A Methodology to Characterize Modal Damping in Vibrating Systems: Application to the Case of Gas Turbine Combustion Chambers

Abstract

The present paper proposes a methodology to characterize the modal damping ratio as a function of frequency in vibrating systems, with a focus on gas turbine combustion chambers. Proposed methodology correlates the modal damping ratio to the system modal eigenfrequencies, accurately considering the damping physical effects on the system dynamics and goes beyond the standard proportional damping hypothesis and similar techniques present in the literature. The methodology has been compared under free-free conditions with standard damping ratio laws that can be found in the literature and which may lead sometimes to an overestimation of the real damping ratio values. Experimental data obtained through dedicated ping tests performed on two components of an industrial gas turbine annular combustion chamber have been used for the methodology validation: comparison between simulated and experimental results seems to be encouraging and the proposed methodology for modal damping ratio characterization turns out to be better, with respect to the standard ones, in reproducing not only the experimentally estimated damping law (as a function of the frequency), but also the typical Force Response Functions (FRFs) of the considered vibrating systems. Finally, since the adopted procedure is numerically efficient and based only on low cost experimental techniques, it can be used for describing modal damping in vibrating systems since the early phases of the design, by increasing the design accuracy and reducing times and costs.