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

Significance Statement

Friction and wear in mechanical systems have been steadily gaining attention over the last decade. Contemporary research efforts address the need for development of environmentally friendly mechanical devices, characterized by lower levels of energy consumption, increased wear resistance and decreased maintenance requirements. Thrust bearings are a vital part of modern machinery, designed for supporting axial shaft loads. Hydrodynamically lubricated thrust bearings are widely used in industrial and marine applications, providing high durability, relatively low friction, and good stability characteristics.

Today, there exist several emerging technologies that aim to improve the behavior of thrust bearings. Among them, the proper implementation of surface treatment technologies, such as artificial surface texturing and surface hydrophobicity, have gained substantial attention. In the present work, the novel concept of spatially varying viscosity is combined with these surface treatment technologies, and the effect on the tribological characteristics of thrust bearings is investigated by means of a CFD-based optimization approach. To this end, a CFD code is coupled to an optimization tool based on genetic algorithms. Six different optimization problems are formulated, with the goal of identifying values of the design variables that maximize load carrying capacity and minimize friction coefficient. The results demonstrate that the introduction of spatially varying viscosity may drastically decrease the value of the friction coefficient, for all of the cases investigated.

Tribological Optimization of Thrust Bearings Operated With Lubricants of Spatially Varying Viscosity. Advances In Engineering

 

Journal Reference

S. K. Pavlioglou, M. E. Mastrokalos, C. I. Papadopoulos , L. Kaiktsis. Eng. Gas Turbines Power, 137(2), 022503 (Sep 10, 2014).

School of Naval Architecture, and Marine Engineering,National Technical University of Athens, Zografos 15710, Greece.

Abstract 

In the present work, a CFD-based 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 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 two-dimensional 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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