Thermo-Elasto-Hydrodynamic lubrication modeling of Tilting Pad Journal Bearings

Sammanfattning: The journal bearing is a critical machine element typically used to support rotating motion in high speed machinery. Through the generation of a hydrodynamic pressure in its thin lubricant film,which is usually in the order of 10-100μmthick depending on the diameter of the journal itself, the bearing is able to withstand large loads, both statically and dynamically, while having a very low rate of wear. It is of course essential that these components provide for a safe operation with as little wear and frictional losses as possible and it is therefore of great interest to develop simulation models of constantly increasing accuracy. Typical relevant quantities when designing a bearing are the load carrying capacity, metal/oil temperature, minimum film thickness, stiffness, damping and power loss. Classical lubrication theory builds upon the Navier-Stokes equations which, with the thin film approximation, can be reduced to a single equation which governs the hydrodynamic pressure build up in the lubricant. Since the problem now has been reduced to solving a single non linear partial differential equation in 2 dimensions, a significant advantage in terms of simulation time compared to the full set of Navier-Stokes equations can be enjoyed with an, in most cases, insignificant error of approximation. However, with time, as the need for bearings capable of operating at higher loads,speeds and with new designs involving more complex geometries, such as, for example, textured surfaces, the applicability of classic thin film theory should not be taken for granted, especially not when there is an increasing amount of turbulence involved.The purpose of the work contained in this thesis is to develop and asses the performance of a state of the art 3D TEHD model using the commercial finite element multi physics software COMSOL Multiphysics. Of special interest is the assessment of the Menter Shear Stress Transport (SST) turbulence model, which is a widely used, standard, 2-equation RANS eddy viscosity model, in predicting characteristic values for a bearing operating in the transition range between laminar and turbulent flow. A comparative study is carried out where the present model is benchmarked against experimental data on a large 4 pad tilting pad journal bearing. The present model is also compared to one of the classic models based on thin film theory. The present model is also used to study the influence of the geometry that constitutes the leading edge groove in a tilting pad journal bearing on the turbulence levels. Finally the possibility of using a shear thinning lubricant for reducing the bearing power loss is investigated. The calculations were all performed using the resources of the super computer cluster at HPC2N at Ume ̊a University. The results clearly show the inadequacy of the SST turbulence model when performing calculations on a bearing operating in the transition range between laminar and turbulent flow. Moreover, the model predicts slightly higher average values of turbulence in a leading edge grooved bearing compared to a conventional one, yet a higher maximum value in the latter.