2013 IR&D Annual Report

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 utilize a gas, such as the process gas of a compressor, for example, 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 both experimental and analytical. An experimental test rig is capable of measuring frequency-dependent stiffness and damping coefficients of foil gas bearings for journal speeds up to 60 krpm. A key component of the test rig is the ability to excite the journal in forward or backward whirl with the use of a bi-directional rotating inner shaft mechanism. A pressure chamber may permit testing of various gaseous working fluids from sub-atmospheric pressures up to 635 psig. The analytical method applies 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 can allow modeling of the complex structures of foil gas bearings, and it may be general enough to be applied to a wide range of foil gas bearing geometries and extensible to other turbomachinery components such as seals.

Model of a bearing using linearized stiffness and damping coefficients
Figure 1. Model of a bearing using linearized stiffness and damping coefficients. Spring and damper elements represent dynamic behavior of lubricating fluid film in series with mechanical structure of top foils and undersprings.

Accomplishments — The design of the test rig was completed, and all features of the rig were fully demonstrated. The algorithms necessary to extract frequency-dependent stiffness and damping coefficients from the measured data have been tested extensively and have been validated with various test cases. The transient analytical method has been demonstrated on a simplified geometry (plain sleeve bearing, centered whirl) for which other established methods are typically applied due to their simplicity. Comparisons of the new and established methods showed excellent agreement for the simple geometry, and parameter studies of transient time-step resolution and mesh density provided insight to optimal simulation settings. A novel “growing rotor” technique for starting a simulation with preloaded foil bearings was developed, and the ability to model compliant, spring-supported bearing foils in a steady-state simulation was demonstrated.

photos show the test rig demonstration setup
Figure 2. These photos show the test rig demonstration setup. The outer shaft is capable of being driven up to 60 krpm, while the inner shaft is capable of being driven at ± 60 krpm.
Diagram: Fluid-structure interaction (FSI) couples the fluid and mechanical simulation results.
Figure 3. Fluid-structure interaction (FSI) couples the fluid and mechanical simulation results.
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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.