Foil thrust bearings have demonstrated significant potential as oil-free alternatives for use in high-speed turbomachinery, largely due to their adaptability to a wide range of environmental conditions. However, increasing the load capacity of foil bearings while maintaining high rotational speeds and minimizing wear remains a critical challenge. To address this limitation, hybridization of foil bearings has been explored by introducing externally pressurized air to maintain adequate bearing clearance. In this study, a performance comparison is conducted between a hybrid rigid thrust bearing (HRTB) and a hybrid thrust foil bearing (HFTB). Both bearings share an outer diameter of 82 mm, along with identical taper angles, orifice locations, and sizes. The performance evaluation was performed at 40,000 rpm under a hydrostatic pressure of 5 bar absolute. Key metrics such as mass flow rate, film thickness, and zero-speed performance were assessed. Additionally, power loss and load capacity were compared at zero and higher rotational speeds. The HFTB demonstrated a load capacity of about 400N at 40,000 rpm with a 5-micron average film thickness limiting criterion. In contrast, the HRTB exhibited pneumatic air hammering during dynamic testing, with the onset of this instability occurring earlier as rotational speed increased, accompanied by corresponding surges in torque measurements.

Bump foils are the most frequently used in foil bearings, but the behavior of bump foils is complicated, and their characteristics have been the subject of research for decades. Bump foils are typically modeled as stiffness elements and damping through frictions with other components. In this paper, the nonlinear stiffness of bump foils in flat configuration has been experimentally measured and compared with various analytical models. These nonlinear characteristics of the bump foil of radial foil bearings can be observed during both manufacturing and operational stages because of their inherent structural properties such as bump geometry, forming process, and complicated contact behavior with bearing housing. These nonlinear characteristics of the bump foils are one of the challenges in analyzing the characteristics of the foil bearings. Furthermore, the same bump geometry but with a curved configuration for actual radial foil bearings was used to measure the structural non-linear stiffness in the bearing level in push-pull set up and compared with analytical prediction. Finally, the non-linear bump stiffness model was adopted to the radial foil bearing to calculate overall bearing stiffness and damping coefficients, and stability characteristics through modal analysis and transient time domain orbit simulations. Lastly, the nonlinear stiffness model of the bump foils will be verified by measuring bearing coefficients using shaker systems.

Small turbo jets with engine thrust below similar to 2000N are designed with fuel-lubricated rolling element bearings with very short lifespan requirement, i.e., just a few hrs. Some expandable engines above 2000N of thrust are designed as turbofan. These expandable turbo fan engines are also designed with rolling element bearings, and they require frequent maintenance, replenishing oil to prevent oil degradation over long period of stocking. Replacing the rolling element bearings with foil bearings on these expandable turbofan engines, especially, on high-speed spools, greatly simplifies maintenance issues, and allows higher operating temperature of the turbine core. The range of engine thrust of expandable turbo fan engines, where foil bearings are applicable, is 2000N similar to 22,250N, considering the shaft size, speed, and rotor weight. Foil bearings were developed for 4450N and 22250N class turbo fan engines, focusing on high-speed spools, where maintenance is difficult and the working environment is also very harsh in temperature. The development of foil bearings for such demanding applications requires not only improving the foil bearing design/analysis tool, but also a comprehensive experimental program. This paper presents design features of the high-speed rotor of 4450N class turbofan engines and engine simulator rig, and the current status of developmental tests. The test rig is designed with environmental temperature control on the bearing compartment mimicking actual engine operating temperature, and axial force to the thrust bearing is applied through balance piston seals.

