Investigation of the Dynamic Behavior of Underplatform Dampers

Vibrations in turbines, due to high-speed flows through its vanes, are of critical importance since they can lead to high-cycle fatigue failures when their frequency is close to the natural frequency of the system. Therefore, small metallic devices, so-called 'underplatform dampers', are used in order to damp such vibrations. The devices are placed between two subsequent blades and pressed against the platforms by the centrifugal force generated by turbine's rotation: vibrational energy is dissipated by friction due to relative motion at the contact interface. At the design stage and for simulation purposes knowing the normal and tangential contact stiffness is of fundamental importance. These forces can be computed and estimated through a curve fitting approach when experimental data is available. This is the challenge as in the design process such data is not available. The main idea is to develop a different approach where can calculate the stiffness parameters starting from the FE model of the damper and a plane area which simulates the contact with the platform. A Guyan condensation is applied providing the reduced mass and stiffness matrix from the FE model, such that they can easily be handled by MATLAB. The numerical method proposed is then applied at this stage, providing in the end the normal force - approach and tangential force - displacement relation from which we can calculate the required stiffness. The results from our simulation are validated by comparing them with the values obtained via a curve fitting process. Experimental data for this case was available from a company turbine that was fully tested at the LAQ AERMEC laboratory at the Politecnico di Torino. The normal and tangential contact stiffness extracted from the experiments are used as inputs in a non-linear numerical tool such that we can determine the forced response of the system and compare it with the curves obtained experimentally.