Formation of Rotating Vortex Rope in the Francis-99 Draft Tube

Abstract

The aim of this research is to understand the mechanism(s) of RVR formation during the changes in operating condition from the Best Efficiency Point (BEP) to Part Load (PL). A Computational Fluid Dynamic (CFD) methodology by the means of ANSYS-CFX is applied on a reduced high head Francis turbine model. The reduced model consists of one stay vane, two guide vanes, one runner blade, one splitter and a full draft tube. Numerical simulation is first performed at BEP as well as PL to ensure the appropriate employment of turbulence models and boundary conditions. In the second step, the inlet boundary conditions are changed linearly from BEP to PL in order to achieve the transient conditions inside the draft tube. The initial condition of the second step is the converged BEP result. The transient simulation is continued until the RVR is fully developed in the draft tube at part load condition. The numerical results for BEP, PL and BEP to PL are in a good agreement with the experimental data. The effect of the RVR is considered from two aspects. The first one is the frequency, and the amplitude of the pressure pulsations induced by the RVR in the draft tube. The second one is the velocity field in the draft tube which is investigated over time during load rejection. Moreover, the flow structure is visualized using the λ2 criterion. The mechanism(s) of RVR formation and damping is accurately investigated by the presented approach. Furthermore, the results provide a better understanding of the physics behind the RVR formation. The obtained results aim to design an effective RVR controlling approach.