Reverse-engineering reconstruction of a worn Francis runner blade for a medium-head
Keywords:
Francis Runner, Blade Reconstruction, Reverse Engineering, Optimization Algorithm, NACA Equations.Abstract
After years of operation with a working fluid that produces not only cavitation but also contains solid particles, the Francis runner blade often shows wear, erosion, cracking, and pitting. When reconstructing very old runners, the original drawings rarely provide detailed technical information about the blade, which, due to its complex shape, cannot be measured using conventional techniques. Therefore, empirical and artisanal methods are often used. Recently, the reverse-engineering process has enabled reconstruction of blade surface geometries through data acquisition, processing, and construction of a 3D model. However, for used and worn blades, the point-cloud data obtained by scanning can be so distorted that the processing technique cannot reconstruct the surface in a continuous, smooth manner that is as close to the original as possible. Consequently, this work aims to reconstruct the blade of an original Francis turbine using a parametric function as the data-processing technique. By manipulating the m, p, and t parameters of the four-digit NACA function using an optimization algorithm, the fitting error was minimized to 0.04 relative to the cross-sections extracted from the scanned digital file. The volume of the solid model obtained with the proposed methodology differs by only 0.25% from the original, while maintaining smooth, continuous surfaces. However, it remains to know whether the hydrodynamic and structural conditions generated by the rebuilt blade could affect the turbine's performance.
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