Automated hydraulic design procedure for a Francis turbine spiral casing

Abstract

Abstract The spiral casing of a Francis turbine distributes the water from the penstock to the stay and guide vanes circumferentially and uniformly, which is important for achieving the required flow conditions at the runner entrance. Moreover, the spiral casing is supposed to provide the required inlet velocity in front of the stay vanes and create minimal hydraulic losses. The dimensions and shape of the spiral casing depend on the hydraulic and energy parameters of the turbine. A calculation methodology for a Francis spiral casing hydraulic design is presented in this paper. The methodology developed is based on the main condition to achieve a uniform water flow rate into the stay vanes system and the wicket gate over the entire perimeter. The free vortex flow theory is implemented in this research, where the design is based on the law of constant velocity moment. The parametric definition of the spiral casing makes the geometries generated for certain input combinations suitable for numerical analysis using commercially available Computational Fluid Dynamics (CFD) software. The calculation procedure can be used for any set of energy and geometry turbine parameters, such as water discharge, angle of streamline departing from the spiral casing, and stay ring diameter and height. The automated approach integrating MATLAB and ANSYS Workbench capabilities is presented as a spiral casing design tool. The product is a design solution proposed on basis of the turbine parameters. The spiral casing geometry generation is followed by a CFD analysis. Considering input parameters of different existing Francis turbines, the spiral casing is redesigned accordingly. One of the obtained spiral casings geometry is numerically tested. The results show that the uniform discharge distribution is achieved. The automation of the design procedure allows further optimization based on chosen input parameters.