Numerical and Experimental Damping Assessment of a Thin-Walled Friction Damper in the Rotating Set-Up With High Pressure Turbine Blades

Under-platform friction dampers are preferably solutions for minimizing vibrations of rotating turbine blades. Solid dampers, characterized by their compact dimensions, are frequently used in real applications and often appear in patents in different forms. A different type of the friction damper is a thin-walled structure, which has larger dimensions and smaller contact stresses on a wider contact area in relation to the solid damper. The damping performance of a thin-walled damper, mounted under the platforms of two rotating, freestanding high pressure turbine blades is investigated numerically and experimentally in this paper. The tangential and normal contact stiffness, that are crucial parameters in optimal design of each friction damper, are determined from three-dimensional finite element (FE) computations of the contact behaviour of the thin-walled damper on the platform including friction and centrifugal effects. The computed contact stiffness values are applied to non-linear dynamic simulations of the analysed blades with the friction damper of a specified mass. These numerical analyses are performed in the modal frequency domain with a code, which is based on the Harmonic Balance Method (HBM) for the complex linearisation of friction forces. The blade vibrations are characterised by a set of the lowest FE mode shapes of one freestanding blade without damper. The dynamic results of the calculated blades with the damper are in good agreement with the measured data of the real mistuned system. In the analysed excitation range, the numerical performance curve of the thin-walled damper is obtained within the scatter band of the experimental results. For the known friction coefficients and available FE and HBM tools, the described numerical process confirms its usability in the design of under-platform dampers.