Properties and Performance of Gas-Expanded Lubricants in Tilting Pad Journal Bearings

Abstract

Lubricants enable proper function and reduce friction in rotating machinery, but they can also contribute to power loss and heat buildup. Gas-expanded lubricants (GELs) have been proposed as tunable mixtures of lubricant and CO₂ under pressure with properties such as viscosity that can be controlled directly in response to changing environmental or rotordynamic conditions. In this work, experimental results of GEL viscosity, gas diffusivity, and thermal conductivity were combined with high-pressure phase equilibrium data to understand how these mixtures will behave in tilting pad journal bearings under a range of industry-relevant high-speed conditions. Simulations were carried out using the experimental data as inputs to a thermoelastohydrodynamic model of tilting pad journal bearing performance. Viscosity could be easily tuned by controlling the composition of the GEL and the effect on bearing efficiency was appreciable, with 14–46% improvements in power loss. This trend held for a range of lubricant chemistries with polyalkylene glycols, polyalpha olefins, and a polyol ester tested in this work. Diffusivity, which drives how readily CO₂ and lubricants form homogenous mixtures, was found to be a function of the viscosity of the synthetic lubricant, with more viscous lubricants having a lower diffusivity than less viscous formulations. Model results for a bearing in a pressurized housing suggested that cavitation would be minimal for a range of speed conditions. Other bearing parameters, such as eccentricity, temperature, and minimum film thickness were relatively unchanged between conventionally lubricated and GEL-lubricated bearings, suggesting that the efficiency improvements could be achieved with few performance tradeoffs.

KEY WORDS:

• Gas-Expanded Lubricants
• TEHD Lubrication
• Phase Behavior
• Tilting Pad Journal Bearings

ACKNOWLEDGEMENTS

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. Additional financial support came from the National Science Foundation (Award No. CBET-0967915) and from the Rotating Machinery and Controls Laboratory at the University of Virginia. Test lubricants were generously donated by Dow Chemical, ExxonMobil, and Chemtura.

Review led by Michael Khonsari