Simulation of Static Performance of Air Foil Bearings Using Coupled FEM and CFD Techniques

Abstract

The main objective of the current work is to determine a relationship between the top and bump foil’s geometry and load carrying capacity in a journal compliant air foil bearing. Static and steady state operation is assumed throughout the analysis. A finite element model is adopted in order to investigate the operational characteristics of the specific bearing. Bump foil’s elastic behavior is modeled using two node linear spring elements. During fluid analysis, compressible viscous steady state Navier-Stokes equations are numerically solved. The material used during the structural analysis is Inconel X750 and it is assumed that it has linear and elastic behavior. Thermal effects are not taken under consideration. Constant ambient pressure is applied at the free faces of the fluid as well as no slip condition at the surface of the fluid that faces the top foil. CFD and structural models are solved separately. At the beginning of the analysis CFD problem is solved with the assumption that the top foil has not yet been deformed. After the solution of the CFD problem, the pressure distribution at the surface of the fluid that faces the top foil is applied at the top foil and then the structural problem is solved. Consequently the deflections of the top foil are applied on the corresponding surface of the CFD model and the algorithm continues until convergence is obtained. As soon as the converged solution for the pressure distribution is obtained, numerical integration is performed along the surface of the bearing in order to calculate its load carrying capacity. The same procedure is repeated for different values of bump foil thickness, height and pitch in order to define a pattern that describes the bearing’s load carrying capacity as a function of the geometric parameters of the structural problem. Static bearing performance characteristics, such as pressure distribution, bump foil deflection and load capacity are calculated and presented. Furthermore fluid film thickness, top foil deflection and fluid pressure are investigated as functions of the bearing angle as well as load carrying capacity as a function of the bump and top foil stiffness. Finally, a simple thermal analysis is incorporated in order to estimate the temperature rise in the CFD domain due to viscous heat.