Numerical investigation of the full load instability in a Francis turbine

Abstract

For the integration of volatile renewable energies like wind or photovoltaics, hydropower plants received increased attention due to their suitability to stabilize the electrical grid. Thus, turbines are often operated at off-design conditions where undesirable flow phenomena like the full load instability can occur. It is of great importance to understand these phenomena in order to assess their hazard potential and consequently define an appropriate operating range. Even though the physical mechanism behind the full load instability has been investigated in the past, it has not been fully understood to this day. For that reason, the main goal of this thesis is to close this gap and develop a complete understanding of this phenomenon. By means of numerical simulations a Francis turbine was analyzed at full load in the region of the instability onset. First, it was investigated for constant cavitation numbers how characteristic quantities like pressure, swirl number or cavitation volume differ between stable and unstable conditions. In the second step, the transition from stable to unstable conditions was investigated by gradually decreasing the pressure. It was then concluded that the full load instability is a result of the interaction between cavitation on the runner blades and the cavitating draft tube vortex. The negative damping of the oscillation is a result of the pressure oscillations traveling at the speed of sound, while swirl variations are transported with the flow. Furthermore, oscillations of the cavitation volume generate pressure fluctuations. All in all, an explanation for the physical mechanism behind the full load instability is given.