Robert J. Bruckner

Design, Fabrication, and Performance of Open Source Generation I and II Compliant Hydrodynamic Gas Foil Bearings

Foil gas bearings are self-acting hydrodynamic bearings made from sheet metal foils comprised of at least two layers. The innermost “top foil” layer traps a gas pressure film that supports a load while a layer or layers underneath provide an elastic foundation. Foil bearings are used in many lightly loaded, high-speed turbomachines such as compressors used for aircraft pressurization and small microturbines. Foil gas bearings provide a means to eliminate the oil system leading to reduced weight and enhanced temperature capability. The general lack of familiarity of the foil bearing design and manufacturing process has hindered their widespread dissemination. This paper reviews the publicly available literature to demonstrate the design, fabrication, and performance testing of both first- and second-generation bump-style foil bearings. It is anticipated that this paper may serve as an effective starting point for new development activities employing foil bearing technology.

Design, Fabrication, and Performance of Foil Gas Thrust Bearings for Microturbomachinery Applications

ABSTRACT Amethodologyforthedesignandconstructionofsimplefoilthrust bearingsintendedfor parametricperformancetestingandlow marginal costs is presented. Features drawn from a reviewof the openliterature are discussed as they relate to bearingper-formance. The design of fixtures and tooling required to fab-ricate foil thrust bearings is presented, using conventional ma-chining processes where possible. A prototype bearing with di-mensionsdrawnfrom theliteratureis constructed,with allfabri-cationsteps described. Aload-deflectioncurvefor thebearingispresented to illustrate structural stiffness characteristics. Start-stop cycles are performed on the bearing at a temperature of425 ◦ C to demonstrate early-life wear patterns. A test of bearingload capacity demonstrates useful performance when comparedwith data obtained from the open literature. Introduction Foil gas bearings represent an enabling technology foradvanced oil-free turbomachinery systems. Operating athigh speeds and temperatures, these next-generation turboma-chines will present tribological challenges that conventional oil-lubricated rolling element bearings may be unable to meet, in-cluding shaft speeds well above three million DN and bearingtemperatures in excess of 400

Remaining Technical Challenges and Future Plans for Oil-Free Turbomachinery

The application of oil-free technologies (foil gas bearings, solid lubricants, and advanced analysis and predictive modeling tools) to advanced turbomachinery has been underway for several decades. During that time, full commercialization has occurred in aircraft air cycle machines, turbocompressors, cryocoolers, and ever-larger microturbines. Emerging products in the automotive sector (turbochargers and superchargers) indicate that a high volume serial production of foil bearings is imminent. The demonstration of foil bearings in auxiliary power units and select locations in propulsion gas turbines illustrates that such technology also has a place in these future systems. Foil bearing designs, predictive tools, and advanced solid lubricants that can satisfy anticipated requirements have been reported, but a major question remains regarding the scalability of foil bearings to ever-larger sizes to support heavier rotors. In this paper, the technological history, primary physics, engineering practicalities, and existing experimental and experiential database for scaling foil bearings are reviewed, and the major remaining technical challenges are identified.

Windage Power Loss in Gas Foil Bearings and the Rotor-Stator Clearance of High Speed Generators Operating in High Pressure Environments

Closed Brayton Cycle (CBC) and Closed Supercritical Cycle (CSC) engines are prime candidates to convert heat from a reactor into electric power for robotic space exploration and habitation. These engine concepts incorporate a permanent magnet starter / generator mounted on the engine shaft along with the requisite turbomachinery. Successful completion of the long duration missions currently anticipated for these engines will require designs that adequately address all losses within the machine. The preliminary thermal management concept for these engine types is to use the cycle working fluid to provide the required cooling. In addition to providing cooling, the working fluid will also serve as the bearing lubricant. Additional requirements due to the unique application of these microturbines are zero contamination of the working fluid and entirely maintenance-free operation for many years. Losses in the gas foil bearings and within the rotor-stator gap of the generator become increasingly important as both rotational speed and mean operating pressure are increased. This paper presents the results of an experimental study, which obtained direct torque measurements on gas foil bearings and generator rotor-stator gaps. Test conditions for these measurements included rotational speeds up to 42000 revolutions per minute, pressures up to 45 atmospheres, and test gases of Nitrogen, Helium, and Carbon Dioxide. These conditions provided a maximum test Taylor number of nearly one million. The results show an exponential rise in power loss as mean operating density is increased for both the gas foil bearing and generator windage. These typical “secondary” losses can become larger than the total system output power if conventional design paradigms are followed. A non-dimensional analysis is presented to extend the experimental results into the CSC range for the generator windage.

