Dynamic behavior analysis of a cracked bulb turbine rotor based on acoustic fluid-structural coupling method

Abstract

Bulb turbine is widely used on the utilization of the hydraulic resources with relatively low head and the tidal energy, and it is troubled by fatigue problems. Thus, it is significant to investigate the influence of the crack on the dynamic response of the rotor to enlarge the service life of the bulb turbine. This paper investigates the modal characteristics and the dynamic response characteristics of a bulb turbine rotor, considering the crack at the flange root. A CFD model and seven FEM models are established, and the boundary conditions for flow and FEM analysis are determined. Firstly, the flow characteristics of the bulb turbine are calculated, and the transient pressure loads on the runner are obtained. Then the effect of added mass, gravity and crack size on the modal characteristics and the dynamic response characteristics of the rotor are discussed. The results show that the crack has little effect on the modal characteristics of the rotor and the frequency spectra of the dynamic stress, but will result in the increase of the mean value and the amplitude of the dynamic stress. In addition, the gravity has a greater effect on the dynamic stress than the added mass of the surrounding water. Results of this study helps to understand the dynamic behavior of the cracked rotor, the proposed method could be used for accurate calculation of the dynamic behavior of the rotor, and possible resonance and fatigue failure problems in future designs of bulb turbine units could be avoided.