Large Eddy Simulations of the Part-Load Instability in a Pump-Turbine in Pump Mode

Abstract To improve grid stability, pump-turbines (PTs) in pump storage plants are used in pump mode further away from their design point. In pump mode, a positive slope on the characteristic curve Hn-Q indicates an unstable zone, which limits the operating range. The numerical prediction of the PT performances of this part-load instability remains challenging. In this work, large eddy simulations (LES) are carried out to enhance the prediction of casing-related instabilities associated with the emergence of a positive slope. Simulations of a full PT geometry are done for several operating points. An arbitrary-Lagrangian–Eulerian approach coupled with a dynamic mesh adaptation (DMA) methodology is used to consider the runner rotation. Moreover, a mesh convergence strategy is used to guarantee the reliability of simulations by ensuring the accuracy of the mean field discretization and the resolution of a sufficient part of the turbulent scales. Specific attention is paid to properly comparing simulations to experiments carried out by General Electric Vernova Hydro Solutions on their test rig. Simulation results agree well with experimental ones and predict stalls in the casing at part-load conditions. Moreover, it is shown that those stalls influence the flow in the runner where eddies are seen close to the trailing edge of the blades. This phenomenon generates losses and is responsible for the positive slope in the pump characteristic curve leading to unstable pump behavior. This study thus provides new insight into part-load instability.