Technology Enhancement for Small Recuperated Turboalternator: Thrust Management Approaches

Because of their potential advantages, investments are ongoing in small (<10 kW) recuperated turboalternators for use in small unmanned systems and man-portable generator sets. Unfortunately, bearings are a challenge in this class of machines due to a combination of very high shaft speeds and high axial loads. High shaft speeds are required for small single-stage gas turbines to maintain adequate pressure ratio for acceptable efficiency. High axial loads result from static pressure acting on various faces on the rotor. This load can be offset using a rotating balance piston but sometimes length, cycle penalty and packaging constraints do not permit this. High shaft speed results in very high n x dM and dN values. With oil lubrication there are associated plumbing challenges and volume and weight penalties. In the case of foil bearings, cooling air can penalize the engine cycle. The reduction of rotor thrust has positive implications for reducing heat, mass and parasitic losses of both bearing types while significantly increasing fatigue life and reliability of rolling-element bearings. The objective of this Phase II effort is to integrate the rotor thrust management technique, studied in Phase I, into an existing turboalternator and demonstrate improvements in bearing life.