Gas-Expanded Lubricants for Increased Energy Efficiency and Control in Rotating Machinery

Bearing lubricants are essential to the efficient and reliable operation of high-speed rotating machinery. Lubricant films separate the rotating shaft and the stationary bearing structure to minimize friction and remove heat while providing stiffness and damping forces to the shaft that enable its smooth operation at high speeds. As the demand for larger, faster machines increases along with the need to push existing equipment to new extremes of operation, the need for lubrication technologies that can support these higher demands is also increased. However, the difficulty in this need lies in the fact that bearing lubricants introduce inefficiencies and other performance limitations of their own. The goal of this work is to develop a novel lubrication technology called gas-expanded lubricants (GELs) which stand to provide real-time control over bearing and rotor dynamics while increasing bearing efficiency and reliability, and without comprising the other important functions that lubricants are needed to provide. GELs are controlled by dissolving carbon dioxide into synthetic lubricants at high pressure. The control of mixture composition provides direct control over the lubricant viscosity. Larger machines often require the use of more viscous fluids to support the weight of the rotor, which can then lead to higher bearing losses and heat generation due to the viscous shear of the fluid. Greater lubricant viscosities also tend to increase bearing stiffness which in many cases can be detrimental to the rotordynamic stability of the machine. The tunability that these fluids impart can be used prior to machine startup or shutdown to maximize rotordynamic stability or to maximize bearing efficiency and reduce operating temperatures during steady operation, thereby increasing the life of the bearing. GELs also provide the ability to adapt to dynamic operating conditions as a result of changing environmental or loading conditions. Other lubricant and bearing technologies have been developed in attempts to improve machine performance, but none of them offer the real-time control of bearing and rotor dynamics of GELs without comprising other important bearing properties such as load capacity.
To fully understand the implications of this lubricant technology on bearing and overall machine performance, it was necessary to develop a fundamental understanding of both the properties of these fluids as well as their expected impacts on machine performance and design. This work applied a combination of experimental and modeling techniques to answer questions related to the development and use of GELs in rotating machinery. The viscosity, diffusivity, and thermal properties of GELs were measured and combined with existing data in the literature to fully characterize their behavior. The mixture viscosity was found to be driven by the amount of carbon dioxide dissolved into the fluid while effects on other properties were found to be minor. This knowledge allowed for the accurate prediction of GEL performance in a variety of machinery bearings operating at a range of loads and speeds. It was found that the changes in lubricant viscosity can significantly affect bearing efficiency, operating temperature, and stiffness and damping forces while keeping other bearing metrics such as eccentricity and minimum film thickness within acceptable ranges. The effects on bearing dynamics were also found to have significant impacts on machine rotordynamic performance metrics including rotordynamic stability, though effects on unbalance-driven vibration amplitudes of the rotor were found to be minor. A number of unique design considerations are discussed in the context of two experimental test rigs that were designed and assembled for the characterization of GEL performance in high-pressure bearings and seals. This work provides the foundation for a novel lubrication technology with the potential to significantly increase performance and control in high-speed rotating machinery.