Gas Foil Bearing Technology Advancements for Closed Brayton Cycle Turbines

Closed Brayton Cycle (CBC) turbine systems are under consideration for future space electric power generation. CBC turbines convert thermal energy from a nuclear reactor, or other heat source, to electrical power using a closed‐loop cycle. The operating fluid in the closed‐loop is commonly a high pressure inert gas mixture that cannot tolerate contamination. One source of potential contamination in a system such as this is the lubricant used in the turbomachine bearings. Gas Foil Bearings (GFB) represent a bearing technology that eliminates the possibility of contamination by using the working fluid as the lubricant. Thus, foil bearings are well suited to application in space power CBC turbine systems. NASA Glenn Research Center is actively researching GFB technology for use in these CBC power turbines. A power loss model has been developed, and the effects of very high ambient pressure, start‐up torque, and misalignment, have been observed and are reported here.

An Assessment of Gas Foil Bearing Scalability and the Potential Benefits to Civilian Turbofan Engines

Over the past several years the term oil-free turbomachinery has been used to describe a rotor support system for high speed turbomachinery that does not require oil for lubrication, damping, or cooling. The foundation technology for oil-free turbomachinery is the compliant foil bearing. This technology can replace the conventional rolling element bearings found in current engines. Two major benefits are realized with this technology. The primary benefit is the elimination of the oil lubrication system, accessory gearbox, tower shaft, and one turbine frame. These components account for 8–13% of the turbofan engine weight. The second benefit that compliant foil bearings offer to turbofan engines is the capability to operate at higher rotational speeds and shaft diameters. While traditional rolling element bearings have diminished life, reliability, and load capacity with increasing speeds, the foil bearing has a load capacity proportional to speed. The traditional applications for foil bearings have been in small, lightweight machines. However, recent advancements in the design and manufacturing of foil bearings have increased their potential size. An analysis, grounded in experimentally proven operation, is performed to assess the scalability of the modern foil bearing. This analysis coupled to the requirements of civilian turbofan engines. The application of the foil bearing to larger, high bypass ratio engines nominally at the 120 kN (∼25000 pound) thrust class has been examined. The application of this advanced technology to this system was found to reduce mission fuel burn by 3.05%.

Conceptual Design of a Supersonic Business Jet Propulsion System

NASAs Ultra-Efficient Engine Technology Program (UEETP) is developing a suite of technology to enhance the performance of future aircraft propulsion systems. Areas of focus for this suite of technology include: Highly Loaded Turbomachinery, Emissions Reduction, Materials and Structures, Controls, and Propulsion-Airframe Integration. The two major goals of the UEETP are emissions reduction of both landing and take-off nitrogen oxides (LTO-NOx) and mission carbon dioxide (CO2) through fuel burn reductions. The specific goals include a 70% reduction in the current LTO-NOx rule and an 8% reduction in mission CO2 emissions. In order to gain insight into the potential applications and benefits of these technologies on future aircraft, a set of representative flight vehicles was selected for systems level conceptual studies. The Supersonic Business Jet (SBJ) is one of these vehicles. The particular SBJ considered in this study has a capacity of 6 passengers, cruise Mach Number of 2.0, and a range of 4,000 nautical miles. Without the current existence of an SBJ the study of this vehicle requires a two-phased approach. Initially, a hypothetical baseline SBJ is designed which utilizes only current state of the art technology. Finally, an advanced SBJ propulsion system is designed and optimized which incorporates the advanced technologies under development within theUEETP. System benefits are then evaluated and compared to the program and design requirements. Although the program goals are only concerned with LTO-NOx and CO2 emissions it is acknowledged that additional concerns for an SBJ include take-off noise, overland supersonic flight, and cruise NOx emissions at high altitudes. Propulsion system trade-offs in the conceptual design phase acknowledge these issues as well as the program goals. With the inclusion of UEETP technologies a propulsion system is designed which performs at 81% below the LTO-NOx rule, and reduces fuel burn by 23% compared to the current technology. BPR F Fn Fn lapse FPR HPC LTO NOx P3 SBJ SLS T3 T4 TOC TTR UEETP Wc

