Numerical investigation of the impact of cavitation on the pressure fluctuations in a Francis turbine at deep part load conditions

Abstract

Abstract Hydropower plants can play an important role to stabilize the electrical grid when the share of volatile renewable energies increases. This goes hand in hand with a more flexible operation of the turbines and longer operation in off-design operating points. The increased pressure fluctuations that occur at off-design conditions may reduce the lifetime of the turbines. Consequently, this should be considered within the design process to develop efficient but also robust turbines. To derive suitable simulation setups that can be used in the design process, it is crucial to have a deep understanding of specific flow phenomena. As pressure fluctuations are of main importance in terms of fatigue, it is important that simulations can capture them well. Within this study, a deep part load operating point of a Francis turbine at model scale is investigated. By using three different meshes the influence of mesh refinement on the predicted pressure fluctuations is analyzed. Furthermore, by means of single- and two-phase simulations it is investigated how the pressure fluctuations are affected by the occurrence of cavitation. The results show only minor differences between the different meshes, whereas significant differences are observed between a single- and two-phase treatment.