Bulk-Flow Analysis of Hybrid Thrust Bearings for Process Fluid Applications
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Advanced cryogenic fluid turbopumps are very compact, operate at extremely high shaft speeds, and require hybrid (hydrostatic/hydrodynamic) radial and thrust fluid film bearings for accurate rotor positioning. Sound design and reliable operation of fluid film thrust bearings also allows for unshrouded impellers with a significant increase in the turbopump mechanical efficiency. A bulk-flow analysis for prediction of the static load performance and dynamic force coefficients of high speed, angled injection orifice-compensated, hybrid (hydrostatic/hydrodynamic) thrust bearings is presented. The model accounts for the bulk-flow mass, momentum and thermal energy transport, and includes flow turbulence and fluid inertia (advection and centrifugal) effects on the bearing film lands and recesses. The performance of a refrigerant hybrid thrust bearing for an oil-free air conditioning equipment is evaluated at two operating speeds and pressure differentials. The computed results are presented in dimensionless form to evidence consistent trends in the bearing performance characteristics. As the applied axial load increases, the bearing film thickness and flow rate decrease while the recess pressure increases. The axial stiffness coefficient shows a maximum for a certain intermediate load while the damping coefficient steadily increases with load. The computed results show the significance of centrifugal fluid inertia at low recess pressures (i.e. low loads) and high rotational speeds, and which can lead to film starvation at the bearing inner radius and subambient pressures just downstream of the bearing recess edge.
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