This paper is focused on the guide vane cascade as one of the most crucial stationary sub-systems of the hydraulic turbine, which needs to provide efficient inflow hydraulic conditions to the runner. The guide vanes direct the flow from the spiral casing and the stay vanes towards the runner, regulating the desired discharge. A parametric design tool with normalized geometrical constraints was created in MATLAB, suitable for generating guide vane cascade geometries for Francis turbines. The goal is to determine the limits of these constraints, which will lead to future faster prediction of initial guide vane configurations in the turbine optimal operating region. Several geometries are developed using preliminary design data of the turbine and are investigated using CFD simulations close to the best efficiency point (BEP) of the turbine. This research is part of the Horizon-2020—HydroFlex project led by the Norwegian University of Science and Technology (NTNU), focusing on the development of a flexible hydropower generation.

The design of radial blade cascades, with the intent to become turbine guide vanes, lies in the basis of satisfying turbine runner needs for a particular turbine design point of interest. For conventional turbine operation, the hydrodynamic parameters in the pre-runner space can be relatively easily estimated. For variable speed turbine operation at the constant head, these parameters change in a way that cannot be easily predicted using conventional techniques. In this paper, the guide vanes, as crucial turbine sub-system which needs to perform efficient runner flow feeding for these operating ranges, are numerically tested in various geometrical shapes and configurations, to observe the cascades behaviour, in a way of estimating which cascade configuration satisfies certain hydrodynamic and efficiency considerations of the operating points of interest. The criteria which the guide vanes need to meet are based on hydrodynamic parameters and expansion of the turbine operating range.