Extended Three-Dimensional Thermo-Hydrodynamic Model of Radial Foil Bearing: Case Studies on Thermal Behaviors and Dynamic Characteristics in Gas Turbine Simulator

Abstract

Environment-friendly microturbomachinery has its broad current and future applications in fuel cells, power generation, oil-free industrial blowers and compressors, small aero propulsions engines for missiles and small aircrafts, automotive turbo chargers, etc. Air foil bearings (AFBs) have been one of the popular subjects in recent years due to ever-growing interests in the environment-friendly oil-free turbomachinery. AFBs have many noticeable attractive features compared to conventional rigid-walled air/gas bearings such as improved damping and tolerance to minor shaft misalignment and external shocks. In addition, the low viscosity of air or gas allows very low power consumption even at high speeds. A turbine simulator mimicking 50 kW power generation gas turbine was designed. The turbine simulator can generate the same thermodynamic conditions and axial thrust load as the actual gas turbine. In this paper, the 3-D thermo-hydrodynamic (THD) model developed for single radial AFB was further extended to the turbine simulator configuration by extending the solution domain to the surrounding structures including two plenums, bearing sleeve, housing, and rotor exposed to the plenums. In addition, a computational fluid dynamic (CFD) model on the leading edge groove region was developed for better prediction of inlet thermal boundary conditions for the bearing. Several case studies are presented through computer simulations for hydrodynamically preloaded three-pad radial AFB in the hot section. It is found that both bearing and rotor should be provided with cooling air to maintain the temperature of both the rotor and top foil below 300 °C. It is also found that the higher thermal contact resistance between the rotor and hot impellers reduces the axial temperature gradient of the rotor. Dynamic performance of the bearing was evaluated using the linear perturbation method for operation at elevated temperature. The softening effect of the bump foil at elevated temperature results in a decrease of both stiffness and damping coefficients compared to the values at room temperature.