Influence of rotation on the modal characteristics of a bulb turbine unit rotor

Bulb turbine units are one of the most installed turbines in run-of river projects with relatively low head. In order to enlarge the useful life of these turbines and avoid fatigue problems and cracks, it is of paramount importance to understand and determine the most relevant parameters and their influence on the dynamic response of the structure. In this paper, the modal characteristics of a bulb turbine unit in operation is numerically investigated, considering the rotation effect. A FEM model including alternator, shaft, runner and fluid is developed and the boundary conditions are determined. Firstly, the modal characteristic of the runner under different blade opening are analyzed. Then the influence of the rotation on the modal characteristic of the shaft and runner is discussed. The numerical method is verified by comparing with experimental results of a rotating and submerged disk. The results show that the runner modes are mainly blade-modes，which can be grouped according to the blade number, one jellyfish mode and four local modes in each group. A modified Campbell diagram of the global modes and a transformation matrix of natural frequency between dry modes and wet modes are proposed. Results of this study helps to understand the most influencing parameters, such as the added mass effect of water combined with the rotation. The proposed modified Campbell diagram could be used for an accurate calculation of natural frequencies and avoid possible resonance problems in future designs of bulb turbine units.