Reports: DNI950554-DNI9: Gas Expanded Lubricants - Improving Energy Efficiency Using 'Smart' Fluids

Andres F. Clarens, PhD, University of Virginia

The goal of this research is to develop a novel lubrication technology that can adapt to changing conditions, minimize power losses, and provide for optimal performance while reducing environmental impacts. This technology, called gas expanded lubricants or GELs, consists of binary mixtures of synthetic lubricant and dissolved CO2. These mixtures can have their properties varied in real time to minimize power losses. The focus of this work is guided by two primary research objectives: (1) Characterize the relationship between GEL composition and bulk properties and (2) Establish the influence of GELs on gear behavior. In order to fulfill these objectives, two principal research tasks have been defined. During the second year of the project period, significant progress has been made in the development of both of these tasks, outlined below.

Task 1. Measure the phase stability and viscosity of a PAG-CO2 GEL formulation

Over the past two years, laboratory efforts have been focused on characterizing the chemical and physical properties of GELs under a variety of bearing-relevant conditions. In Year 2, the phase behavior of the mixtures was characterized with a goal of compiling results and inputting them to an industry standard model of bearing performance. The results from this analysis are the basis for a paper under review in Tribology Transactions. A brief overview of some of the data that was collected is presented here.


CO2 diffusivity into a variety of representative lubricants was measured and the resulting diffusion coefficients are plotted as a function of lubricant viscosity in Figure 1. These results indicate greater diffusivity of CO2 into the lubricants with lower viscosities. Polyalkylene glycols, polyalpha olefins, and ester-based synthetic lubricants of the same viscosity have also been found to have the same degree of CO2 diffusivity. These results are consistent with our hypothesis and they will provide valuable practical understanding in the design of gas exchangers and other unit operations needed to support GEL technology deployment.

Figure 1. Carbon dioxide diffusion coefficients as a function of lubricant viscosity.

Oxidative Stability

Oxidative stability of GELs has been measured over the past year and the results suggest that the lubricants are highly resistant to oxidation over time undergoing less oxidation than conventional petroleum based lubricants, even in the presence of high-pressure CO2. Our results showed little change in the viscosity of GELs and synthetic lubricants alone over the six-week period. Modest increases in viscosity were observed for the petroleum-based lubricants over the same time period and under similar temperature conditions. Visually, the GELs showed no change during this period while the petroleum-based lubricants quickly became darker in appearance as higher molecular weight sludge formed. CO2 has been shown to accelerate oxidation of other compounds.


Extensive efforts to characterize the viscosity of a library of GELs were initiated in Year 1 and completed in Year 2. The viscosity of various mixtures of lubricant and CO2 was measured for a range of pressure and temperature conditions. Different base viscosities were selected to illustrate the effect that CO2 can have on these mixtures. Mass fraction values were obtained by volumetric readings on the carbon dioxide pump and mass balance calculations. The experimental data shows a good fit with an Arrhenius type mixing relationship. This mass fraction-viscosity relationship has been found to accurately predict the viscosity of GELs of all lubricant types and base viscosities for a wide range of applicable carbon dioxide concentrations. The viscosity-composition data being generated here will form the basis for the controllers that will be developed to regulate the composition of the GELs being delivered to the bearing.

Task 2. Develop an experimental testbed to measure bearing performance using GELs

During Year 2, we completed the design and began construction and assembly of the mechanical test rig that will be used to measure the performance of GELs in tilting pad bearings. The only remaining design detail that we are finalizing is related to the high pressure seals that will be used to maintain the high pressure in the bearings. These have to be custom built and we are currently conducting computational fluid dynamic (CFD) testing of the seal design. These CFD simulations are providing us with leakage rates and we expect that the design will constitute novel contributions for the literature since no off-the-shelf technology currently exists to maintain high pressures under the speeds we expect to test.  The test rig will use a 10 hp motor capable of reaching 10,000 rpm to drive a 1.5" rotor of approximately 42" in length. This rotor will be supported by two tilting-pad journal bearings supplied by Lufkin-RMT, Inc., who is currently working to manufacture the individual components. A major focus this past year has been on developing the gas and lubricant delivery system that will generate the GELs and supply them to the bearings. Lubricant will be fed into the system at a rate of 2 gpm from an oil reservoir while CO2 will be provided by one or more liquid CO2 canisters. Two pumps will supply these mixtures to each housing. After exiting the bearing housing, the lubricant will flow back into the reservoir where CO2 can be removed in an open-loop design. The design of this delivery system was completed during this second year of the project and the system has been received, pictured below in Figure 2. The installation of this system is currently taking place as we prepare to receive the rest of the test rig. The installation of the test bed and preliminary experiments will be completed in the final year of the project.

Description: Final Pictures1.JPG

Figure 2. Delivery system, built in Year 2, for pumping GELs to the test rig bearings.