Anti-Swirl Arrangements Prevent Rotor/Seal Instability

Fluid involved in rotative motion (fluid “swirl”) within seals of high speed fluid handling machines is a recognized source of rotor instability. This type of instability leads to the limit cycle of rotor lateral precessional self-excited vibrations of a subsynchronous frequency. The anti-swirl concept is based on the injection of an additional flow to the seal, in the tangential direction, opposite to the direction of the shaft rotation. This flow causes a decrease of the shaft rotation-generated circumferential velocity of the fluid, and improves rotor/seal stability. In this paper the anti-swirl concept is outlined. The mathematical model of the rotor/seal system is analyzed. The analysis and experimental tests establish the physical basis of the anti-swirl concept. The fluid force model, based on the fluid circumferential average velocity of the flow, proves to be an adequate way to represent the seal fluid dynamic forces.