Aaron M. Rimpel

Rotordynamic Performance of Flexure Pivot Tilting Pad Gas Bearings With Vibration Damper

Recently, gas-lubricated bearings have drawn enormous attention for clean energy conversion/process systems such as fuel cells, micro-gas-turbines, gas compressors, etc. Among many different types of gas bearings, tilting pad gas bearings have many attractive features such as high rotor-bearing stability and less severe thermal issues (due to multipad configurations) than foil gas bearings. However, extension of the application of the tilting pad gas bearings to flexible rotors and harsh environments with external vibrations/impacts poses significant design challenges. The design problem addressed in this paper is the vibration damper to be integrated with the flexure pivot tilting pad gas bearing (FPTPGB) with and without pad radial compliance. Linear and nonlinear dynamic models of the FPTPGB with vibration damper were developed, and rotordynamic performance was evaluated to prescribe design guidelines for the selection of bearing shell mass and damper properties. Direct numerical integration (time-domain orbit simulations) and linear analyses were employed to predict rotordynamic responses and other interesting behaviors relevant of rotor-bearing systems with the vibration damper. Rotor-bearing systems showed better performance with larger damper stiffness for both with and without radial compliance. However, bearing shell mass showed different tendencies; lower bearing shell mass was shown to be ideal for bearings with radial compliance, while the opposite trend was observed for bearings without radial compliance. Although increasing the degrees of freedom of the system by allowing the bearing shell to move introduces additional natural frequencies, careful design considerations could allow the placement of the natural frequencies outside of the operating range.

Design and Manufacturing of Mesoscale Tilting Pad Gas Bearings for 100–200 W Class PowerMEMS Applications

This paper introduces a design and manufacturing of mesoscale flexure pivot tilting pad gas bearing with a diameter of 5 mm and a length of 1―2.5 mm for PowerMEMS (micro electromechanical systems for power generation) applications with power ranges of 100― 200 W. Potential applications include power source for unmanned air vehicles, small robots, microgas turbines to be harnessed by very small solid oxide fuel cells, microblowers/compressors for microfuel cells, etc. The design studies involve scaling analysis, time-domain orbit simulations for stability analyses, and frequency-domain modal analyses for prediction of rotor-bearing natural frequencies. Scaling analysis indicates that direct miniaturization of macroscale tilting pad gas bearing can result in a large bearing number, which may render the rotor-bearing system unstable. However, the scaling analysis provides the baseline design from which the final design can be derived considering manufacturing issue. The generalized modal analysis using impedance contours predict damped natural frequencies close to those from orbit simulations, providing high fidelity to the developed numerical methods. It was predicted that the designed mesoscale tilting pad gas bearings would show very stable operation up to a maximum simulated speed of 1,000,000 rpm. The designed mesoscale tilting pad gas bearings were manufactured using X-ray lithography and electroplating.

MANUFACTURING AND TESTING EXPERIENCE WITH DIRECT METAL LASER SINTERING FOR CLOSED CENTRIFUGAL COMPRESSOR IMPELLERS

Direct Metal Laser Sintering (DMLS) is an additive manufacturing process that utilizes a high-powered laser to build up a metal part by selectively melting thin layers of metal powder. This process is attractive for the manufacturing of parts with complex geometry such as closed centrifugal compressor impellers. DMLS allows closed impellers to be made in a single piece and eliminates the shroud joint that results from two-piece manufacturing processes. Using a monolithic impeller can allow higher tip speeds with improved fatigue characteristics compared with two-piece and three-piece designs. Prototype parts can be made more economically than investment casting when considering the tooling costs. Manufacturing costs for DMLS parts are marginally higher than for two-piece machined impellers, but qualification efforts for the braze/weld joint at the cover are circumvented. The DMLS process introduces several factors that must be considered in the impeller design to achieve a successful build with the proper strength and surface finish. This paper describes the authors’ experience with manufacturing and testing multiple closed impeller designs constructed from Inconel 718, 17-4 PH Stainless Steel, and Titanium 6Al-4V. A detailed discussion of design factors and manufacturing experience with a DMLS vendor is included for the various metals. Dimensional, post-test destructive inspection, and material test results are provided showing that the DMLS process can produce an impeller with good dimensional accuracy, surface finish, and material strength. Finally, overspeed test results up to maximum tip speeds of over 1400 ft/s (425 m/s) and aerodynamic performance test results are presented and discussed.

