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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.

This paper presents a revolutionary approach of using a fluid-lubricated bearing for both traditional functions (load support, damping, and heat removal) and to actively control the rotor dynamics of rotating machinery. We will discuss how its use in the design of next generation turbomachinery can yield dramatic benefits. This includes an increase in efficiency, operational life, fault diagnostic, and reductions in machine size, weight, and cost. With the use of hydrostatic instead of hydrodynamic lubrication, traditional lubricants can be replaced by fluids more friendly to the process and environment. In this paper a comparison between the new hydrostatic bearing (Bently ServoFluid™ Control Bearing) and active magnetic bearings (AMB) will be presented. The Bently ServoFluid™ Control Bearing is an active controlled externally pressurized (a hybrid hydrostatic) bearing using fluid restoring force to compensate for rotor-related forces. It has the positive features of rolling element, fluid film and magnetic bearings with fewer negative attributes. The fluid restoring forces provide static and dynamic motion control similar to magnetic bearings, but with significantly larger compensation forces and with higher stiffness control. This revolutionary approach enables machinery owners to identify, understand and compensate for rotor system forces, an improvement over simply using vibration (motion) information for machinery diagnostics. This allows more complete diagnostics and prognostics of machine health. The bearing can be used to apply known perturbation forces to the rotor. Perturbation forces enable the determination of rotor system stiffness, and subsequent changes, thus improving machinery diagnostics. It can also provide information, such as the mechanical parameters governing the motion, system linearity, and stability margins for more accurate modeling of machines. Test results will be included to show experimentally determined transfer functions of each of the control loop elements, and predicted rotor forces. A typical root locus plot will be shown demonstrating how the characteristics change with bearing stiffness. Prototype machines, with both low viscosity fluid (water) and typical viscous fluid (T-10 turbine oil), have been built, tested, and successfully operated.

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Rotor/seal full annular rub, including synchronous (forward) and reverse (backward) precessions, has been investigated both experimentally and analytically. Of particular interest is the finding of reverse precessional full annular rub (dry whip) that occurs repeatedly in small clearance cases without any outside disturbance. The experimental results include rub triggering mechanism, mass unbalance, and rotative speed effects. A simplified mathematical model is used to interpret experimental results. Nonlinear solutions for both synchronous and reverse precessions are obtained along with instability zones. Mass unbalance effect on shifting from synchronous response to reverse rub and destabilizing factors such as dry friction, rotor damping, and seal stiffness, are discussed.