Dynamic characteristics of rotating components for Francis turbine during load rejection considering fluid structure interaction

Ensuring safe operation of Francis turbine during load rejection is critical due to strong hydro-mechanical coupling. This study investigates the dynamic behavior of rotating components using a two-way fluid structure interaction method, including fluid-blade and oil film-guide bearing coupling for the first time. Results show that unbalanced hydraulic excitation and generator inertia are the main vibration sources. Displacements of rotating components amplify along the axial direction, with lower guide bearing amplitudes 26-30.5 times those at the water guide bearing, and further increasing by 1.7 times at the generator compared with that at the lower guide bearing. The overall dynamic response is dominated by low-order double bending vibration modes, and eccentric motions of the rotating components evolve from a regular ellipse to a complex irregular pattern. Contact surfaces between guide bearing and oil film exhibit stable elastic deformation, with larger deformations at low-stiffness edges and smaller deformations in well-supported inner regions. Significant stress concentration occurs at the blade trailing edge and crown connection during initial load rejection, with a maximum equivalent stress reaching 33.3 MPa and maximum blade deformation of 0.08 mm. These findings clarify the vibration transmission mechanism and provide a basis for improving the dynamic stability and safety of Francis turbines during load rejection.