Effect of Cavities on Impeller Aeromechanical Forcing in a Low Pressure Ratio Centrifugal Compressor Stage

Previous experimental and CFD investigation on a GE Oil & Gas centrifugal compressor stage with a vaneless diffuser revealed a complex excitation mechanism caused by aero-acoustic interaction between three blade rows. Recent published studies have indicated that cavities enclosing shrouded impellers may strongly amplify the acoustic excitation of the impeller by Tyler-Sofrin modes. In this current research therefore, the previous CFD study is expanded to include a model of the disk and shroud cavities. A linearized Navier-Stokes frequency sweep in forced response mode shows a well-defined peak in cavity acoustic activity at a certain frequency. It also shows that the inclusion of cavities noticeably increases the modal forcing on the impeller, qualitatively confirming findings in existing literature. A significant difference in modal force magnitude is found between the shroud-disk out-of-phase mode and the in-phase mode, which is consistent with experimental measurements of vibratory response.