CFD Simulation of the turbine start-up of a pump-turbine with investigation of the usage of a porous domain for modelling very small guide vane angles

Abstract With increasing number of transient operations, such as start and stop procedures in pump-turbines, CFD simulations of these sequences have become more important to understand the occurring flow phenomena and evaluate their impact on the pump-turbines lifetime. In the present work, the numerical setup for transient CFD simulations is discussed and a CFD simulation of a turbine start-up using a 29 million node mesh and the SBES turbulence model is carried out. The results demonstrate reasonable accuracy when comparing the overall trends with the available measurement data. However, significant underestimation of torque fluctuations as well as draft tube pressure fluctuations during the initial phase of the start-up is observed. In a subsequent study, the modelling of very small guide vane angles by a porous domain is investigated. An analytical function for the loss coefficient of the porous domain is applied up to different threshold values of the guide vane angle. A guide vane angle of 1° is identified as the best option regarding accuracy and computational effort.