Design Strategy of Thrust Gas Bearings for Refrigerant Compressors Considering Effects of Centrifugal Force and Convective Inertia

This paper studies and compares 3D thermo-hydrodynamic (THD) performance of rigid thrust gas bearings lubricated with R1234ze gas using Reynolds Equation and commercial computational fluid dynamics (CFD) software. The gas bearing consists of six pads operated in both hydrodynamic and hybrid modes at high ambient pressures typical to the bearing compartment of refrigerant compressors. The CFD simulations adopted an advanced turbulence model and Peng-Robinson real gas equation of state while the Reynolds Equation-based THD model adopted NIST Refprop for real gas model. Results from the two models are compared, and detailed observations on gas bearing performance are presented including load capacity, pressure profiles, streamlines, effect of convective inertia and Mach number, etc. Further, the effects of inlet dead volume (IDV) region in such modeling are included to capture complete detailed flow behaviors for the presented high-density refrigerant gas flows within such bearings. The CFD results demonstrate that convective inertia increases load capacity significantly for refrigerant gases. Both models capture temperature drops around the orifice curtain area from isentropic expansion, but CFD predicts more accurate isentropic expansion not only in temperature drop but also in pressure drop. Furthermore, convective inertia effects are not substantial for air bearings at lower ambient boundary pressures but are still significant for refrigerants at low ambient boundary pressures. Another important feature captured from CFD is the possibility of sub-ambient pressure at certain inner and outer boundaries of the bearing due to high Mach numbers of the leakage flows, and resultant sub-ambient static pressures and lower load capacity when ambient pressure is too low. However, such sub-ambient pressure is not prominent in typical operating conditions of refrigerant gases.