LectureMinimizing fluid induced forces through the secondary leakage flow paths in shrouded centrifugal impellers is crucial to improve the rotordynamic stability of high-pressure centrifugal gas compressors. Swirl brakes (SBs) are widely used in turbomachinery to reduce the fluid induced forces and enhance the system rotordynamic behavior. Prior to the present study, swirl-brakes of a high-pressure multistage compressor were designed using 3D steady-state computational fluid dynamics (CFD) analysis and verified through multiple fullscale closed loop tests for a compressor, operating with and without the SBs. In the present study, a 3D transient CFD analysis is conducted to calculate the fluid induced rotordynamic coefficients of a Teethon- Rotor (TOR) seal and the impeller shroud cavity. Full 360o fluid domains including the impeller, the shroud cavity, and TOR seals with either a swirl-brake or without it are numerically modeled. A mesh deformation technique is employed to follow the whirling rotor motion of the impeller. To evaluate the capability of the transient CFD model, comparisons are made between the prior test results and predictions from the current numerical study. The cross-coupled destabilizing forces as calculated from CFD are then compared with the methodologies recommended by API 617 and a few other researchers in the literature. In addition, the destabilizing forces calculated through the CFD methodology are used in rotordynamic simulations of the rotor, with and without SBs. Comparisons of simulation results to test results and field performance show good qualitative correlation. CFD predicts forces in the secondary flow path (without SBs) that cause early signs of instability observed in the compressor. The same methodology with SBs predicts cross-coupling forces that are benign, which is also confirmed by stable operation of the same compressor when rebuilt with SBs. These results are included in the paper
