Numerical Analyzes and Experimental Investigations of Air Foil Journal Bearings

Self-acting air foil journal bearings are an important component in high-speed, oil-free turbomachinery applications. For rotors supported in air foil journal bearings, subharmonic vibrations and fluid-film induced instabilities of the rotor-bearing system may occur. In order to decrease the amplitude of these subharmonic vibrations and to increase the onset speed of the fluid-film induced instabilities, a preload is often introduced in air foil journal bearings. In this study, the effect of an assembly preload on the thermo-elasto-gasdynamic behavior of a three-pad air foil journal bearing is investigated numerically and experimentally. The 3D thermo-elasto-gasdynamic bearing model comprises the description of bump and top foil deflection and the calculation of the pressure and temperature distribution in the air film as well as the temperature distributions in the surrounding structure, namely in the rotor, in the bearing sleeve and in the top and bump foil. Temperature distributions in the air film and in the surrounding structure are obtained from the 3D energy equation and appropriate heat equations. To reduce the computational effort for the solution of the governing system of nonlinear integro-partial differential equations, different approaches for a mathematical reduction of the 3D energy equation and the generalized Reynolds equation are applied and compared with respect to computation time and accuracy. The governing equations are solved by the finite element method. In order to validate the numerical results, different experiments are accomplished on two different test rigs. Using the first test rig - the hysteresis device - hysteresis curves of differently preloaded three-pad air foil journal bearings are measured at zero running speed. The numerical model of the elastic foil structure is validated by a comparison between measured and predicted hysteresis curves, foil structural stiffness and frictional loss (damping). On a second test rig - the automated high-speed test rig - the effect of the assembly preload on the drag torque and on the thermal behavior of three-pad air foil journal bearings is investigated experimentally. For the purpose of validating the thermal model of the preloaded three-pad air foil journal bearing, measured and predicted bearing temperatures are compared