Author(s): Kaveh Amiri; Michel J. Cervantes; Berhanu Mulu
Keywords: Bearing load; blade pressure measurement; flow-induced vibration; Kaplan turbine; prototype measurement; spiral casing performance
Abstract: This paper reports on unsteady pressure measurements on the runner blades of a Kaplan turbine model as well as torque and radial load bearing measurements on the corresponding prototype at several operating points to investigate the sources of periodic loads exerted on the runner when operating at the best efficiency point and off design. Pressure measurements on the model runner blades indicated that the spiral casing delivers a poorly conditioned flow to the guide vanes close to the lip-entrance junction, resulting in flow separation on the guide vanes. The asymmetric flow delivered to the runner induces large oscillations with respect to the guide vane passing frequency, runner frequency and its harmonics to the runner blades. The torque measurements on the prototype also revealed an asymmetric flow at the distributor outlet. The bearing radial load measurements performed on the prototype support the torque measurement results. The asymmetric hydraulic loads on the runner result in shaft wobbling, and the oscillatory forces exerted on the blades are transferred to the main shaft and bearings. Another source of oscillating forces exerted on the runner blades is the rotating vortex rope (RVR) formation that occurs at part-load operation of the turbine and induces pressure fluctuations at two sub-synchronous frequencies to the runner.