Lubrication of conformal contacts with surface texturing
Sammanfattning: Conformal contacts like those found in journal or inclined slider bearings are widely encountered in various types of machines from small engines to large turbines. These contacts involve convergent gaps that are used to separate the surfaces in relative motion and generate pressure in the lubricant film. The contacts have to carry a load while keeping friction as low as possible. Environmental and economic concerns require the machines to operate with minimal power loss. A number of design modifications have been proposed over the years in order to decrease bearing power consumption. There are indications that surface topography can have a significant and positive influence on hydrodynamic performance. Texturing of a load carrying surface is a process that would be beneficial in lubricated conformal contacts as it would be possible to obtain thicker films and reduced frictional losses. Textured lubricated contacts are analysed with Computational Fluid Dynamics (CFD) code through different geometries. The effects of the texture on hydrodynamic performance of the contact are investigated. For the simulations performed, the full Navier-Stokes equations are solved under steady-state conditions. The flow is taken to be laminar and two- dimensional. A journal bearing with several dimples created on the stationary part is firstly investigated. A realistic multiphase flow cavitation model is introduced and successfully validated. It is shown that the coefficient of friction can be reduced if dimples of suitable depth are introduced. This can be achieved either in the region of maximum hydrodynamic pressure for a bearing with high eccentricity ratio or just downstream of the maximum film for a bearing with low eccentricity ratio. A new effect of pressure build-up, generated by the surface texture, has been identified at low eccentricity ratios. This pressure build-up effect is more extensively investigated through the study of an inclined slider bearing with a texture located at the inlet part of the contact, i.e. just downstream of the maximum film. The governing mechanism at the origin of an optimum in load carrying capacity for a smooth slider bearing is analysed and the effects of the texture on the pressure build-up and load carrying capacity are explained. The energy received by the fluid from the moving wall is converted into pressure in the first part of the converging contact and into losses in the second part. Convergence ratio can be increased until the limit where flow recirculation begins to occur is reached in order to get the greatest pressure gradient. The texture appears to reach its maximum efficiency when its depth is such that the velocity profile is stretched at its maximum without incoming recirculating flow. Thus, the wall profile shape controlling the velocity profile can be optimized for many hydrodynamic contacts. For such contacts, heat is usually produced due to shearing of the flow and the lubricant is subjected to temperature variations. Since the dynamic viscosity of the lubricant is temperature dependent, variation of the viscosity as well as frictional losses and load carrying capacity is expected. Thermal effects are analysed for different shear rates in this inlet textured slider bearing. Shear losses and subsequent heating reduce load carrying capacity compared with isothermal conditions. Texture has a positive effect in a parallel surface contact when thermal effects are considered. It has been found that for the different convergence ratios considered, the texture enables the sustaining of a load only until a certain critical shear rate is reached. This critical shear stress depends on a number of factors such as the convergence ratio and lubricant parameters including the viscosity-temperature coefficient and the reference dynamic viscosity.
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