Water Injection for Reduction of Vortex Rope Effects in Francis Turbine Draft Tube and Offering New Method for Selecting Nozzle Diameter

In this paper, fluid flow in Francis turbine draft tube is simulated with and without water injection condition in unsteady state using actual and straight diffuser geometry of draft tube. Also a new method for selecting nozzle diameter for injection is proposed. Simulation is carried out using Fluent software and k-ε and SST k-ω turbulence models in the straight and actual geometries of the draft tube, respectively. In two considered geometries, results of proposed method for selecting the nozzle diameter, which is based on the ratio of the total loss to the pressure recovery factor, have been compared with the method used in the previous researches, which was based on the total loss calculation. Inlet boundary conditions and validation are based on the experimental data. Results show that selecting nozzle diameter is depended on the geometry of the draft tube, and using the ratio of total loss to pressure recovery factor approach for selecting nozzle diameter will improve the size of selected nozzle diameter up to 33%, velocity fluctuations up to 16.3% and pressure fluctuations amplitude up to 19% in draft tube.