In gas and steam turbine applications a common approach to prevent the blades from high cycle fatigue failures due to high vibration amplitudes is the usage of friction damping elements. Besides the intended amplitude reduction this procedure also features some possibly unwanted side effects like a shift in resonance frequencies due to stiffening effects caused by the contact. Thus, as an alternative an eddy current based noncontacting damping concept for the application in turbomachinery is investigated. In this paper two different types of eddy current dampers are considered. Theoretical models for both are established by applying electromagnetic-mechanical theory. The theoretical models are compared with forced response measurements that are performed at a stationary test rig.
In the dynamics of turbomachinery, the mechanical damping of the blading has been the focus of research for the last decades to improve the dynamic performance in terms of high cycle fatigue issues. In addition, an increased mechanical damping can produce a higher flutter safety margin such that stable operation conditions are achievable in a bigger range. Hence, novel damping techniques are considered besides the well known friction based damping devices. In this paper, an extended analysis of the eddy current based damping device for a last stage steam turbine blading presented in GT2009-59593 is conducted. A transient electromagnetic finite element analysis of the eddy current damper is performed, and the resulting damping forces are compared to their analytical solution. Parameter studies are carried out, and equivalent damping factors are calculated. Furthermore, for the validation of the finite element model, a test rig was built that allows for the direct measurement of damping forces when forcing a sinusoidal relative motion. Forced response measurements and simulations are used to demonstrate its dynamic performance and predictability.
Abstract In turbomachinery applications, the rotating turbine blades are subjected to high static and dynamic loads. The static loads are due to centrifugal stresses and thermal strains whereas the dynamic loads are caused by the fluctuating gas forces resulting in high vibration amplitudes, which can lead to high cycle fatigue failures. Hence, one of the main tasks in the design of turbomachinery blading is the reduction in the blade vibration amplitudes to avoid high dynamic stresses. Thus, coupling devices like underplatform dampers and tip shrouds are applied to the blading to reduce the vibration amplitudes and, therefore, the dynamic stresses by introducing nonlinear contact forces to the system. In order to predict the resulting vibration amplitudes, a reduced order model of a shrouded turbine blading is presented including a contact model to determine the nonlinear contact forces. To compute the forced response, the resulting nonlinear equations of motion are solved in the frequency domain using the multiharmonic balance method because of the high computational efficiency of this approach. The transformation from the time domain into the frequency domain is done by applying Galerkin’s method in combination with a multiharmonic approximation function for the unknown vibration response. This results in an algebraic system of nonlinear equations in the frequency domain, which has to be solved iteratively in order to compute the vibration response. The presented methodology is applied to the calculation of the forced response of a nonlinear coupled turbine blading in the frequency domain.
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