A FEM Structural Analysis of a Francis Turbine Blade Parametrized Using Piecewise Bernstein Polynomials

Abstract

Several methodologies have successfully described the runner blade shape as a set of discrete sections joining the hub and shroud, defined by 3D geometrical forms of considerable complexity. This task requires an appropriate parametric approach for its accurate reconstruction. Among them, piecewise Bernstein polynomials have been used to create parametrizations of twisted runner blades by extracting some cross-sectional hydrofoil profiles from reference CAD data to be approximated by such polynomials. Using the interpolating polynomial coefficients as parameters, more profiles are generated by Lagrangian techniques. The generated profiles are then stacked along the spanwise direction of the blade via transfinite interpolation to obtain a smooth and continuous representation of the reference blade. This versatile approach makes the description of a range of different blade shapes possible within the required accuracy and, furthermore, the design of new blade shapes. However, even though it is possible to redefine new blade shapes using the aforementioned parametrization, a remaining question is whether the parametrized blades are suitable as a replacement for the currently used ones. In order to assess the mechanical feasibility of the new shapes, several stages of analysis are required. In this paper, bearing in mind the standard hydraulic test conditions of the hydrofoil test case of the Norwegian Hydropower Center, we present a structural stress–strain analysis of the reparametrization of a Francis blade, thus showing its adequate computational performance in two model tests.

Keywords

• turbine blade
• finite element method
• piecewise Bernstein polynomial
• stress–strain analysis