58835 - Surrogate Models for the Prediction of Damping Ratios in Coupled Acoustoelastic Rotor-Cavity Systems 

The oil and gas, chemical, and process industries employ centrifugal compressors for a wide range of applications. Due to this, the conditions, under which centrifugal compressors have to operate, vary significantly from case to case. Gas pipeline compressors, for example, may feature discharge pressures well over 100 bar. In other fields of application, like gas injection for enhanced oil recovery, discharge pressures over 600 bar and gas densities over 300 kg/m^3 are not uncommon. During the last decades, comprehensive research was conducted on the impact of high pressure operating conditions on the vibrational behavior of centrifugal compressor wheels. In multiple studies, acoustic modes building up in the side cavities were found to be a potential source of high cycle fatigue in radial compressors. Nowadays, it is well-known that an increase in gas pressure levels leads to a more pronounced fluid-structure interaction between the side cavities and the impeller resulting in a frequency shift of the acoustic and structural modes.

For the safe operation of compressors, it is necessary to predict these coupled natural frequencies accurately. The state-of-the-art approach to achieve this objective is the finite element method. In a recently published paper, the authors presented a generalized model to predict the natural frequencies and mode shapes of acoustoelastic rotor-cavity systems. This approach reduces the computational cost significantly while retaining the accuracy of a finite element simulation. So far, the model was only validated using measurement data of an impeller at standstill under varying cavity pressures. In this study, the authors show that the generalized model can predict the natural frequencies of rotating systems with sufficient accuracy by using measurement data of a disk spinning at multiple rotational speeds in a cylindrical cavity.

As it is not always possible to avoid operating close to or accelerate through a resonance of the compressor, it is crucial to know the damping present within the system that limits the amplitudes for a given excitation force. While many studies focus on the identification of damping ratios in axial turbomachines, only a few publications concentrate on the damping of radial impellers. Therefore, the authors present measurement data acquired from the test rig at University Duisburg-Essen, Chair of Turbomachinery, which reveals the damping behavior of a spinning disk under varying operating conditions. Three surrogate models are proposed to predict the identified damping behavior. The first one is based solely on a one-dimensional piston model. The second approach uses an enhanced version of the generalized method, while the third one is a combination of both. After deriving these three models, the measurement data is used to validate the surrogate systems. The paper concludes with a discussion of the measurement results and the benefits and limitations of the proposed models.