Start-up response of fluid film lubricated cryogenic turbopumps
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Reusable primary power cryogenic turbopumps (TPs) implement externally pressurized fluid film bearings to support the expected large thrust and lateral radial loads. Compact low count part TPs operate super critically at exceedingly high shaft speeds (180 krpm) and with large pressure differentials. Hybrid journal bearings (HJBs) enable smaller and lighter turbopumps through no bearing DN life limitation. The growth of an "all-fluid-filmbearing" technology for reusable and less costly (per launch) TPs demands the development of analytical models and design tools accompanied by the testing of components. The paper presents a computational model for the prediction of the start-up performance of a flexible rotor supported on hydrostatic radial bearings. The transient response of rotor-bearing systems is of importance to determine safe operation and reliable dynamic performance under extreme loading conditions. In the start-up operation of a cryogenic TP, the fluid supply and discharge pressures, as well as the radial loads acting on the bearings, depend on pump rotor speed. The designed aerodynamic performance of the whole turbopump determines the schedule of rotor speed ramp-up. The start-up event is quite short in nature, lasting a few seconds at most. The bearing reaction forces are calculated from the numerical solution of unsteady bulk-flow equations including fluid inertia, flow turbulence, variable fluid properties and thermal energy transport. The equations of motion for the rotor and the fluid film bearings are solved numerically with local linearization at each integration time step. Predictions for the transient start-up response of a test rotor supported on water hydrostatic bearings are presented. The numerical results evidence the effect of rotor mass on the rotordynamic stability of the rotor-bearing system.
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