Modal parameter estimation of turbomachinery in operation taking into account friction damping

Operational modal analysis is used to identify modal parameters of a rotating machine, more specifically an axial compressor. The application of operational modal analysis to rotating machines is considered a challenge, since the rotation disturbs the output measured and can be falsely identified as structural characteristics. Thus, an effective approach to handle the rotor speed harmonics consisting of a modified technique for harmonic removal and a criterion for the identification of harmonics is presented. Additionally, the axial compressor investigated in this work is characterized by numerous joints introducing friction. Thus, localized nonlinear structural behavior needs to be taken into account, when determining the sought-after modal parameters. In order to deal with the friction influence leading to varying eigenfrequencies and damping ratios with increasing response amplitudes, an operating point dependent modal parameter identification process is proposed. The damping values, which are identified in operation, decrease by up to one order of magnitude from the modal damping in standstill condition. These results demonstrate the relevance of an operating point dependent analysis of the structural behavior of rotating machines.