Platform Centered Reduction: A Process Capturing the Essentials for Blade-Damper Coupled Optimization

Purpose of this paper is to further the research line of the authors in the direction of developing a tool attractive to designers in the early design stage of the damping of turbomachinery blades by guiding their initial choice of an optimal dry friction damper. The paper shows how, to this purpose, certain reasonable simplifications are introduced in the procedure and in the model, leaving the customary full high fidelity computations to the final design verification analysis.

The key simplifications here considered are:

- the blade neck is modelled with Euler beam finite elements (FE) so to speed up the updating of its dimensions during the optimisation process;

-  the contact forces exerted by the dampers on the two sides of the blade platform are reduced to concentrated forces and moments applied to the platform’s center, associated to its displacements and rotations;

- as an improvement to the model proposed in the paper presented at Turbo Expo 2019, the airfoil is now obtained from a full 3D FE model after a component mode synthesis reduction; this choice is justified by the facts that the airfoil is by large the item with most complex shape and that during the coupled optimization of the damper the airfoil is considered to be of fixed shape.

This hybrid model is then employed in the process where the domains of optimal matching between the damper and the blade is searched for by exploring the influence of blade neck thickness (flexibility), damper shape and damper mass.  Such a purposely simplified process allows a clear identification of links between relevant blade features and response/fatigue life and, at the same time, an assessment of the interplay between blade parameters and damper parameters in determining the modal features and the damping capabilities. It is shown how different optimal solutions may be identified depending on the expected forcing level on the blade.