Abstract
Friction dampers are used to reduce vibration amplitude of turbine blades. The dynamics of these assemblies (blades + dampers) is nonlinear and the analysis is challenging from both the experimental and the numerical point of view.The study of the dynamics of blades with a tip damper is the aim of the present paper. The blades with axial-entry fir tree attachment carry a damper in a pocket between the blade covers. Pin dampers significantly affect the resonance frequency of the first blade bending mode and introduces non linearity due to friction contacts.A test rig, made of two blades held in a fixture by an hydraulic press with one damper between the blades was used for the experimental activity. Three different types of dampers (cylindrical, asymmetrical, wedge) have been experimentally investigated and experiments have shown that asymmetrical damper performs better than the others.The response of the blades with the asymmetrical damper was then simulated with a non linear code based on the Harmonic Balance Method (HBM). In the analysis, both the blade and the damper are modelled with the Finite Elements and then the matrices reduced with the Craig- Bampton Component Mode Synthesis (CB-CMS), while the periodical contact forces are modelled with state-of-the-art node-to-node contact elements. Numerical analysis has shown a strong influence of the actual extent of the contact area on the dynamics of the assembly. A model updating process was necessary. In the end, the numerical predictions match very well with the experimental curves.
Highlights
Due to the aggressive conditions during operation the forced vibrations of the blades in power turbines still represent one of the most common causes of failure
A part from the dampers, modelled in the numerical simulation, the other main damping source is represented by the blade root joints
The case of tip dampers, as the one investigated in this work, is even more critical than underplatform dampers, due to the location of the damper that increases the effect of the damper on the blade dynamics
Summary
Due to the aggressive conditions during operation the forced vibrations of the blades in power turbines still represent one of the most common causes of failure. Published under licence by IOP Publishing Ltd contribution) and in the resonant amplitude (damping contribution) due to the friction contact Most of these papers deal with the modelling of underplatform dampers (UPD), which are metal devices compressed by the centrifugal force under the blade platforms which dissipate vibrations. For every level of centrifugal force, the following test procedure was used: 1) application of the dead weights on the loading plates (Fc); 2) excitation by the shaker (stepped sine) of the blade #1 at the different excitation force amplitude (Fe), at each frequency step the excitation force amplitude is controlled and kept constant at the target value (with a tolerance of about 1%); 3) disassembly of the damper from the blade, cleaning of the damper; 4) repetition of point 1) to 3) for three times. A stiffening effect is observed and the resonance frequency moves to the range of 2500-3000 Hz. The forced response on the assembly with the damper was measured in order to determine the best performing damper.
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