Impact of Mistuned Underplatform Dampers On the Nonlinear Vibration of Bladed Disks

Before the final experimental validation and certification of a turboengine, designers perform a numerical simulation of its vibratory properties, among other things, in order to estimate its lifespan and adjust the design in an optimization process. One possible practical solution to decrease the vibratory response is to add underplatform dampers to the system. These components dissipate energy by friction and are widely employed in turbomachinery. However, a specific underplatform damper is usually efficient only for a specific mode. The purpose of this work is to investigate the possibility of adding different kinds of underplatform dampers to the cyclic structure in order to decrease the vibratory energy over a larger panel of modes. Different methods exist to determine the vibrations of nonlinear cyclic symmetric systems, but creating a robust methodology to account for the additional effect of mistuning remains a big challenge in the community. In this paper, the structure is mistuned through the friction coefficient of the dampers and not by altering its geometry, as is usually done in the literature. First, assuming a cyclic symmetric structure, the performance of the dampers is assessed for specific modes. Then, employing a method recently developed, the efficiency of an intentional mistuning pattern of underplatform dampers is studied and an optimal pattern proposed.

Impact of Mistuned Underplatform Dampers on the Nonlinear Vibration of Bladed Disks

Before the final experimental validation and certification of a turboengine, designers perform a numerical simulation of its vibratory properties, among other things, in order to estimate its lifespan and adjust the design in an optimization process. One possible practical solution to decrease the vibratory response is to add underplatform dampers to the system. These components dissipate energy by friction and are widely employed in turbomachinery. However, a specific underplatform damper is usually efficient only for a specific mode. The purpose of this work is to investigate the possibility of adding different kinds of underplatform dampers to the cyclic structure in order to decrease the vibratory energy over a larger panel of modes. Different methods exist to determine the vibrations of nonlinear cyclic symmetric systems, but creating a robust methodology to account for the additional effect of mistuning remains a big challenge in the community. In this paper, the structure is mistuned through the friction coefficient of the dampers and not by altering its geometry, as is usually done in the literature. First, assuming a cyclic symmetric structure, the performance of the dampers is assessed for specific modes. Then, employing a method recently developed, the efficiency of an intentional mistuning pattern of underplatform dampers is studied and an optimal pattern proposed.

On the Efficiency of a Conical Underplatform Damper for Turbines

Underplatform dampers (UPDs) are commonly used in aircraft engines to limit the risk of high-cycle fatigue of turbine blades. The latter is located in a groove between two consecutive blades. The dry friction contact interface between the damper and the blades dissipates energy and so reduces the vibration amplitudes. Two common geometries of dampers are used nowadays, namely wedge and cylindrical dampers, but their efficiency is limited when the blades have an in-phase motion (or a motion close to it), since the damper tends to have a pure rolling motion. The objective of this study is to analyze a new damper geometry, based on a conical shape, which prevents from this pure rolling motion of the damper and ensures a high kinematic slip. The objective of this study is to demonstrate the damping efficiency of this geometry. Hence, in a first part, the kinematic slip is approximated with analytical considerations. Then, a nonlinear dynamic analysis is performed, and the damping efficiency of this new geometry is compared to the wedge and the cylindrical geometries. The results demonstrate that the conical damper has a high damping capacity and is more efficient and more robust than the two others.

Best practices for underplatform damper designers

The purpose of this paper is to offer a practical demonstration of how essential preoptimization is in the design of underplatform dampers for turbine blades. Preoptimization can be thought of as a “prescreening” which allows excluding, since the early design stages, a high number of poorly performing damper–platform configurations. This concept, previously presented by the authors, is here extended and its generality for all blade bending modes is rigorously demonstrated. The paper addresses a test case where the introduction of curved-flat underplatform dampers is necessary to avoid a dangerous resonance crossing in the operating rotational speed range of a real turbine disk. It is shown how preoptimized dampers are the only ones that manage to satisfy all functional requirements, including those in the nonlinear operating regime. The same set of dampers may have been identified by exploring, through hundreds of computationally intensive nonlinear calculations, the performance of all possible damper configurations. The latter approach, i.e. iterative design, is unpractical and has to be repeated for each new set of blades since it is based on a test case-specific trial-and-error procedure. Preoptimization substitutes iterations with knowledge of the damper behavior and can therefore be considered as “informed design”: viable damper configurations are instantly singled out through simple but insightful considerations on the damper equilibrium of forces and moments.