Nonlinear Vibration by Asynchronous Excitation Force in Friction Damper of Turbine Blade

Turbine blades are used under increasingly severe conditions in order to increase the thermal efficiency of the gas turbines in operation. Friction dampers are often used to reduce the vibration of the blade and improve the plant reliability. Blades designed to generate friction between platforms and dampers have been widely adopted in gas turbines. It is important to predict the vibration characteristics of such damper blades analytically during the design phase, and many analysis methods have been proposed vigorously. The excitation force which induces vibrations to the gas turbine front stage blades mainly includes nozzle wake, potential interaction, and specific harmonic excitation force generated from the number of combustors. It is a general study to deal with resonance passing where the natural frequency of the turbine blade coincides with the frequency of specific harmonic excitation force while increasing the turbine rotation speed. Many analytical approach using single harmonic balance methods have been investigated as a technique to efficiently predict the vibration characteristics of a friction damper with respect to the harmonic excitation force.
However, there are asynchronous components of excitation forces in addition to synchronous components caused by specific harmonic excitation forces in turbine blades. Although the other researches dealing with friction dampers in turbine blades have basically considered only resonance induced by specific harmonic excitation force, asynchronous excitation force are considered at the same time in this paper. In addition, many excitation tests using simulated blade have been conducted, but basically only sweep excitation assuming a specific harmonic excitation force is considered.
In this study, a new method for predicting the characteristics of nonlinear vibration under excitation force including not only synchronous but also asynchronous component force is developed. In order to investigate the effect of asynchronous components, time history response analysis considering nonlinear vibration and experiments using simulated turbine blades were conducted. Analysis results showed that the friction damper slipped even at low synchronous excitation force by the existence of the asynchronous excitation force components. This is because friction slipping occurs when the relative vibration amplitude of entire frequency band exceeds the limit. Moreover, it is possible to predict vibration characteristics considering friction slipping by estimating the ratio of overall excitation force to single harmonic excitation force.
In order to verify the effect of asynchronous excitation force and the validity of the proposed correction method, verification tests using simulated blade were conducted. The experiment results show that friction slipping occurred by small harmonic excitation force when there was asynchronous excitation force and provide good agreement with the analysis results. Moreover, the validity of the proposed method which corrects the prediction analysis of the single harmonic method is confirmed. Therefore, this paper shows that it is necessary to consider the asynchronous excitation force in the friction damper blades of the turbine blades when not only the harmonic excitation force but also the considerable asynchronous excitation force is generated.