Rotordynamics of a 105,000 rpm Oil-Free Compressor-Expander for Subsurface Natural Gas Compression and Reinjection

An aerodynamic and rotordynamic performance test stand has been developed to test a 105,000 rpm compressor-expander which is intended for subsurface natural gas reinjection. The machine consists of a two-stage centrifugal compressor which is driven by a single-stage expansion turbine, and the rotor is supported by foil gas journal bearings and a spiral-groove gas thrust bearing. The test stand is configured for open-loop testing of the overall compressor and turbine performance with air as the working fluid and atmospheric pressure at the compressor suction and turbine discharge locations. The dynamic performance of the test rig was monitored with proximity probes (relative rotor displacements) and accelerometers (housing acceleration) during tests. An instability of the conical rigid rotor mode was observed at or near the designed operating speed of 105,000 rpm which prevented testing of the machine at the maximum speeds required by the aerodynamic performance test matrix (110% of the designed operating speed). Various rotordynamic analyses investigated the effects of hole-pattern and labyrinth seals on stability, including a hole-pattern seal taper sensitivity study and the effect of higher compressor discharge densities. Based on the conclusions of the low-pressure condition rotordynamic analyses, the hole-pattern seals were replaced with labyrinth seals in the open-loop test rig, and the stability issue was no longer present up to the maximum speed.Copyright

A Rotordynamic, Thermal, and Thrust Load Performance Gas Bearing Test Rig and Test Results for Tilting Pad Journal Bearings and Spiral Groove Thrust Bearings

A high speed gas bearing test rig was developed to characterize rotordynamic, thermal, and thrust load performance of gas bearings being developed for an oil-free turboexpander. The radial bearings tested in this paper were tilting pad journal bearings with radial compliance features that allow the bearing bore to increase to accommodate shaft growth, and the thrust bearings were a spiral groove type with axial compliance features. The thrust bearing accounts for over 90% of the combined bearing power consumption, which has a cubic relationship with speed and increases with case pressure. Radial bearing circumferential pad temperature gradients increased approximately with speed to the fourth or fifth power, with slightly higher temperature rise for lower case pressure. Maximum steady state bearing pad temperatures increase with increasing speed for similar cooling mass flow rates; however, only the thrust bearing showed a significant increase in temperature with higher case pressure. The thrust bearings were stable at all speeds, but the load capacity was found to be lower than anticipated, apparently due to pad deformations caused by radial temperature gradients in the stator. More advanced modeling approaches have been proposed to better understand the thrust bearing thermal behavior and to improve the thrust bearing design. Finally, the radial bearings tested were demonstrated to be stable up to the design speed of 130 krpm, which represents the highest surface speed for tilting pad gas bearings tested in the literature.© 2016 ASME

OPEN-LOOP AERODYNAMIC PERFORMANCE TESTING OF A 105,000 RPM OIL- FREE COMPRESSOR-EXPANDER FOR SUBSURFACE NATURAL GAS COMPRESSION AND REINJECTION

An aerodynamic performance test stand has been developed for validation of the performance of a 105,000 rpm compressor-expander which is intended for subsurface natural gas reinjection. The turbomachine consists of a two-stage centrifugal compressor, which is driven by a single-stage expansion turbine. The rotor is supported by foil gas journal bearings and a spiral-groove gas thrust bearing. The test stand is configured for open-loop testing of the overall compressor and turbine performance with air as the working fluid and atmospheric pressure at the compressor suction and turbine discharge locations. Several performance curves were generated for each component ranging from 73,500–115,500 rpm (70–110% of design speed). In general, measured compressor head was slightly lower than predictions, while measured efficiencies were close to predicted values. The turbine had higher flow than predicted, due in part to a larger flow area in the turbine. The turbomachine has shown acceptable performance on the open-loop test stand, and further testing at higher-pressure closed-loop conditions are planned.Copyright

Experimental and Analytical Studies on Flexure Pivot Tilting Pad Gas Bearings With Dampers Applied to Radially Compliant Pads

Hydrodynamic flexure pivot tilting pad gas bearings (FPTPGBs) can enable successful operation of oil-free microturbomachinery, and FPTPGBs with radially compliant pads (FPTPGB-Cs) permit rotor centrifugal and/or thermal growth to exceed original bearing clearances and achieve higher speeds. This work presents the experimental and analytical study of such bearings and the application of dampers behind the pad radial compliance structure. A time domain orbit simulation method was implemented as the primary analysis tool to predict rotor-bearing response to imbalance, the presence and location of critical speeds, etc., and compare with test results. Experiments demonstrate the stable hydrodynamic operation of FPTPGBs with a ∼28.6 mm, 0.8 kg rotor above 120 krpm for the first time. The rotor-bearing system was intentionally destabilized in tests by increasing bearing clearances, and viscoelastic dampers added behind the FPTPGB pads delayed the onset of subsynchronous vibrations (from 43 krpm without damper to above 50 krpm with damper). Midrange subsynchronous vibrations of the destabilized system initiated at ∼20 krpm were suppressed by ∼25 krpm due to the stabilizing effect of rotor centrifugal growth. The viscoelastic dampers had a negligible effect on suppressing these midrange subsynchronous vibrations in experiments, but this was not demonstrated in simulations, presumed to be due to much lower stiffness contribution of the damper at lower frequencies.Copyright