Experimental study on the detection of vibrations of an operating turbine runner with sensors on the casing

Hydraulic turbine runners experience strong vibrations in operation, especially when resonances between runner natural frequencies and hydraulic excitations occur. Precise determination of prototype runner frequencies in operation is essential, but direct measurements are costly and technically challenging. This study presents an experimental investigation on a reduced-scale Francis model to evaluate the use of nonintrusive stationary sensors to measure runner vibrations. The runner was instrumented with accelerometers and piezoelectric patches, and an accelerometer was installed on the casing. The natural frequencies of the operating runner were excited with the patches and the vibration was measured simultaneously in the rotating and stationary structures. Results prove the feasibility of monitoring runner vibrations externally and demonstrate the correlation between rotating and stationary frames. Runner natural frequencies change and split due to confined rotation. The stationary sensor measures them with a frequency shift depending on rotating speed and nodal diameters of the excited mode shape.