Understanding the modal response of a full-scale 200 MW Francis turbine runner

Under off-design operating conditions, the flow through hydraulic turbines presents fluidodynamic disturbances that can excite a natural frequency and provoke a resonance. Determining with precision the modal properties of the runner is thus of paramount importance. The current article presents an experimental modal analysis of a 200 MW Francis runner located in the Oksla hydropower plant in Norway. During the test, the runner was bolted to the still shaft supported by the bearings and an instrumented hammer was used to excite it. Then, several accelerometers were fixed to the band and blades to measure its modal response. Subsequently, frequency response functions between the hammer impulsive force and the vibration acceleration signals were computed to determine the natural frequencies and mode shapes of the Oksla runner. Even though the main modes corresponded to nodal diameters, other more complex modes were also identified on the blades. Nevertheless, a significant disagreement was found when comparing the experimental results with the numerical predictions from a similar but not exactly equal Francis geometry, which confirms the need to scan the actual shapes and dimensions of the runner in order to obtain reliable simulated results.