Investigation of Francis Turbine Part Load Instabilities using Flow Simulations with a Hybrid RANS-LES Turbulence Model

The operation of Francis turbines in part load condition causes high pressure fluctuations and dynamic loads in the turbine as well as high flow losses in the draft tube. Owing to the co-rotating velocity distribution at the runner blade trailing edge a low pressure zone arises in the hub region finally leading to a rotating vortex rope in the draft tube. A better understanding and a more accurate prediction of this phenomenon can help in the design process of a Francis turbine. The goal of this study is to reach a quantitatively better numerical prediction of the flow at part load and to evaluate the necessary numerical depth with respect to effort and benefit. As standard practice, simulation results are obtained for the steady state approach with SST turbulence modelling. Those results are contrasted with transient simulations applying a SST as well as a SAS (Scale Adaptive Simulation) turbulence model. The structure of the SAS model is such, that it is able to resolve the turbulent flow behaviour in more detail. The investigations contain a comparison of the flow losses in different turbine components. A detailed flow evaluation is done in the cone and the diffuser of the draft tube. The different numerical approaches show a different representation of the vortex rope phenomenon indicating differences in pressure pulsations at different geometric positions in the entire turbine. Finally, the turbulent flow structures in the draft tube are illustrated with several evaluation methods, such as turbulent eddy viscosity, velocity invariant and turbulent kinetic energy spectra.