Mechanism of extraordinary stress on blade during start-up of the Francis turbine

Abstract

This paper describes the mechanism of extraordinary transient stress that occurs during start-up of a Francis turbine by investigating the detailed analysis with measured stress of prototype runner and transient response calculation. According to the dynamic stress measurement with high sampling frequency, it has been clear that a unique phenomenon occurs during start-up especially at the low rotation speed. From the macro perspective, impulsive stress occurred at blade outlet instantaneously and sporadically. Time interval of the event can be considered close to random, and there is no synchronization with the rotation speed. From the micro perspective, the strain occurs owing to the entire vane vibration, and the impulsive loads bump blade outlet from the side of the pressure surface. A Blade shows dumped free vibration in its natural mode in water. As the conclusion, the phenomenon that runner blades are struck by impulsive load on the pressure surface during start-up is compared to a drum beaten by drumsticks randomly in short time, and we named the unique phenomenon “Hydro Drum”