Effect of Operating Conditions on the Elastohydrodynamic Performance of Foil Thrust Bearings for Supercritical CO2 Cycles

Abstract

In this paper, a quasi-three-dimensional fluid–structure model using computational fluid dynamics for the fluid phase is presented to study the elastohydrodynamic performance of foil thrust bearings for supercritical CO2 cycles. For the simulation of the gas flows within the thin gap, the computational fluid dynamics solver Eilmer is extended, and a new solver is developed to simulate the bump and top foil within foil thrust bearings. These two solvers are linked using a coupling algorithm that maps pressure and deflection at the fluid structure interface. Results are presented for ambient CO2 conditions varying between 0.1 and 4.0 MPa and 300 and 400 K. It is found that the centrifugal inertia force can play a significant impact on the performance of foil thrust bearings with the highly dense CO2 and that the centrifugal inertia forces create unusual radial velocity profiles. In the ramp region of the foil thrust bearings, they generate an additional inflow close to the rotor inner edge, resulting in a higher peak pressure. Contrary to the flat region, the inertia force creates a rapid mass loss through the bearing outer edge, which reduces pressure in this region. This different flow fields alter bearing performance compared to conventional air foil bearings. In addition, the effect of turbulence in load capacity and torque is investigated. This study provides new insight into the flow physics within foil bearings operating with dense gases and for the selection of optimal operating condition to suit CO2 foil bearings.