Numerical investigation of cavitation properties for hydraulic turbines

Francis turbines are the most common water turbine type in the world due to their large head range and higher power capacity. Francis turbines, as other hydraulic turbines, are custom-designed for nominal operating conditions specific to each power plant. They also need to be operated at off-design flow conditions because of variable head and flowrate. Operating the turbine at off-design points may cause cavitation, which may cause erosion, reduction in efficiency, instability of operation, severe noise, and vibration. By the help of rapidly developing computer technology, Computational Fluid Dynamics (CFD), having outstanding abilities and accuracy, became an irreplaceable tool in designing, examining performance and cavitation properties. In this thesis, the guide vane and runner geometries of Kepez-1 Francis turbine are redesigned with CFD tools in order to obtain high efficiency and minimize cavitation in its operating range. The hill-chart and cavitation limits of the rehabilitated turbine are determined by performing single-phase, steady-state simulations for the sixty operating conditions in its operating range. When compared with the CFD results of the existing turbine, it is obtained a 2.9% efficiency increase reaching 94.48% turbine efficiency in the design point, and the performance targets are accomplished by providing 90% or more turbine efficiency in the entire operating range. The result of steady state simulation shows that travelling bubble and draft tube vortex cavitations are observed at the overload operating condition. The same cavitation properties are not observed when two-phase, unsteady simulation is performed at the same operating condition. Additionally, turbine efficiency is predicted higher. This is because frozen rotor interface used for steady-state simulations overestimates flow irregularities. In the results of unsteady simulations, this causes less cavitation and hydraulic losses, therefore higher turbine efficiency is obtained. Efficiency and torque fluctuations, rotor-stator interactions, and pressure fluctuations are investigated in detail with simulations performing at both design and partload operating points in addition to overload operating point. As a result, new Kepez-1 turbine can be operated at its design point with 95.3% turbine efficiency. Additionally, when compared with the results of steady-state simulations, it has higher turbine efficiency and less cavitation properties in its operating range.