Evaluation of RSI-induced stresses in Francis runners

Modern design of hydraulic turbines aims to achieve very high levels of efficiency and structural integrity in the environment of highly variable loading conditions. To combine those requirements, a profound knowledge of static and dynamic loads acting on hydraulic components is necessary. Dynamic loadings of Francis runners strongly depend on head range and required operating conditions. For higher head (low specific speed) Francis runners, the main dynamic loading is caused by Rotor-Stator Interaction (RSI). The knowledge of dynamic loads and stresses in Francis runners is often derived from strain gauge measurements and/or simulation of flow interacting with structural components. Reliable simulations of Fluid-Structure Interaction (FSI) are necessary to predict the dynamic behavior, especially in terms of hydrodynamic mass and damping properties. This contribution describes the equipment and procedure of strain gauge measurements in Francis prototype runners, including the challenges to overcome during such tests. It provides an overview of strain gauge test results for Francis runners in different head ranges and describes the characteristic differences depending on specific speed. Measurement data is compared with state of the art simulations of Francis runner dynamics to predict dynamic stresses caused by RSI. The contribution underlines the fact that a calibrated analysis procedure is the key to optimize the design of Francis runners.