Pressure measurements inside a Francis turbine runner
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Over the last years, there have been several incidents with cracks in high head Francis turbines. These cracks are understood to be related to pressure pulsations, vibration modulus and combination of these. In this paper, an investigation of pressure pulsations in a High Head model turbine is performed with the use of onboard pressure sensors and strain gauge. Earlier onboard measurements have mainly utilized blade-mounted sensors. In this paper, a setup with hub-mounted pressure sensors are described. In addition, a strain gauge mounted on the trailing edge of one blade is used to relate the measured pressure to the loading on the blade. Results from the measurements gave a good but not complete picture of the wave propagation through the runner. The most dominating frequency was the guide vane passing frequency on both the pressure measurements and the strain measurements. The measurements did not provide all necessary information for full analysis of the onboard pressure pulsations. On the other hand, the setup is considered as a good reference for computational fluid dynamics validation (CFD) and is considered less extensive for evaluating the onboard pressure pulsations compared to blade-mounted sensors.