Calibration of strain gauges on a model runner blade combining numerical and experimental data

Abstract

In this paper, we present the calibration setup, method, and results for a set of strain gauges installed on the trailing edge of the runner blades of a model Francis turbine. The calibration work is done as a step in the HydroFlex project, with the goal of taking experimental data to validate numerical models to better estimate the reduction in lifetime from more flexible operation. Due to the complex geometry of the blade, an analytical solution for the stresses for a given load is not possible to obtain, so calibration is needed. A combination of strain measurement and numerical analysis is used to correlate the response from the strain gauges directly to the strains parallel to the trailing edge. The strain gauges are installed on the suction side of the blades, and close to the hub and shroud, and the calibration is done by applying known weights on the shroud using a tailor made blade fixture. The signal from the sensors is passed through a set of miniature amplifiers that fits in the hub of the model runner. The numerical setup is done in ANSYS Mechanical, and is set up to fully replicate the conditions of the physical setup. The results shows that the method is viable, however there is a challenge with the response, and thus required amplification, due to the stiffness of the runner blade material.