A theoretical, numerical and experimental analysis of S-shape instabilities in reversible pump-turbines: Resultant strategies for improving operational stability

Abstract

In order to be able to draw conclusions on preventing S-shape instabilities in reversible pump-turbines during normal operation, it is important to fully understand their complex nature. In particular for the design, it is essential to predict occurring instabilities as accurate as possible and to fathom their underlying mechanisms properly. Therefore, the first section of this work incorporates a short CFD study based on the design of a reversible pump-turbine scale model. The numerical investigations are done according to the methodology represented in [7], comprising unsteady flow simulations under steady and transient operating conditions. The hydraulic machine is spatially discretized in model size. The turbulence modeling is based on the Explicit Algebraic Reynolds Stress Model. Related measurements are executed on a test rig at ASTROE (Hydraulic Engineering and Laboratory) in Graz, Austria, providing experimental data of integral quantities at different guide vane openings. Besides the CFD study, this work mainly aims at comprehensively analyzing underlying mechanisms of S-shape instabilities in reversible pump-turbines. An analytical description of flow phenomena is intended to provide a better understanding of mechanisms such instabilities are based on. A synthesis results in potential strategies for improving operational stability at individual operating points.