Part load resonance risk assessment of francis hydropower units

While operating at part load, Francis turbines feature a precessing cavitation vortex rope in its draft tube. The precession of this vortex in the elbow of the draft tube acts as a pressure pulsation source which frequency can match the first hydroacoustic eigenfrequency of the hydraulic system in some cases. Resonance phenomenon can be predicted by using reduced scale physical model tests and numerical simulations, but it remains challenging. This paper proposes a procedure to estimate the risk of part load resonance at the early stage of a hydropower project. The proposed procedure uses the hydroacoustic properties of a given reduced scale physical model and applies them to a large number of turbine designs and power plant configurations to assess the risk of resonance for each one of them. Results show that resonance are likely to occur in hydropower plants in a certain range of turbine rated head and rated discharge values. These results can then indicate if more detailed investigations in some specific hydropower projects are necessary.