Analysis of unstable flow characteristics in the blade region of Francis pump-turbine under non-design conditions

Under non-design situations, the Francis pump-turbine, a frequently employed core energy conversion device in pumped storage power plants, plays a significant role in ensuring their stable operation. This article examines the unstable flow inside a Francis pump-turbine using energy gradient theory and numerical computations and model experiments. Research has indicated that forced impacts and flow separation between fluids and blades are significant causes of deteriorating mechanical energy gradients, whereas non-optimal angles between stay and guide vanes can readily cause flow separation. The strong helical flow at the blade inlet is the primary cause of the unstable flow, and the hub and blade outlet are the primary locations for the high shear force work area brought on by large-scale vortex structures, and the middle portion of the impeller blade is where the unstable flow field begins and needs more attention.