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Research Papers: Gas Turbines: Turbomachinery

# Gas-Expanded Lubricant Performance and Effects on Rotor Stability in Turbomachinery

[+] Author and Article Information
Brian K. Weaver

Department of Civil and Environmental Engineering,
University of Virginia,
Charlottesville, VA 22904
e-mail: bkw3q@virginia.edu

Timothy W. Dimond

Rotor Bearing Solutions International, LLC,
3277 Arbor Terrace,
Charlottesville, VA 22911
e-mail: tim.dimond@rotorsolution.com

Jason A. Kaplan

Department of Mechanical and Aerospace
Engineering,
University of Virginia,
122 Engineer's Way,
Charlottesville, VA 22904
e-mail: jak6j@virginia.edu

Alexandrina Untaroiu

Department of Mechanical and Aerospace
Engineering,
University of Virginia,
122 Engineer's Way,
Charlottesville, VA 22904
e-mail: au6d@virginia.edu

Andres F. Clarens

Department of Civil and Environmental Engineering,
University of Virginia,
Charlottesville, VA 22904
e-mail: aclarens@virginia.edu

Contributed by the Turbomachinery Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received September 23, 2014; final manuscript received October 13, 2014; published online December 17, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(7), 072601 (Jul 01, 2015) (11 pages) Paper No: GTP-14-1560; doi: 10.1115/1.4028846 History: Received September 23, 2014; Revised October 13, 2014; Online December 17, 2014

## Abstract

Gas-expanded lubricants (GELs) are tunable mixtures of synthetic oil and carbon dioxide that enable dynamic control of lubricant viscosity during bearing operation. This control can help reduce bearing power loss and operating temperatures while also providing direct control over bearing stiffness and damping, which can enhance rotordynamic performance. In this work, the bearing and rotordynamic performance of two representative high-speed machines was evaluated when different lubricants, including GELs, were supplied to the machine bearings. The machines chosen for this analysis, an eight-stage centrifugal compressor and a steam turbine-generator system, represent a wide range of speed and loading conditions encountered in modern turbomachinery. The fluids compared for machine performance were standard petroleum-based lubricants, polyol ester (POE) synthetic oils, and POE-based GELs. The performance simulations were carried out using a thermoelastohydrodynamic bearing model, which provided bearing stiffness and damping coefficients as inputs to finite element rotordynamic models. Several bearing performance metrics were evaluated including power loss, operating temperature, film thickness, eccentricity, and stiffness and damping coefficients. The rotordynamic analysis included an evaluation of rotor critical speeds, unbalance response, and stability. Bearing performance results for the compressor showed a 40% reduction in power loss at operating speed when comparing the GEL to the petroleum-based lubricant. The GEL-lubricated compressor also exhibited lower operating temperatures with minimal effects on film thickness. GELs were also predicted to produce lower bearing stiffness when compared to standard fluids in the compressor. Rotordynamic results for the compressor showed that the fluid properties had only minor effects on the unbalance response, while GELs were found to increase the stability margin by 43% when compared with standard fluids. The results from the turbine-generator system also demonstrated increases in low-speed bearing efficiency with the use of GELs, though at higher speeds the onset of turbulent flow in the GEL case offset these efficiency gains. Rotordynamic results for this system showed a contrast with the compressor results, with the GELs producing lower stability margins for a majority of the modes predicted due to increased bearing stiffness in the high-speed turbine bearings and negative stiffness in the lightly loaded, low-speed pinion bearings. These results suggest that GELs could be beneficial in providing control over a wide range of machine designs and operating conditions and that some machines are especially well suited for the tunability that these fluids impart.

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## Figures

Fig. 1

Power losses increase drastically as the flow regime transitions from laminar to turbulent flow (adapted from Ref. [4])

Fig. 2

GELs provide real-time control over lubricant viscosity

Fig. 3

Finite element model of the eight-stage compressor

Fig. 4

Finite element model of the steam turbine-generator system: (a) generator, (b) low-speed pinion, (c) high-speed pinion, (d) flexible coupling, and (e) steam turbine (adapted from Ref. [31])

Fig. 5

Reynolds numbers for Pad 2 in the compressor bearings

Fig. 6

Compressor bearing power loss as a function of speed

Fig. 7

Minimum film thickness as a function of speed

Fig. 8

Journal eccentricity ratio as a function of speed

Fig. 9

Critical speed map of the centrifugal compressor

Fig. 10

Unbalance response of the first bending mode

Fig. 11

Compressor stability margin

Fig. 12

Dynamic stiffness (a) and damping (b) coefficients for the compressor bearings

Fig. 13

Power loss as a function of speed in the high-speed pinion bearings

Fig. 14

Reduced operating temperatures in the high-speed pinion GEL case have significant effects on (a) fluid film thickness and (b) eccentricity ratio

Fig. 15

Reynolds numbers for Pad 2 in the coupling-end turbine bearing

Fig. 16

Power loss as a function of speed in the coupling-end turbine bearing shows the onset of turbulent flow at 10,000 rpm

Fig. 17

Mode 5 of the generator and low-speed pinion for the (a) GEL and (b) ISO fluid cases

Fig. 18

Unbalance response of the turbine first bending mode

Fig. 19

Steam turbine stability margin

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