Damping Measurements on a Multi-Blade Cascade with Multiple Degrees of Freedom: A Francis-99 Test Case

Abstract

Due to thinner blades and higher demands for flexibility, the high-head Francis runners designed today face considerable challenges that severely affect the runners’ expected lifetime. For many high-head Francis runners, the leading cause of fatigue is blade cracking due to Rotor-Stator Interaction, which cause vibrations in the runner blades.Accurate prediction of the vibration magnitudes in a turbine is paramount in designing a reliable Francis runner. The understanding of the interaction between the hydrodynamic forces and the internal stresses in the runner is not yet sufficient to make this prediction. Previous investigations have identified some key parameters that affect dynamic behaviour in water, such as added mass, as well as added stiffness and damping from moving water. These parameters affect the natural frequency and damping of a structure, which in the end will affect what vibrations magnitudes the runner will be subjected to for a given frequency of excitation. The behavior of these parameters have recently been investigated by several researchers, but the effect of neighboring blades is yet not understood.A multi-blade cascade has been tested for four of its different modes of vibration. The results indicate that the slope of the damping with respect to the inverse Strouhal number is constant. This slope was found to be the same as for several single-blade tested performed, both in the same rig and in other works. The implication is that the product of added mass and mode shape does not change significantly.