Thermohydrodynamics of sliding contacts with textured surfaces
Sammanfattning: Hydrodynamic sliding contacts like those found in journal or thrust bearings are frequently encountered in various types of machinery from computers to large turbines. These contacts, involving a variation in film thickness, are used to generate pressure in the lubricant film and separate the surfaces in relative motion. The contacts must carry a load while maintaining friction as low as possible. Environmental and economic concerns require the machines to operate with minimal power consumption. A number of design modifications have been proposed over the years in order to improve performance of such hydrodynamic contacts. There are experimental indications that textured surfaces, composed of a pattern of well-defined identical shapes, can improve hydrodynamic performance. There is therefore a need to understand and explain the effects of textured surfaces on hydrodynamic contact performance. A Computational Fluid Dynamics (CFD) analysis of the flow field can provide such an understanding. The full Navier-Stokes equations are solved using CFD code for both a slider and a journal bearing. Thermal and cavitation effects are considered. Numerical techniques that deform the computational grid in time are used to recalculate the film gap and simulate the motion of a microgroove located on a moving surface. For a texture located in the inlet part of the stationary surface of an inclined slider, the pressure build-up mechanism is investigated. Such a texture decreases losses locally and allows for increased pressure and a higher load carrying capacity. A critical value of the texture depth separates positive and negative effects of inertia on the load carrying capacity of the slider. The texture studied here provides maximum efficiency when its depth is such that the lubricant flow occurs at the onset of recirculation. In 3D, the texture length should be close to the pad length to generate the highest load carrying capacity. Improvements in performance are shown for different operating conditions. One important effect is that the load carrying capacity of the slider can be increased by up to 16% under severe thermal operating conditions. A journal bearing textured with microgrooves on the stationary surface is investigated. The coefficient of friction can be reduced if grooves of suitable depth are introduced. Under light loading (eccentricity ratios less than 0.15), shallow microgrooves (with a depth less than the minimum film thickness) placed in the maximum film region increase the minimum film thickness while reducing the friction force. The load carrying capacity is enhanced based on the same principle as for the inlet textured slider. Under high loading (eccentricity ratios greater than 0.5), deep microgrooves (with a depth greater than the minimum film thickness) placed in the maximum pressure region reduce both friction force and minimum film thickness. Nevertheless, for high loading, the texture provides better reduction in friction force than a smooth bearing operating with a thinner lubricant. A microgroove positioned on the moving surface, i.e., on the shaft of a journal bearing is also studied. A 2D isothermal case is investigated to explain the effect of the microgroove on the pressure profile and the behaviour of the load carrying capacity over one shaft revolution. The microgroove decreases pressure locally at every circumferential position, resulting in a decrease in the averaged load carrying capacity. A 3D thermal case is analysed to show how lubricant transport is modified by a microgroove. The microgroove affects thermal mixing as it carries a greater amount of cold lubricant to regions with high temperatures. This effect, more pronounced with deeper microgrooves, is due to a global flow recirculation inside the microgroove, which improves mixing.
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