Reconstruction of the Francis 99 main runner blade using a hybrid parametric approach.

Abstract

The runner blade shape has been described as a set of discrete sections joining hub and shroud, resulting in such 3D free geometrical forms of considerable complexity that the search for an appropriate parametric approach for its reconstruction and optimization is still a matter of research field. For this reason, the main purpose of this study has been to reconstruct a real and twisted runner blade with a required accuracy through a methodology that uses several algorithms to combine sections and surfaces parametric approaches. Using the surface approach, the free-form shape of the 3D camber surface was defined by a multi-directional interpolation procedure. Using the section approach a Direct Profiling method was tested to fitting the hydrofoil section to the real one. For the blade reconstruction, ten profiles were stacked along the spanwise direction of the real blade to obtain by interpolation the pressure and suction sides using a lofting procedure, thus achieving an acceptable continuous and smooth surface definition. We assume that this methodology could be efficient to reconstruct a damaged and worn free-form blade, especially when the original surface geometry implies sections with different Thickness Distribution along the spanwise direction.

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

“…The reconstruction methodology process can be summarized in three main stages: data extraction, blade reconstruction, and numerical blade evaluation (see Figure 4) [1,3,8].…”

“…One of the most critical aspects of turbomachinery design is to find the optimal shape of the turbine blades to reach their highest possible performance [1,2]. Once an actual optimized blade has been operating for a while, it is necessary to take corrective measures after the natural wear of the blades due to their operating conditions and the consequent reduction in the turbine efficiency.…”

“…Blade parametrization methodologies can be classified into two main groups: those that use a sectional approach and those that use surface patches [5][6][7]. The sectional approaches are useful to describe strong shape variations; however, a high number of sections required to define a twisted blade shape involve a large number of parameters [1]. The 3D radial-axial blades of a Francis turbine are such twisted shapes that they can be considered free-form surfaces.…”