Thin Gas Film Isothermal Condensation in Aerodynamic Bearings

High-speed small-scale turbomachinery for waste heat recovery and vapor compression cycles is typically supported on gas-lubricated bearings operating close to the saturation conditions of the lubricant. Under particular conditions, the gas film might locally reach the saturation pressure with potentially hazardous effects on the performance of the gas bearing. The present work introduces a model based on the Reynolds equation and the development of cavitation modeling in liquid-lubricated bearings for condensing gas bear- ings. The effect of condensation on load capacity and pressure and density profiles is inves- tigated for two one-dimensional bearing geometries (parabolic and Rayleigh step) and varying operating conditions. The results suggest that the load capacity is generally nega- tively affected if condensation occurs. An experimental setup consisting of a Rayleigh-step gas journal bearing with pressure taps to measure the local fluid film pressure is presented and operated in R245fa in near-saturated conditions. The comparison between the evolu- tion of the fluid film pressure under perfect gas and near saturation conditions clearly sug- gests the occurrence of condensation in the fluid film. These results are corroborated by the very good agreement with the model prediction.