Compliantly Damped Tilting Pad Porous Carbon Gas Bearing: Design of a Novel Test Rig For MW-Class Oil-Free Turbomachinery

Abstract

This project presents the design of a compliantly damped tilting pad porous carbon hybrid gas bearing (CHGB), and a test rig to facilitate component and system level testing. The intent of such a bearing is to operate as a process-lubricated bearing in high-speed machinery. A CHGB utilizes porous carbon to prevent catastrophic failure in the event of intermittent contact or loss of air pressurization. The use of a compliant bearing support structure allows the bearing to accommodate misalignment, centrifugal growth, large rotor excursion, and aids in development of large damping forces by integrating an external damper working in series with the gas film. Simultaneous use of external pressurization and porous gas delivery provides several benefits during various regimes of operation. External pressurization supply ports in the pad allows for leveraging of hydrostatic and hydrodynamic effects during bearing operation. Porous operation at small film thicknesses offers additional protection prior to rotor contact with the bearing. A compliant structure is designed to yield specific stiffness properties for system level operation. Design of a component and system test rig is necessary to evaluate the non-rotating dynamic performance of the bearing assembly and eventually support a turboexpander dummy rotor. The completed work presents finite element analysis (FEA) and experimental results for development of a porous carbon mechanical mate, along with FEA results for the compliant bearing support structure, and test rig performance in component level configurations. Non-rotating uniaxial tests aid the dynamic characterization of a single tilting pad and external damper. The radial stiffness of the compliant supports correlates well to FEA results (within 5%). Static loading tests show that the compliant supports dominate the stiffness of the system which agree well with predictions. Stiffness of the carbon pad affects the equivalent stiffness of the system and requires further investigation. Mechanically preloading the pad through external loading results in an increased equivalent stiffness, showing a direct correlation between preload and gas film stiffness. The external damper demonstrates high damping levels at low frequencies which indicates good transmissibility between the pad structure and damper. Frequency dependent behavior and the damper design represent areas of improvement.