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

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.

Piston ring performance in two-stroke marine diesel engines: Effect of hydrophobicity and artificial surface texturing on power efficiency

“…The present work builds on top of recent research related to the effect of surface treatment methods on the tribological performance of hydrodynamically lubricated contacts. In particular, two different methods will be evaluated, those of hydrophobicity and artificial surface texturing, which have been applied in journal and thrust bearings giving substantial potential of performance improvement, as it can also be seen in Fatu et al, 13 Pavlioglou et al, 14 and Guo-Jun et al 15 and Etsion et al, 16 Papadopoulos et al, 17 and Xiong and Wang 18 respectively. Hydrophobic surfaces draw their origins from the lotus leaf, and, due to their particular nanostructure, they are wetting resistant, exhibiting low levels of friction during fluid flow.…”

“…Hydrophobic surfaces draw their origins from the lotus leaf, and, due to their particular nanostructure, they are wetting resistant, exhibiting low levels of friction during fluid flow. In order to modify Reynolds equation so as to account for hydrophobic surfaces, the no-slip boundary condition on the fluid-wall interface should be replaced with proper slip boundary conditions, see Fatu et al 13 and Guo-Jun et al 15 Instead of using Reynolds equation, Computational Fluid Mechanics (CFD) simulations can also be used, like in Pavlioglou et al, 14 and it has been confirmed that the corresponding results match sufficiently.…”