Dynamic behavior of hydraulic turbine runners

Resumen

In the hydraulic turbine machinery, the runner is a key component where the energy conversion takes place. During operation, the runner is closed inside a casing full of water and suffers the hydraulic excitations. Many damages were observed on different types of runners and vibration problems were induced to the whole machine, which badly impacted the operation stability and the generation of powerplants. Accordingly, to know the dynamic behavior of the machine, especially the turbine runner, is of high interest for both manufacturers and powerplants. This thesis presents an investigation on the dynamic behavior of different types of hydraulic turbine runners. Numerical simulations were carried out on two runners, one reduced Francis model and one prototype pump-turbine runner. Corresponding experiments were also performed and results were included to verify the numerical method. Hydraulic turbine runner has very complex geometry and works under very complex conditions. These bring some big challenges for the numerical simulation. Overcoming the geometrical complexity, the high quality finite element mesh was built totally by hexahedral elements to couple the structure and fluid domain. With the numerical model verified by corresponding experiments in air and in water, more complex conditions were considered, such as the nearby boundaries and shaft connection. Natural frequencies and modal behavior of the runners were obtained considering the effects of different conditions. The surrounding fluid significantly reduces natural frequencies the runners, depending on the behavior of the mode. This effect may be significantly increased due to the presence of the nearby boundary and constraint conditions. The significance also depends on the behavior of the modes. This surrounding fluid effect was determined both numerically and experimentally. The phenomenon was physically explained using energy principles. A non-dimensional factor was derived, which can be extrapolated to geometrical similar runners independent of the dimensions and materials. This is practically valuable for the runner development. In addition, the different types of runners represent different behaviors, due to the difference of geometrical characteristics. The nodal diametrical modes of the two runners were analyzed and compared carefully. Accordingly, the effects due to the fluid presence and other conditions are also different. Finally, a numerical procedure using harmonic response analysis was developed to determine the dynamic response of the runner excited by rotor stator interaction. The characteristics of excitations were derived based on the theory of rotor stator interaction