Numerical investigation of multiscale flow-induced vibration and fatigue life prediction of a large Francis turbine

Hydropower has been assuming a progressively vital role in regulating power system dominated by renewable energy sources. Accordingly, hydropower units routinely operate beyond their high-efficiency zones into wide-load regimes to actively enable dynamic load regulation. This study focuses on mechanical issues induced by multiscale unsteady flow in a large Francis turbine under high-head conditions. A two-way fluid–structure interaction method was used to model the complex interplay between multiscale fluid dynamics and structural behaviors. The analysis aimed to clarify propagation mechanism of pressure pulsations and assess the impact of radial force on runner fatigue life and shafting vibrations. Results show that radial force fluctuations from the multiscale fluid on the runner dominantly trigger runner peripheral fatigue and shafting transverse vibration. Pressure pulsations from wake vortex rope and rotor–stator interaction (RSI) between runner blades and stationary turbine components spread throughout the multiscale flow channel. In contrast, pressure pulsations from the RSI between guide vanes and runner were confined to the meter-scale flow channel. Regions particularly prone to fatigue crack initiation were identified as the mid-blade crown junction and blade trailing edge. Furthermore, the dynamic misalignment of the rotating shaft system, observed in this study, led to non-integer multiples of the rotational frequency in the vibration response of the shaft. This study highlights interplay between multiscale unsteady flow and structural responses, providing valuable insights into improving the stability and service life of large Francis turbines. It also offers guidance for future investigations on turbine vibrations and fatigue across various operating conditions.