Abstract: During part-load operation, francis turbines often experience adverse phenomena such as degraded internal flow patterns and increased vibration. To gain deeper insight into the flow characteristics within different components of the turbine under these conditions, this paper conducts a three-dimensional transient numerical simulation of the full flow passage for a specific francis turbine unit. A systematic comparative analysis of the flow characteristics under three typical operating conditions is performed. The results indicate that significant flow instability occurs during part-load operation. Specifically, a large positive attack angle at the runner inlet leads to flow separation, the shear between the jet flow from guide vanes at small openings and the main stream generates large-scale turbulent vortex structures, and the residual swirl at the runner outlet results in the formation of an eccentric vortex rope in the draft tube, thereby inducing intense pressure pulsations with substantially increased amplitude. The results of this study contribute to a better understanding of the complex unsteady flow phenomena inside francis turbines operating under part-load conditions, providing a theoretical basis for the safe operation and structural optimization of francis turbines.

Key words: francis turbine, part-load operation, pressure pulsation, unsteady flow