Characterization of the Fluid Damping in Simplified Models of Pump-Turbines and High Head Francis Runners

Abstract

In order to satisfy the power demand in the electrical grid, hydraulic turbine units frequently work under off-design operation conditions and pass through transient events. These operation conditions can lead to high vibration amplitudes in the turbine runners, decreasing their useful life, and in some cases to premature failures. To determine and to understand the behaviour of the fluid damping is a relevant topic, because this parameter limits the maximum amplitude in resonance conditions. The runner of some types of turbines, such as reversible pump-turbine and high head Francis turbine, can be modelled as a disk-like structure, due to their similar mode shapes. Because of this, in this work, the fluid damping of a vibrating disk was studied. The disk was submerged in water and was put in a resonant state at different vibration amplitudes. Moreover, this structure was excited at different distances to a rigid surface, in order to analyse the effects of the distance between the runner and the casing. The main effects on the fluid damping were determined and characterized, showing a dependency of the fluid damping ratio on the different parameters.

A Field Test Study on the Instability of a Pump-Turbine under Turbine Mode

“…In terms of vibration, Cao et al [7] studied the wet mode of the runner under cavitation state, found that cavitation will affect the asymmetry of the added mass matrix. Moraga et al [8], through simplified disc experiments, speculated that the unit speed will affect the fluid damping of rotating components. Some scholars have also attempted specific methods to improve the stability of pump turbine, such as water injection in the vaneless space [9].…”