Modelling of the cavitation vortex rope in a Francis turbine at full-load conditions

Hydropower plants providing generation flexibility can experience several issues when operate in off-design conditions. In Francis turbines, the development of a cavitation vortex rope at the outlet of the runner may compromise the unit stability and prevent the operation. 

The vortex rope characteristics are influenced by the swirling flow at the outlet of the runner depending on the operating conditions. At full-load, the vortex rope may enter into an unstable regime causing vibrations in the whole hydraulic system.

The goal of this study is to investigate the volume and velocity field of the cavitation vortex rope at full-load, and to model these characteristics as a function of the operating parameters.

For this purpose, an experimental campaign has been performed on a reduced-scale model of a Francis turbine. A characterisation based on high-speed videos of the cavitation vortex rope has been implemented to estimate the cavitation volume variations at several full-load conditions. The reliability of the method is ensured by verifying the correlation between the computed volume, the pressure in the draft tube and the flow velocity field measured by Particle Image Velocimetry. This also allows studying the dynamics of the vortex to develop a predictive model of the flow stability.