Rotating turbine blades in turbomachinery are subject to high loads, including centrifugal loading, fluctuating aerodynamic force and high temperature. Thus, the structure is susceptive to resonance or overlarge forced response with the wide excitation frequency range and weak material damping, which can result in high cycle fatigue (HCF) failure. In order to reduce the risk of failure, it is necessary to decrease the vibration stress during the design phase. Friction damping is an effective vibration reduction technology by adding contact interfaces to dissipate the vibration energy. Underplatform dampers are widely used as friction damping structures in the field of turbomachinery.

This paper proposes a method for damping evaluation based on Energy Method, and verifies it by calculating the nonlinear forced response. From the perspective of damping ratio of the damper and vibration stress of the blade, the method avoids computing the time-consuming nonlinear vibration response in which the excitation during engine operation needs to know in advance. This method is conducted during the damper design phase, and the design objective is that the damper can provide a larger damping ratio on a given range of vibration stress if resonance occurs.

For the high-pressure turbine blades with semi-circular underplatform dampers, two 1D macro-slip friction models are firstly introduced, constant normal load and variable normal load. The corresponding hysteresis curves are derived respectively and the contact stiffness is calculated by Finite Element (FE) Method. Besides, micro-slip friction contact model is also adopted for comparison, which shows that it can precisely depict the damping effect with the small vibration stress of blade. Secondly, the relative motion on the contact surface between damper and adjacent blades is analyzed. Based on a specific eigen-mode, the relative displacement is given according to the proportional relationship between the modal and vibration parameters. The equivalent damping ratio of dampers is formulated based on the Energy Method. Thirdly, numerical results applied on the turbine blades were presented, and the characteristic curve of damping ratio for underplatform damper was obtained. Different from the performance and optimization plot, this curve was the relationship of damping ratio on the vibration stress. The damper can be optimized to provide a better damping on a given vibration stress. Nevertheless, the feasibility of the proposed method was verified by comparing with the method of calculating the nonlinear forced response. Finally, the effects of design parameters on the damping ratio were studied. It showed that under different rotating speed, the design parameters such as damper mass, contact stiffness and friction coefficient can improve the damping quality. The optimum damper mass and peak damping ratio under design condition was given with the allowable vibration stress. For a given allowable vibration stress amplitude and target damping ratio, the range of values for the corresponding design parameters can be calculated.


An Energy Method for Assessing the Damping of Turbine Blade Underplatform Damper and Forced Response Verification

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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