Compliant Foil Journal Bearing Performance at Alternate Pressures and Temperatures

An experimental test program has been conducted to determine the highly loaded performance of current generation gas foil bearings at alternate pressures and temperature. Typically foil bearing performance has been reported at temperatures relevant to turbomachinery applications but only at an ambient pressure of one atmosphere. This dearth of data at alternate pressures has motivated the current test program. Two facilities were used in the test program, the ambient pressure rig and the high pressure rig. The test program utilized a 35 mm diameter by 27 mm long foil journal bearing having an uncoated Inconel X-750 top foil running against a shaft with a PS304 coated journal. Load capacity tests were conducted at 3, 6, 9, 12, 15, 18, and 21 krpm at temperatures from 25°C to 500°C and at pressures from 0.1 to 2.5 atmospheres. Results show an increase in load capacity with increased ambient pressure and a reduction in load capacity with increased ambient temperature. Below one-half atmosphere of ambient pressure a dramatic loss of load capacity is experienced. Additional lightly loaded foil bearing performance in nitrogen at 25°C and up to 48 atmospheres of ambient pressure has also been reported. In the lightly loaded region of operation the power loss increases for increasing pressure at a fixed load. Knowledge of foil bearing performance at operating conditions found within potential machine applications will reduce program development risk of future foil bearing supported turbomachines.

Advanced Rotor Support Technologies for Closed Brayton Cycle Turbines

Closed Brayton Cycle (CBC) turbine generators are a leading power conversion candidate for long life space missions. In these systems, a recirculating gas is heated by nuclear, solar or other heat energy source then fed into a high speed turbine which drives an electrical generator. For closed cycle systems such as these, the working fluid also passes through the bearing compartments thus serving as a lubricant and bearing coolant. Compliant surface foil gas bearings are well suited for the rotor support systems of these advanced turbines. Foil bearings develop a thin hydrodynamic gas film which separates the rotating shaft from the bearing preventing wear. During start-up and shut down when speeds are low, rubbing occurs. Solid lubricants are used to reduce starting torque and minimize wear. Unlike fixed geometry rigid gas bearings, foil bearings are flexible and readily accommodate misalignment, thermal gradients and other system effects. Further, their stiffness and damping properties can be tailored for specific applications. In this paper the use of foil bearings as the rotor support system for a CBC will be reviewed. Bearing performance in inert gas and the bearing integration process will be discussed. In addition, a proven four step bearing-rotor integration process will be introduced which includes rotordynamic studies, bearing development and testing, experimental rotor simulation tests and finally system level turbine tests.

Thermal Management Phenomena in Foil Gas Thrust Bearings

Thrust foil gas bearings operate at high speeds on a very thin fluid film which experiences high shear. Shear induced viscous losses result in localized heating which must be managed to prevent thermal distortions and failure. The current work examines the need for thermal management in thrust foil bearings as evidenced by reduced performance in uncooled bearings. Measured bearing power loss is a few hundred watts from twenty-five to fifty-five krpm (89–196 m/s runner surface velocity based on mean bearing diameter). Modeling of the thrust runner demonstrates a potential for stress-induced running surface deformations greater than the gas film thickness (>10 μm), and radial temperature gradients in the bearing foils can exceed a few degrees Celcius per millimeter. Air flow forced through the foil structure achieves improvements in bearing load capacity and demonstrates a need for increased understanding of the thermal environment in these bearings.© 2006 ASME