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Research Papers: Lubricants

# Feasibility of Gas-Expanded Lubricants for Increased Energy Efficiency in Tilting-Pad Journal Bearings

[+] Author and Article Information
Andres Clarens, Shibo Wang

Civil and Environmental Engineering, University of Virginia, Charlottesville, VA 22904

Amir Younan, Paul Allaire

Mechanical and Aerospace Engineering, University of Virginia, Charlottesville, VA 22904

J. Tribol 132(3), 031802 (Jul 21, 2010) (8 pages) doi:10.1115/1.4001648 History: Received October 19, 2009; Revised April 19, 2010; Published July 21, 2010; Online July 21, 2010

## Abstract

Lubricants are necessary in tilting-pad journal bearings to ensure separation between solid surfaces and to dissipate heat. They are also responsible for much of the undesirable power losses that can occur through a bearing. Here, a novel method to reduce power losses in tilting-pad journal bearings is proposed in which the conventional lubricant is substituted by a binary mixture of synthetic lubricant and dissolved $CO2$. These gas-expanded lubricants (GELs) would be delivered to a reinforced bearing housing capable of withstanding modest pressures less than 10 MPa. For bearings subject to loads that are both variable and predictable, GELs could be used to adjust lubricant properties in real time. High-pressure lubricants, mostly gases, have already been explored in tilting-pad journal bearings as a means to accommodate higher shaft speeds while reducing power losses and eliminating the potential for thermal degradation of the lubricant. These gas-lubricated bearings have intrinsic limitations in terms of bearing size and load capacity. The proposed system would combine the loading capabilities of conventional lubricated bearings with the efficiency of gas-lubricated bearings. The liquid or supercritical $CO2$ serves as a low-viscosity and completely miscible additive to the lubricant that can be easily removed by purging the gas after releasing the pressure. In this way, the lubricant can be fully recycled, as in conventional systems, while controlling the lubricant properties dynamically by adding liquid or supercritical $CO2$. Lubricant properties of interest, such as viscosity, can be easily tuned by controlling the pressure inside the bearing housing. Experimental measurements of viscosity for mixtures of polyalkylene glycol $(PAG)+CO2$ at various compositions demonstrate that significant reductions in mixture viscosity can be achieved with relatively small additions of $CO2$. The measured parameters are used in a thermoelastohydrodynamic model of tilting-pad journal bearing performance to evaluate the bearing response to GELs. Model estimates of power loss, eccentricity ratio, and pad temperature suggest that bearings would respond quite favorably over a range of speed and preload conditions. Calculated power loss reductions of 20% are observed when compared with both a reference petroleum lubricant and PAG without $CO2$. Pad temperature is also maintained without significant increases in eccentricity ratio. Both power loss and pad temperature are directly correlated with $PAG-CO2$ composition, suggesting that these mixtures could be used as “smart” lubricants responsive to system operating conditions.

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## Figures

Figure 1

The viscosity of a lubricant-gas mixture can be lowered by several orders of magnitude by controlling the composition using system pressure. These data, for PAG+CO2 binary mixtures (at 35 °C), are from Refs. 14,16.

Figure 2

Schematic of lubricated bearing systems under a conventional static setup (a) and in the presence of a GEL (b) where pressurized gas/lubricant mixtures can be delivered in response to variable operating conditions

Figure 3

Schematic of tilting-pad journal bearing with process parameters used in the modeling framework reported here

Figure 4

Published modeling and experimental results demonstrate the effectiveness of the model in predicting journal temperature (left) and eccentricity (right) data published in Ref. 24

Figure 5

Viscosity of PAG-CO2 mixtures as a function of mixture composition at 40°C and 100°C. The measured results were obtained experimentally while the predicted value is obtained using Eq. 1. The modeled values were input to the TEHD model of bearing performance.

Figure 6

Phase behavior of PAG-CO2 mixtures as a function of mixture composition with GEL regions highlighted for several relevant temperatures. Experimental results are adapted from Ref. 14.

Figure 7

Power loss as a function of speed. Efficiency improvements of nearly 20% are observed over a range of operating speeds when using GELs compared with a reference lubricant.

Figure 8

Pad temperature as a function of speed. A desirable reduction in pad temperature can be achieved using GELs due largely not only to the superior heat removal characteristics of the PAG lubricant but also to the reduction in power loss provided by GELs.

Figure 9

Journal eccentricity ratio as a function of speed. The presence of CO2 in the GELs does increase the eccentricity ratio as expected but not outside the normal operating range for this type of bearing.

Figure 10

Bearing power loss under two different preloading conditions suggests that the effect of GELs is independent of bearing geometry/preload

Figure 11

Minimum film thickness as a function of speed for GELs and a reference fluid as compared with the allowable film thickness as estimated by Martin and Garner (29)

Figure 12

Bearing stiffness coefficients for the reference fluid, PAG, and PAG+CO2 under preloads of 0.47 (left) and 0.15 (right)

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