Dynamic Loads on Francis Turbines: An Experimental Study

Abstract

There is an increasing need for stable and flexible renewable energy. This calls for better lifetime predictions for Francis runners, which in turn requires more knowledge on the dynamic loads on the runner blades. This thesis outlines an experimental approach to further the understanding of the dynamic loads a Francis runner is subjected to, during off-design operation, transient operation, and during resonance conditions. The objective of the thesis is to provide verification data for numerical analysis, as well as to quantify how the different operating regimes affect the lifetime of a Francis runner, through dynamic loads on the runner. Experiments have been performed on a high head Francis model runner, both for deep part load (DPL), and for part load (PL) with a fully developed Rotating Vortex Rope. The measurements show that the DPL condition causes a large region of back flow, and the PL condition has high amplitudes of the Rehinegan’s frequency. However, these effects are not significant when compared to the pressure amplitude at the runner blade inlet as a result of Rotor-Stator Interactions (RSI). A transient condition was examined, with the turbine undergoing load rejection from the Best Efficiency Point (BEP) to PL, and the RSI amplitude remained dominant at the turbine blades. In order to investigate the RSI phenomena more closely, tests were conducted on a simplified runner blade (a hydrofoil with an asymmetric trailing edge), mounted in a square high-speed channel with angle of attack. The tests show that the damping factor of the hydrofoil increases linearly, with a transition in the lockin region. Particle Image Velocimetry (PIV) measurements showed that the vortex shedding of the hydrofoil exhibited large stream-wise fluctuations in velocity, likely due to wandering of the upper separation point. Further testing on a different hydrofoil geometry reveal that the damping undergoes the same transition through lock-in, even when the vortex shedding amplitude is minimal. Additionally, the tests showed that the two hydrofoils exhibited the same slope in the damping factor change, when plotted against the reduced velocity, or the inverse Strouhal number. Computational Fluid Dynamics (CFD) matches the experimental data, and indicates that the trend continues up to at least 45m=s. A multi-bladed cascade has been tested, for four modes of vibration, and the same transition through lock-in is observed. More interesting, the slope of the damping change coincides with the previously tested hydrofoils, in addition to conforming with the slopes of hydrofoils tested in other works. This suggests that the product of added mass fraction and mode shape integral remains relatively unchanged for fixed-beam hydrofoils. This holds even for blades of differing natural frequencies (one order of magnitude in difference) and for multiple blades with modes coupled through water. The implication of this is that the analysis of a single blade can be extended to a runner, with predictable results.