Fluid-Structure Coupling Effects on the Dynamic Response of Pump-Turbine Guide Vanes

Hydraulic pump-turbines are subject to a high periodic excitation due to the Rotor-Stator Interaction, RSI. Basically, the RSI is caused by the impeller blade passage in the wake of the guide vanes in generating mode, or upstream from the guide vanes in pumping mode. Therefore, the structural parts, notably the guide vanes, suffer from high cycle fatigue strength. The dynamic behavior of the guide vanes is influenced by the surrounding flow. Additional inertia and dissipation strongly affect the structural vibrations; the added mass and the hydrodynamic damping being of the same order of magnitude as the structural mass and damping. In addition, should the entire guide vane cascade be considered, the neighboring guide vanes are influencing each other through the fluid medium. Their eigenfrequencies as well as the vibration amplitudes close to resonance may, thus, be strongly modified. A poor assessment of their dynamic behavior during the design stage may lead to premature failures due to RSI in the early stage of commissioning. So far, researchers have studied the RSI phenomenon, but have not established an analytical description. They have also investigated the added mass, especially the one acting on vibrating runner blades. However, few studies are related to the hydrodynamic damping in hydraulic machines. Moreover, to the author's knowledge, researchers have not yet considered neither the influence of the guide vane vibrations on the pressure fluctuations arising from the RSI nor the coupling between the guide vanes. Therefore, the present experimental work considers the response of the guide vanes in a pump-turbine reduced scale model to the RSI excitation. The pump-turbine is operated at the Best Efficiency operating Point, BEP, in turbine mode. The guide vane cascade consists of a complex mechanical system featuring many degrees of freedom. The study aims to show that the cascade may be viewed as a 2nd order mechanical system. The impulse response of immersed guide vanes is enabled with the use of a spark plug flush mounted in the bottom ring in a guide vane channel. This type of measurements is successfully undertaken in water, model at rest, and model in operation. Keeping the operating conditions of the BEP constant, the impeller rotation frequency is then swept and the guide vanes are therefore excited by the RSI over a wide frequency range. The combination of zb impeller blades with zo guide vanes makes apparent many different rotating diametrical pressure modes. The guide vanes respond up to the RSI 5th harmonic, but are mostly excited at the frequencies corresponding to the RSI fundamental f = zbn and the 2nd harmonic f = 2zbn. The amplitude of the fluctuating bending displacement and torsion angle of the guide vanes is strongly varying across the impeller frequency range. The ranges of the 1st and the 5th RSI harmonic frequency contain the frequency of the 1st bending eigenmode and the 1st torsion eigenmode, respectively. The pres