Resonances Of Dry-friction Damper Turbine Blades Under Multi-harmonic Excitation

One of the common structural failure of blades is due to excessive stresses caused by large amplitude vibrations. To reduce the vibratory stress of blades, the damped blades with integral wire or shroud are widely used by turbine works. As the vibratory energy is dissipated by the dry friction between the dampers, the vibration of blades is reduced. The dynamic characteristics of the dry friction damped blades influenced by the multi-harmonic excitations will be discussed in this paper. Part one, introduction and analysis of the damped mechanism and the excitation force of turbine blades.The mechanism and corresponding models, especially the interface friction, of many kinds of dampers are introduced and analyzed. The analytical friction force model with variation of normal pressure between friction surfaces will be considered in the following analysis. The excitation force is one of the most important elements for research of dynamic characteristics of damped blades. Among various-order harmonics, the harmonic component, which approaches the natural-frequency of blade, can have the main influence on the safety of the blades. The first, second and Z-order harmonics are considered in this paper to analyze the dynamics of blades.Part two, dynamics of the one- and two-dof models. Firstly, the one-dof blade model is studied by the average method. The average equations are determined, and the amplitude-frequency curves in the first and second-order resonances are presented. The influences of the system parameters on the amplitude- frequency curves are discussed. Secondly, taking into account the coupling of flexion-torsion, the two-dof blade model is studied by the harmonic balance method to obtain the response equations. The influences of the system parameters on the amplitude- frequency curves of the first and second-order resonances are discussed. Numerical solutions are used to prove the accuracy of the analytical analyses. The results in this paper show that the dry friction in blades plays a very important role in reducing vibration amplitude of the blades. The normal force between the rubbing surfaces can influence the damping effect of blades. Through the study we found that there is an optimum value for the normal force, under which the vibration of blades can be damped effectively. The damped blades is a nonlinear dynamic system, so its natural frequency can be influenced by external force. Besides, the influence of shear stiffness of rubbing surfaces, the place of the damper on the blades, and the friction coefficient on dynamic characteristics of blades are also discussed.