Effect of Foil Geometry on the Static Performance of Thrust Foil Bearings

Abstract

Gas foil bearings use ambient air as working fluid, which forms the hydrodynamic wedge between the rotor surface and the compliant foil structure, therefore they are oil-free and environmental friendly. Compared to traditional bearings, gas foil bearing can operate in extreme conditions such as high temperature and high rotating speed. They also provide better damping and stability characteristics and have larger tolerance to debris and rotor misalignment. Gas foil bearings have been successfully applied to micro and small sized turbomachinery, such as micro gas turbine and cryogenic turbo expander. In the previous decades, a lot of theoretical and experimental work have been conducted to investigate the properties of gas foil bearings. However, very little work has been done to study the influence of the foil bearing pad geometry configuration using computational fluid dynamics (CFD). This study proposes a robust approach to analyze the effect of the foil geometry on the performance of a six pads thrust foil bearing. In this study, a 3D CFD model for a parallel six pads thrust foil bearing is built using ANSYS-CFX software. The full Navier-Stokes equations are solved in the fluid domain. Taking into account the rotational periodicity, the computational model is simplified and only one pad of the thrust foil bearing is analyzed. In order to predict the thermal property, the total energy with viscous dissipation is used. Practical boundary conditions are used in the baseline CFD model. The results of the baseline modes is further compared with a modified Reynolds equation solution. Based on this model, the geometry of the thrust foil bearing is parameterized and analyzed using the design of experiments (DOE) methodology. In this paper, the selected geometry parameters of the foil structure include: minimum film thickness, inlet film thickness, the ramp extent on the inner circle, the ramp extent on the outer circle, the arc extent of the pad, and the orientation of the leading edge. A factorial design technique is employed to sample the design space in DOE. The objectives in the sensitivity study are selected as load force and friction torque. An optimal foil geometry is derived based on the results of the DOE process by using a goal driven optimization technique to maximize the load force and minimize the friction torque. The results show that the geometry of foil structure is a key factor for foil bearing performance. The numerical approach proposed in this study is expected to be useful from the thrust foil bearing design perspective.