Abstract
At part load operation of a Francis turbine, vortex rope at the center of flow of runner outlet often generates vibrations and noises. Recently it was reported that serious vibration measuring in whole turbine passage occurred under special condition. In this study, experiments with a high specific speed model turbine were performed to investigate the phenomenon. The model tests were carried out changing guide vane opening, rotating speed and cavitation coefficient. Pressure fluctuations were measured at 7 points, such as spiral case, wicket gate outlet, upper draft and draft elbow, and FFT analyses were performed to the obtained data to evaluate the interactions. In addition, animations were taken by high-speed video to reveal the relation between the hydraulic instability and motion of the vortex rope. It was confirmed that section profile of the vortex rope had an elliptical form when the hydraulic instability occured.
