Advanced design methods and non-ideal gas lubrication applied to aerodynamic bearings

Clean and energy-efficient turbomachinery are playing an increasingly important role in the decarbonization of heating systems, waste heat recovery and transportation, as the use of small-scale heat pumping and fuel cells spreads. Such machines typically employ aerodynamic bearings because of their olfreeness and very large lifespan. In particular, grooved bearings enjoy a high stiffness and repeatable performance under similar manufacturing conditions and are particularly well suited for small-scale rotors. However, such gas bearings are usually modeled under the assumption that the lubricant follows the ideal gas law, which may not be valid in conditions met in heat pumping (high pressure condensable gases) and ambient air compression (condensation of moisture). In addition, a strict separation between grooved and compliant bearings exist without justification and optimization methods used for the design of such elements ignore the concept of robustness. This thesis explores these shortcomings. First, the classical modeling theory for grooved bearings, the Narrow-Groove Theory (NGT), is experimentally validated in dynamic conditions. A test rig is built, consisting of a 16mm rotor rotating at 100 krpm on two herringbone grooved journal bearings (HGJBs) driven by an impulse turbine. The bearing bushings are excited in two orthogonal directions using piezo-electric shakers, allowing the deduction of the frequency-dependent force coefficients. The measurements are in good qualitative agreement with the theory, although stiffness and damping coefficients were observed 23 and 29% lower than the predicted values, respectively. The influence of real-gas effects and humid air on the static and dynamic performance of plain bearings and HGJBs is theoretically and numerically investigated on a wide range of operating conditions. Real-gas effects were found to negatively affect the load capacity and either have a positive or negative influence on the stability depending on the operating conditions. Humid air effects have a negligible yet negative influence on the static performance and can decrease the critical mass of HGJBs by 25%. The effects of pure-fluid condensation are assessed theoretically using a 1D slider bearing. Further, an experimental setup consisting of a 20mm three-pad Rayleigh step journal bearing operating at 30 krpm in pressurized R245fa is built to experimentally validate the proposed model by measuring and comparing the pressure in the thin gas film. Theoretically- predicted characteristic features of a condensing gas film are observed experimentally, suggesting sustained operation of a gas lubricated bearing with local condensation. The path toward hybrid foil and grooved bearings is explored, as a tentative to combine the best of the two worlds. A model based on the simple foundation approach is pro- posed and the improvement potential of spiral grooves applied on foil thrust bearings is numerically assessed using multi-objective optimization of the grooved pattern. Results suggest that the load capacity can be improved while reducing the drag torque. However, the ultimate load capacity does not systematically benefit from the presence of spiral grooves. Finally, a novel design procedure maximizing the robustness of aerodynamic bearings against manufacturing deviation is proposed and applied to HGJBs.


Advisor(s):
Schiffmann, Jürg Alexander
Year:
2020
Publisher:
Lausanne, EPFL
Keywords:
Laboratories:
LAMD


Note: The status of this file is: Anyone


 Record created 2020-02-27, last modified 2020-04-20

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