibration phenomenon.Within this contribution, a fluid-structure-interaction (FSI) technique is applied to identify the mechanism of self-excitation. The investigation is carried out on a disk that represents the simplified geometry of a Kaplan runner. Computational fluid dynamics (CFD) simulations are used in order to obtain parameters in terms of stiffness, damping and inertia of the fluid to be applied in the following shaft line analysis. Damped modal analyses of the shaft line allow for evaluation of the stability of natural frequencies and corresponding mode shapes of the shaft line. A resulting negative damping indicates the occurrence of auto-oscillation behavior of the shaft line. For the operating condition under investigation, mode shapes with negative damping are obtained with this approach and a dynamic bending moment on the runner due to lateral movement in combination with a tilting at the runner in these mode shapes is identified as source of instability.Based on these findings, an analytical approach for modeling the relevant effect is derived and compared to CFD result as well as results from acoustic finite element modelling. Finally, conclusions for avoidance of self-excited vibrations at Kaplan runners are proposed.