On the Physical Mechanisms that Cause the Full Load Instability in Francis Turbines

Abstract

Abstract Today, hydropower receives increased attention for the safe integration of volatile renewable energies as it is able to stabilize the electrical grid. For that reason, turbines are more often operated at off-design conditions with the drawback that undesirable flow phenomena like the full load instability can occur. To assess their hazard potential, it is of great importance to understand those phenomena. Even though the physical mechanisms behind the full load instability have been analyzed in the past, they are not yet fully understood. The goal of this study is to close this gap and to develop a complete understanding of the full load instability. Two-phase simulations of a Francis turbine at model scale are performed for a full load operating point that becomes unstable when the cavitation number falls below a certain threshold. The unsteady simulations are performed using the Zwart cavitation model and the SST turbulence model with applied curvature correction. For simulations at different cavitation numbers it is investigated how characteristic quantities like swirl number or cavitation volume differ between stable and unstable conditions. This allows getting a deeper insight into characteristic changes of the flow for the transition from stable to unstable conditions. It can be identified that the interaction between cavitation on the runner blades and the cavitating draft tube vortex plays a key role in the development process of the full load instability. All in all, a complete explanation for the development of the full load instability can be given. The new findings of this study allow proposing an adapted 1D model, which considers the occurrence of blade cavitation.