This paper focuses on the set up and the validation of a numerical model for the analysis of the flow in reversible Francis pump-turbines in the S-Shape region. For such operating conditions the flow inside the pump-turbine is characterized by highly unsteady flow separations and complex rotor/stator interactions. Unsteady numerical computations were performed on scale model pump-turbines. Turbulence modelling was achieved by the SAS-SST turbulence model which allows the resolution of the largest flow instabilities. Three different configurations of pump turbine with the same specific speed (nq≅40) were considered in this study. Several operating conditions from an operating point close to the maximum of power output to zero discharge condition for a given large guide vane opening were computed. Validation of the computational methodology was done by comparison of the CFD results with the existing model test results. For all the considered configurations and operating conditions, the numerical model gave accurate results for the estimation of quantities such as the head of the pump-turbine or the torque on the runner. Moreover, the pressure fluctuations and the forces acting on the rotating part of the pump-turbine were also evaluated (or predicted). The analysis of the numerical results allows more in depth knowledge of the instabilities of the flow in pump-turbine in the S-Shape region.

Clément Jacquet
SuperGrid Institute, Villeurbanne, France