On the External Effects Affecting Torsional Modes in Guide Vanes

Abstract

Rotor stator interaction in high-head Francis turbines has led to several failures in recent years. Increasing efficiency demands require design optimization of the turbine components, which may lead to thinner profiles. Not only can component not withstand the given loads; quite often one or more of their natural frequencies coincide with that of the rotor-stator interaction. Most of the research published has been on runners, while other parts of the turbine are less studied. Even though guide vanes have torsional modes with frequencies which may be close to the exiting frequency from the rotor stator interaction with the runner, no significant failures due to resonance have been reported. This paper investigates some of the possible mechanisms which may negate resonance in the torsional modes of the guide vanes including; hydrodynamic damping from the flowing water and friction in the guide vane bearings. A case study is conducted on a guide vane where the calculated natural frequency is within 10% of the excitation frequency, while no significant vibrations have been reported. Further, the findings are generalized to Francis turbines of different specific speeds. The results indicate that the dynamics in the bearings are especially important to consider to be able to predict the vibration levels of the guide vane. Having the correct friction factor in the bearing may lead to significant damping and almost eliminate any excitation of torsional eigenmodes.