Transient analysis of load rejection for a high-head Francis turbine based on structured overset mesh

Abstract Tremendous development and integration of intermittent renewable energy resources have forced the hydraulic turbines to experience transient processes more frequently and thus endangered the stability of the machine. The principal objective of this paper is to present the numerical investigation into load rejection towards a high-head model Francis turbine based on the structured overset mesh, which provides greater flexibility over the standard mesh techniques and adequate mesh quality during the movement of the guide vanes. The numerical results show good agreement with experimental measurements. The formation and evolution process of the vortex rope in the draft tube are elaborated by the velocity invariant Q and the swirl number S reaching its maximum value due to the formation of precessing vortex rope. Evidence is presented which shows that significant influence of the rotor-stator interaction is captured, and the pressure fluctuation amplitudes caused by the changing of angular position of guide vane are also powerful. Furthermore, the velocity profiles in axial and radial imply that an increasing recirculating flow region is developed at the center of the draft tube during the transient process. The findings presented in this paper enhance the insight into transient features of load rejection process.