Foil bearings are one of the candidates as oil-free bearing technology for small expandable turbojet engines or high-speed spool of small turbo fan engines below engine thrust below 22,500N (5000lb). Corrugated bump foils are the most popular underlying structure for the foil bearing providing stiffness and damping to the bearing. The bump foil flow resistance has been modeled using experimentally measured Darcy friction factors for the typical bump foil channels in the previous work. The flow resistance model of the bump foil bearings and flows around the bearings (leakage, leading edge groove flows, etc) were implemented to overall secondary flow network for a system incorporating multiple foil bearings. The secondary flow network model was proven very effective to find various cavity pressures along the secondary flows. For complete analysis of thermal behavior of foil bearings and the whole system, the flow network model developed in the previous work has to be combined with energy equations applied to the each secondary flow cavity to solve not only cavity pressures but also cavity temperatures. The energy balance equations for each cavity include not only enthalpy flux associated with all the secondary flows and bump channel flows and convective cooling of rotating surface exposed to the secondary flow paths. This paper presents complete mathematical formulations for the secondary flow temperatures compatible to the mass balance equations applied to the secondary flow cavities of typical turbojets and micro gas turbine rotors incorporating foil bearings. The formulations have been developed for general real gases, i.e., non-ideal gas with specific heat having strong dependency on both temperature and pressure.

The helium turbo-compressor is one of the core technologies for small modular nuclear reactor (SMR), where the helium gas is a primary coolant of the next generation high temperature gas reactor. General Atomics Electromagnetic System (GA-EMS) is developing a 100-MW thermal Gas-cooled Fast Reactor (GFR) as a part of the Department of Energy (DOE) ARC-20 program, and the helium turbo-compressor is a part of power conversion unit coupled to 44MW permanent magnet generator developed by GA-EMS. The power cycle is a closed loop Brayton cycle with a recuperator. The design speed of the turbo-compressor is 11,400rpm with compressor inlet pressure of 3.1MP and pressure ratio of 2.25. The turbine inlet temperature and pressure are 800 degrees C and 7MPa, respectively. This paper presents novel design features of the helium turbo-compressor with very high aerodynamic efficiency and unique mechanical architecture, which incorporates compact modular design concept with active magnetic bearings (AMBs). The compressor is designed as a two-module (low-pressure compressor and high-pressure compressor) with an intercooler between them for improved cycle efficiency. The entire compressor module and turbine module are designed with their own sets of AMBs and flexible coupling connecting the two rotors for improved rotordynamics and easy maintenance. For effective axial load balancing and thermal management, the axial AMBs were designed with an integrated balance piston. The design point operating speed is below the first bending mode for both compressor and turbine rotors allowing reliable operation of the AMBs for entire operating speed range.

A modified Reynolds Equation including centrifugal force of gas film was derived, and it was used to study the effect of gas film centrifugal force and associated streamline on the load capacity of both rigid and foil thrust bearings operating in hydrodynamic and hybrid modes. The thrust bearings have 6 pads with an outer diameter of 82 mm and an inner diameter of 47 mm with typical aper-flat geometry. The setup for the simulations is a single-acting air thrust bearing operating at various ambient pressures and isothermal temperature. Air was chosen for the gas film for the investigations, but the modified Reynolds Equation can handle any general gas films through non-dimensional parameter governing the centrifugal force. The simulations were performed with varying ambient pressure from 1 to 9 bar. The simulation results at different ambient pressures and temperatures are presented in forms of pressure profiles, streamlines, and bearing’s load capacity. The bearings’ load capacity becomes worse when the centrifugal force is considered at very low temperature, and the reduction of the load capacity grows more noticeable with the increase of the ambient pressure and the decrease of the ambient temperature. However, at higher temperature where centrifugal force is not large enough to create large leakage, the centrifugal force helps to redistribute the streamline to favorable way to increase the load capacity.

Proton Exchange Membrane (PEM) Fuel cell Systems (FCS) are rapidly growing technology in the field of electric cars, buses, and trucks. In such applications, a motor-driven oil-free air compressor is one of the most critical auxiliary subsystems for the FCSs. Foil bearings are the perfect choice as oil-free bearings for FCS compressors due to high rotordynamics stability and shock-resistance. The platform of a motor-driven oil-free compressor can also be used as an electric turbocharger for traditional internal combustion engines. All electromechanical subsystems for electric vehicles must satisfy vibration endurance requirements following ISO 16750-3:2012, characterized by certain g-loading at specific frequencies. FCS air compressors also follow the same ISO 16750 standard. For the compressor to satisfy ISO 16750 requirement, detailed rotordynamics simulations with externally excited compressor housing under certain g-loading are essential to design the foil bearings considering the static and dynamic loads to the bearing and rotordynamics stability. The main objectives of the current research are 1) to develop a six degree of freedom (6-DOF) dynamic model of the rotor supported by foil bearings under external excitation to the compressor housing, 2) to simulate linear and non-linear rotordynamics of the compressor rotor supported by two radial foil bearings, and 3) to provide appropriate design guideline such as bump stiffness and axial length of the foil bearings. The simulations show that the rotor is stable for both under single frequency excitation and simultaneous excitations of all the frequencies in ISO 17650 standard. 4-DOF modal analyses were applied to identify natural mode of the system under the housing excitation.

