Design Space of Foil Bearings for Closed-Loop Supercritical CO₂ Power Cycles Based on Three-Dimensional Thermohydrodynamic Analyses

The closed-loop Brayton cycle with supercritical CO₂ (S-CO₂) as an operating fluid is an attractive alternative to conventional power cycles due to very high power density. Foil gas bearings using CO₂ are the most promising for small S-CO₂ turbomachinery but there are many problems to address: large power loss due to high flow turbulence, lack of design/analysis tool due to nonideal gas behavior, and lack of load capacity when they are used for large systems. This paper presents high-level design/analysis tool involving three-dimensional (3D) thermohydrodynamic (THD) analyses of radial foil bearings considering real gas effect and flow turbulence inside the film. Simulations are performed for radial foil bearing with 34.9 mm in diameter and lubricated with CO₂ and N₂ under various ambient conditions up to above 40 bar gauge pressure. The simulation results using the turbulence model still underpredict the measured data in open literature. However, the error between the prediction and measurements decreases as either speed or ambient pressure increases. In addition, general behavior of substantial increase in power loss with ambient pressure agrees with the measured data. The simulation results indicate the importance of detailed THD analysis of the foil bearings for prediction of power loss under severe turbulent condition. A conceptual layout of rotor system for 10 MWe S-CO₂ loop is also presented along with realistic rotor weight and bearing load. A hybrid foil bearing with diameter of 102 mm is suggested for gas generator rotor, and its power losses and minimum film thicknesses at various operating conditions are presented.