Effect of guide vane opening speed on multiscale flow characteristics in a Francis turbine during load variation

As a core regulating component in multi-energy complementary system, Francis turbines (FTs) are required to frequently undergo variable load processes (VLPs) to mitigate power fluctuations from wind and photovoltaic generation. To clarify the influence of guide vane opening speed (GVOS) on hydraulic stability during VLPs, numerical simulations are conducted for three GVOSes over the load variation from 30% to 100% rated power. The evolution of internal flow structures and pressure pulsation (PP) characteristics in the flow passage components is systematically analyzed. A flow field decomposition framework based on complementary ensemble empirical mode decomposition (CEEMD) is further introduced to reveal formation mechanism of high-amplitude PPs of draft tube (DT) and the evolution of their dominant modes. The results indicate that the influence of GVOS on runner blade PPs is mainly concentrated near the blade inlet edge, and the peak-to-peak value of PP decreases as the opening speed increases. With faster opening speeds, both vortex rope volume and turbulent kinetic energy in the DT are reduced. High-amplitude PPs are primarily concentrated at 0.27fn and its second harmonic 0.54fn, where 0.27fn corresponds to the vortex rope frequency. Pressure field decomposition shows that a higher opening speed delays the moment when the vortex rope reaches its most unstable state and shortens its duration. The formation intensity and energy diffusion of the dominant vortex rope are significantly weakened, resulting in improved hydraulic stability during the VLP. These findings provide useful guidance for enhancing the transient stability of FTs under variable load operation.