Simulation and modeling of the hydrodynamic, thermal, and structural behavior of foil thrust bearings

A simulation and modeling effort is conducted on foil gas thrust bearings. A foil bearing is a self acting hydrodynamic device capable of separating stationary and rotating components of rotating machinery by a film of air or other gaseous "lubricant". Although simple in appearance these bearings have proven to be complicated devices. They are sensitive to fluid structure interaction, use a compressible gas as a lubricant, may not be in the fully continuum range, and operate in the range where viscous heat generation is significant. These factors provide a challenge to the simulation and modeling task.; The conservation equations of mass, momentum, and energy are applied to the problem. The traditional Reynolds equation is developed with the addition of a Knudsen number effect due to thin film thicknesses. The energy equation is simplified by applying the thin layer assumptions such that fluid properties do not vary through the film. Heat transfer between the lubricant and the surroundings is also taken into consideration. The structural deformations of the bearing are modeled with a single partial differential equation. The equation models the top foil as a thin, bending dominated membrane whose deflections are governed by the biharmonic equation. A linear superposition of hydrodynamic load and compliant foundation reaction is included. The stiffness of the compliant foundation is modeled after a set of discrete springs that support the topfoil. The system of governing equations is solved numerically by a computer program written in the Mathematica computing environment. A generalized hydrodynamic analysis is conducted to systematically analyze each of the individual effects included in the development of the governing equations.; Previous analytic work on foil thrust bearings includes the modeling of the Reynolds equation with an isothermal density model and an additional model to predict top foil deflections. This work finds a substantial difference between bearing performance based on traditional lubricant models and that based on the energy equation model. Qualitative and quantitative comparisons are produced that demonstrate the utility of the current approach, which couples the Reynolds, energy, and structural equations.