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Closed Brayton Cycle (CBC) turbine generators are a leading power conversion candidate for long life space missions. In these systems, a recirculating gas is heated by nuclear, solar or other heat energy source then fed into a high speed turbine which drives an electrical generator. For closed cycle systems such as these, the working fluid also passes through the bearing compartments thus serving as a lubricant and bearing coolant. Compliant surface foil gas bearings are well suited for the rotor support systems of these advanced turbines. Foil bearings develop a thin hydrodynamic gas film which separates the rotating shaft from the bearing preventing wear. During start-up and shut down when speeds are low, rubbing occurs. Solid lubricants are used to reduce starting torque and minimize wear. Unlike fixed geometry rigid gas bearings, foil bearings are flexible and readily accommodate misalignment, thermal gradients and other system effects. Further, their stiffness and damping properties can be tailored for specific applications. In this paper the use of foil bearings as the rotor support system for a CBC will be reviewed. Bearing performance in inert gas and the bearing integration process will be discussed. In addition, a proven four step bearing-rotor integration process will be introduced which includes rotordynamic studies, bearing development and testing, experimental rotor simulation tests and finally system level turbine tests. |
