Chain-reaction collapse of cavitation clouds in Francis turbine runner during start-up

This paper elucidates the hydrodynamic generation mechanism of the extraordinary stress in Francis turbine runner that occurs during start-up, which is called "Hydro Drum", by unsteady CFD analysis with a cavitation model and model test. The analysis results revealed the pressure wave generated by the collapse of one cavitation cloud caused the collapse of other cavitation clouds. In addition, the magnitude of the pressure wave generated by collapse of the cavitation cloud resulting from the chain-reaction was much larger than that of the cavitation cloud that triggered the chain-reaction, resulting in an impulsive pressure rise. Model tests were conducted to measure the relationship between the extraordinary stress and the pressure on blades, and the mechanism was verified. In conclusion, it is elucidated that the extraordinary stress on runner blades during start-up of a Francis turbines is caused by impulsive pressure rises accompanied by the chain-reaction of collapse of cavitation clouds.