An Experimental Facility and Analytical Methodology for Determining Frequency-Dependent Force Coefficients of Foil Gas Bearings, 18-R8189
Principal Investigator
Aaron Rimpel
Inclusive Dates: 10/01/10 – Current
Background — Accurate knowledge of linearized stiffness and damping coefficients of bearings is a critical aspect in the successful design of high-performance turbomachinery. In recent years, improvements in foil gas bearing technology have led to their increasing application in the expanding oil-free turbomachinery market (current applications include air cycle machines, auxiliary power units, automotive turbochargers, micro gas turbines, refrigeration compressors, etc.). Foil gas bearings use a gas, such as the process gas of a compressor, as the lubricant that separates the rotor from the stationary bearing surfaces. Thus, the need for a separate lubrication circuit with seals, as required for traditional oil lubrication, is eliminated. Foil gas bearings are also not limited by precessing-inertia speed limits as with rolling element bearings, nor do they require expensive control systems as with active magnetic bearings. The relatively low damping of foil gas bearings, when compared to oil lubrication, is mitigated through the use of friction damping mechanisms in the compliant support structures within the bearing. Foil gas bearings of various types are the main focus of gas bearing research today, and they are also the most common gas bearings currently found in commercial applications. Despite the growing popularity of foil gas bearings, there is considerable uncertainty regarding their stiffness and damping coefficients.
Approach — The approach used for this project is experimental and analytical. An experimental test rig will be capable of measuring frequency-dependent stiffness and damping coefficients of foil gas bearings for journal speeds up to 60 krpm and for lubricating air supplied at ambient pressures up to 200 psig (although not in the scope of the current project, different gases and higher pressures up to 635 psig may be tested in future work). The analytical method will apply transient fluid-structure interaction (FSI) modeling techniques to simulate the gas film and structural components of the foil gas bearing via coupled computational fluid dynamics (CFD) and finite element analysis (FEA). The transient FSI method will allow modeling of the complex structures of foil gas bearings, and it will be general enough to be applied to a wide range of foil gas bearing geometries and extensible to other turbomachinery components such as seals.
Accomplishments — The design of the test rig was completed, and the construction is nearly complete. Commissioning of the main test rig subsystems (electric motor drive, bi-directional impulse turbine-driven exciter shaft, test enclosure/pressure vessel, instrumentation, etc.) is still in process. The algorithms necessary to extract frequency-dependent stiffness and damping coefficients from the measured data have been tested extensively and are ready for implementation. The transient method has been demonstrated on a simplified geometry (plain sleeve bearing, centered whirl) for which other established methods are typically applied due to the simplicity (steady-state intermediate reference frame simulation). Comparisons of the new and established methods show excellent agreement for the simple geometry. Parameter studies of transient time-step resolution and mesh density have been conducted to provide insight to optimal simulation settings. The next step is to apply the transient FSI method to foil gas bearing geometry, which will be compared to measured data.
