Abstract
Undesirable pressure fluctuations in Francis turbines are caused by precession of the vortex rope in the draft tube under conditions of low flow rate. These pressure fluctuations can cause a resonance that seriously undermines the stability of the turbine system. In the previous study, the effects of anti-swirl fins in the draft tube were investigated to suppress the vortex rope. However, despite the installation of anti-swirl fins, the precession of vortex rope is occurred near the draft tube cone, and an additional suppression factor for vortex rope is required. Therefore, this preliminary study investigates the effect of air-injection to select flow rate of the air-injection for suppressing the vortex rope in the draft tube cone. Three-dimensional steady-state Reynolds-averaged Navier-Stokes analyses were conducted with a shear stress transport turbulence model to investigate the performance and internal flow characteristics according to the flow rate of air-injection. The efficiencies were decreased by the air-injection into the draft tube, however, the effect of vortex rope suppression was exhibited when the flow rate of air-injection was above 0.5%Q.
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