Mechanism of unsteady flow and fluid–structure coupled vibration during rapid startup of a large Francis turbine

Abstract

Rapid startup of large Francis turbines involves strongly coupled guide-vane motion, runner acceleration, water-hammer propagation, cavitation evolution, and structural vibration. However, the fluid–physical pathway through which these transient flow processes generate runner dynamic stress remains insufficiently clarified. In this study, a one-dimensional–three-dimensional computational fluid dynamics–finite-element method coupled model is developed for a prototype large Francis turbine to investigate the complete startup process. The results reveal that rapid guide-vane opening produces a high-speed annular swirling jet in the vaneless space because the guide-vane opening rate is not synchronized with runner acceleration. This jet impinges on the runner inlet and drives the early-stage vortex formation, flow separation, and impact cavitation. As the runner accelerates, the vortical structures migrate downstream along the blade passages, and the cavitation pattern evolves from impact cavitation to vortex cavitation and cloud cavitation near the blade trailing edge. The dominant pressure-pulsation components also shift from low-frequency water-hammer-related fluctuations to rotor–stator interaction frequencies. The resulting hydraulic excitation is transmitted to the runner structure, producing pronounced dynamic stress near the blade trailing edge and the crown/band junctions. The suction-side trailing edge shows the strongest stress response, indicating that vortex-cavitation evolution is a primary source of local flow-induced vibration during rapid startup. These findings clarify the transient coupling chain from guide-vane motion to swirling flow, cavitation, pressure pulsation, and runner stress response, providing physical guidance for startup regulation and vibration-resistant runner design.