Internal flow characteristics in the draft tube of a Francis turbine model by air injection under low flow rate condition

Abstract

Abstract Undesirable internal flow features under low flow rates in the draft tube of a Francis turbine are caused by the vortex ropes with precession. These vortex ropes cause performance degradation and increase the instability of turbine systems. The effects of anti-swirl fins on the wall of the draft tube were observed in the previous study to suppress the generation of the vortex rope. However, vortex ropes still remained near the cone of the draft tube. Thus, extra methods to suppress the vortex rope are needed. In this study, the influences of air injection were investigated from the cone of the runner to suppress the vortex rope characteristics in the draft tube of a Francis turbine model. For investigating the unsteady flow and pressure phenomena by injecting the air in the draft tube, unsteady-state Reynolds-averaged Navier–Stokes calculations were carried out with a turbulence model of a scale-adaptive simulation shear stress transport. By injecting the air into the draft tube, the hydraulic efficiency was reduced, however, the visible vortex ropes in the draft tube were suppressed. Additionally, the unsteady pressure phenomena also were reduced by injecting the air.