Non-uniform Design of Guide Vanes for Variable Speed Francis Turbines

Increased demand of energy production leads to the requirements for flexibility in the operation of hydropower plants, which can be achieved through variable speed turbine operation and multiple start/stops a day. Variable speed operated turbines in hydropower plants tend to run with improved performance at off-design operating points. The shape of the guide vanes directly influences the turbine operating range and they can influence the maximal efficiency. Analysing the guide vanes, as a part in direct correlation with the turbine runner, as variable speed operation is required, the tendency is to expand the operating region of the turbine, with a target to improve the lower efficiencies at off-design points, i.e. to improve off-design energy production. Uniform guide vanes blade profiles in a standard design procedure, show that at off-design operation of the turbine, the efficiency drops significantly, as the blades trailing edges profile do not follow the runner needs at increased or decreased rotational speed. A contribution has been made towards defining a non-uniform blades of the guide vanes, especially at their trailing edges, where several parts of the blade are estimated to guide the flow at the desired inlet angle for the runner at certain operating speed. Using CFD simulations, the results are presented in a way of estimating the turbine performance and expanding the operating range by using uniform and non-uniform blades for variable speed operating conditions, compared on the basis of Francis 99 turbine model from the Waterpower Laboratory at the Norwegian University of Science and Technology (NTNU) as a reference turbine model of this research.