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Research Papers: Gas Turbines: Structures and Dynamics

Tribological Optimization of Thrust Bearings Operated With Lubricants of Spatially Varying Viscosity

[+] Author and Article Information
S. K. Pavlioglou

School of Naval Architecture
and Marine Engineering,
National Technical University of Athens,
Zografos 15710, Greece
e-mail: solon.pav@gmail.com

M. E. Mastrokalos

School of Naval Architecture
and Marine Engineering,
National Technical University of Athens,
Zografos 15710, Greece
e-mail: mytikas@central.ntua.gr

C. I. Papadopoulos

School of Naval Architecture
and Marine Engineering,
National Technical University of Athens,
Zografos 15710, Greece
e-mail: chpap@central.ntua.gr

L. Kaiktsis

School of Naval Architecture
and Marine Engineering,
National Technical University of Athens,
Zografos 15710, Greece
e-mail: kaiktsis@naval.ntua.gr

1Corresponding author.

Contributed by the Structures and Dynamics Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received July 8, 2014; final manuscript received July 18, 2014; published online September 10, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(2), 022503 (Sep 10, 2014) (11 pages) Paper No: GTP-14-1334; doi: 10.1115/1.4028371 History: Received July 08, 2014; Revised July 18, 2014

In the present work, a computational optimization study of thrust bearings lubricated with spatially varying viscosity lubricants is presented, with the main goal of minimizing friction coefficient. In practice, spatial variation of viscosity could be achieved by utilizing electrorheological or magnetorheological fluids. The bearings are modeled as two-dimensional (2D) channels, consisting of a smooth moving wall (rotor) and a parallel or inclined stationary wall (stator), which can be (i) smooth, (ii) partially textured with rectangular dimples, and (iii) smooth and partially hydrophobic. The bearings are considered to be operated with an ideal lubricant that exhibits different values of viscosity in two distinct regions of the fluid domain: a high viscosity area is considered at the channel inflow, with the viscosity acquiring a reference (low) value farther downstream. The flow field is calculated from the numerical solution of the Navier–Stokes equations for 2D incompressible isothermal flow. The bearing geometry is defined parametrically. Three optimization problems are formulated, corresponding to: (I) a conventional smooth converging slider, (II) a parallel slider with artificial surface texturing at part of the stator surface, and (III) a parallel or converging slider with hydrophobic properties at part of the stator surface. Here, the geometry parameters, as well as the increased viscosity value and the corresponding application regime, form the problem design variables. Bearings are optimized for maximum load capacity and minimum friction coefficient. Optimal solutions are compared against corresponding ones for operation with constant viscosity. It is demonstrated that, by using spatially varying viscosity, a substantial reduction of friction coefficient can be achieved, for all optimization problems considered. This decrease is shown to be a consequence of a sharp pressure rise in the high viscosity regime, resulting in a corresponding rise in load capacity, accompanied by a less pronounced increase in wall shear stress, and thus in total friction force.

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Figures

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Fig. 1

(a) Sketch of a thrust bearing pad and (b) corresponding 2D flow domain and details of the finite volume mesh utilized in the present study

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Fig. 2

Sketch of 2D geometry of (a) a smooth converging slider, (b) a parallel slider with artificial surface texturing at part of the stator surface, and (c) a slider with hydrophobic properties at part of the stator surface. In all cases, the area of increased viscosity is highlighted.

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Fig. 3

Mesh study for the case of an optimal hydrophobic slider with spatially varying viscosity

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Fig. 4

Sketch of velocity profile near a solid boundary, for no-slip condition (left), and slip condition with slip length b (right)

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Fig. 5

Smooth converging sliders with spatially varying viscosity: (a) values of load capacity—friction coefficient, color-coded by the viscosity ratio values, for all solutions. Points on the Pareto front are highlighted (X symbols). (b) Details of (a) in the region of low friction coefficient values.

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Fig. 6

Smooth converging sliders with constant and spatially varying viscosity, corresponding to the optimal cases of Table 3: (a) sketch of sliders, with the regime of increased viscosity highlighted, (b) distribution of pressure at the moving wall, and (c) distribution of shear stress at the moving wall

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Fig. 7

Parallel textured sliders with constant and spatially varying viscosity, corresponding to the optimal cases of Table 4: (a) sketch of sliders, with the regime of increased viscosity highlighted, (b) distribution of pressure at the moving wall, and (c) distribution of shear stress at the moving wall

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Fig. 8

Optimal parallel textured sliders with constant (top) and spatially varying (bottom) viscosity: velocity profiles at a number of cross sections. The regime of increased viscosity is highlighted.

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Fig. 9

Hydrophobic sliders with constant and spatially varying viscosity, corresponding to the optimal cases of Table 5: (a) sketch of sliders, with the regimes of hydrophobicity and increased viscosity highlighted, (b) distribution of pressure at the moving wall, and (c) distribution of shear stress at the moving wall

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Fig. 10

Optimal hydrophobic sliders with constant (top) and spatially varying (bottom) viscosity: velocity profiles at a number of cross sections. The regimes of hydrophobicity and increased viscosity are highlighted.

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Fig. 11

Load carrying capacity and friction coefficient versus nondimensional slip length, b*, for hydrophobic sliders with constant and spatially varying viscosity. The two geometries correspond to the optimal cases for b* = 10 (Table 5).

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Fig. 12

Slip velocity at the slider stator versus streamwise coordinate for hydrophobic sliders with (a) constant and (b) spatially varying viscosity, for different values of nondimensional slip length, b*. Here, the geometries of Table 5 are maintained.

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Fig. 13

Optimal hydrophobic slider with spatially varying viscosity: variation of friction coefficient versus the design variables (k, ls, lv, α), in the regime of the friction coefficient optimum

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Fig. 14

Distribution of pressure at the moving wall for the optimal thrust bearings of the present study: smooth converging slider with (a) constant viscosity and (b) spatially varying viscosity; parallel textured slider with (c) constant viscosity and (d) spatially varying viscosity; hydrophobic slider with (e) constant viscosity and (f) spatially varying viscosity

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