Multiscale & Multiphysics Modelling of Thrust Pad (Air) Bearings

Abstract: Without lubrication, machines are not imaginable to perform over a long period of time and complete their designated operations. With its omnipresent availability, the air is capable of functioning as a lubricant in long operations very efficiently. Moreover, thrust bearings support axial loads and transmit power at the same time under heavy loads. Therefore, to provide separation under heavy loads in lubricated rotating devices such as thrust pad bearings keeping the power losses at a minimum, film thickness and pressure distribution are very important to investigate at the bearing interfaces. Thrust pad gas (air) bearings are being used in very high-speed rotating machines. Usages of these air bearings are increasing nowadays in industries. In this thesis project, simulations of lubricated contacts of a thrust pad air bearing are performed utilizing multiphysics phenomena and surface textures as mathematical functions. Structural mechanics and fluid mechanics physics are used to model multiphysics functionality. Ideal surface texture models defined by mathematical functions are utilized. More efficient techniques such as homogenization techniques to model the influences of surface roughness are introduced for multiscale study. The current work also presents the Reynolds equation for incompressible and iso-viscous Newtonian fluid flow and formulation for a stationary study. The air bearing with three pads is presented and a virtual twin of this model is built for simulation in COMSOL Multiphysics software. Simulation results are obtained using a single pad from the air bearing considering periodicity of the mathematical formulation. Numerical solutions for pressure build-up and film thickness distributions are achieved from a stationary study performed in COMSOL Multiphysics. MATLAB is used for rigid body solutions. Numerical verification is carried out between the rigid body solutions from MATLAB and fluid physics solutions from COMSOL Multiphysics only for the simulations with tilting pad configuration. Obtained rigid body solutions are also compared to the trends of thrust pad bearing design diagrams to verify the modelling approach and the results. A tilting pad lubricating configuration is used for the thrust pad bearing first. Then pocket geometries for optimization of the bearing pads are explored. For that purpose, separate digital models of the bearing pad are built in COMSOL and analysed for the best performances. Material properties of steel AISI 4340 and Polylactic Acid (PLA) material are used to model virtual bearing pads. To understand the performance of the bearing better, its performance parameters such as load carrying capacity (LCC), relative power loss, and coefficient of friction torque (COT) solutions from the simulations of lubricated contacts of the thrust pad air bearing are analysed. To characterize the performance of the bearing, dimensionless LCC, relative power loss, and COT are explicitly formulated and computed from the pressure and film thickness solutions obtained in the simulations. Relative power loss and COT are resulted from the development of shear stresses in the lubricating fluid due to motion. Parametric analysis is also performed for these parameters in COMSOL Multiphysics. Additionally, performances of several pocket geometry design configurations are also analysed for the best values reached such as the maximum LCC. Pockets with shallower depths are found to have provided higher LCC in general than deeper pocket geometries and plane pads with tilting pad lubricating configuration. Finally, a physical model of an air thrust pad bearing with 3D-printed bearing segments made of PLA material is tested. The physical bearing performed very well in achieving full film separation in the test.