This work presents a novel design of a hybrid thrust foil bearing (HTFB) with an outer diameter of 154mm, along with simulation and test results. The HTFB incorporates a high-pressure air/gas injection to the taper portion of the thrust foil bearing, boosting the hydrodynamic effect on the main bearing surface and hydrostatic load capacity at zero speed. As a result, this bearing has high load capacity, low power loss, and no friction/wear during startup and shutdown. Firstly, push-pull tests were conducted on a double-acting HTFB configuration to evaluate the nonlinear structural stiffness of the bump foil structure. The measured nonlinear stiffness model was adopted to predict the overall bearing performance under various speeds and external loads. The bearing performance was predicted at 40krpm, 3000N axial load, and 6 bar absolute hydrostatic pressure in both hydrodynamic and hybrid modes. The simulation uses an advanced model which predicts the 2D plate top foil deflection with physical bump locations mapped from the actual hardware. The simulation confirms that the hybrid operation significantly increases the minimum film thickness compared to hydrodynamic bearing due to the boost effect of hydrodynamic pressure in the main film. The preliminary experimental measurements on load capacity at zero speed agree with the prediction. Rotational tests were conducted at 10krpm, 15krpm and 20krpm. Ultimately, the novel HTFB shows excellent static performance with potential for use in Organic Rankine Cycle (ORC) generators and other large oil-free turbomachines.

An analytical study of a power conversion unit consisting of a dual compressor and a turbine operating on a closed Brayton cycle with helium is presented. The turbomachinery was a constant hub design with two compressor modules and a turbine module. The design code included the capability to perform sweeps of input parameters such as the polytropic efficiency or loss coefficient, degree of reaction, etc. It is required that the pressure rise of each of the two compressor modules be approximately equal. An optimization procedure for the low-and high-pressure compressors resulted in both of them having seven stages. The performance for each of the compressor stages is almost identical. Another optimization procedure for the turbine yielded six stages. Likewise, the performance for each of the turbine stages is almost identical. Performance maps were developed individually for each of the compressor modules and the turbine. Mathematical constraints based on problem physics were used in conjunction with the component maps to trace turbomachinery operating points. Results show that the low-pressure compressor operates close to the surge line. It is recommended to implement a recirculation path to avoid possible surge in compressor.

This paper studies and compares 3D thermo-hydrodynamic (THD) performance of rigid thrust gas bearings lubricated with R1234ze gas using Reynolds Equation and commercial computational fluid dynamics (CFD) software. The gas bearing consists of six pads operated in both hydrodynamic and hybrid modes at high ambient pressures typical to the bearing compartment of refrigerant compressors. The CFD simulations adopted an advanced turbulence model and Peng-Robinson real gas equation of state while the Reynolds Equation-based THD model adopted NIST Refprop for real gas model. Results from the two models are compared, and detailed observations on gas bearing performance are presented including load capacity, pressure profiles, streamlines, effect of convective inertia and Mach number, etc. Further, the effects of inlet dead volume (IDV) region in such modeling are included to capture complete detailed flow behaviors for the presented high-density refrigerant gas flows within such bearings. The CFD results demonstrate that convective inertia increases load capacity significantly for refrigerant gases. Both models capture temperature drops around the orifice curtain area from isentropic expansion, but CFD predicts more accurate isentropic expansion not only in temperature drop but also in pressure drop. Furthermore, convective inertia effects are not substantial for air bearings at lower ambient boundary pressures but are still significant for refrigerants at low ambient boundary pressures. Another important feature captured from CFD is the possibility of sub-ambient pressure at certain inner and outer boundaries of the bearing due to high Mach numbers of the leakage flows, and resultant sub-ambient static pressures and lower load capacity when ambient pressure is too low. However, such sub-ambient pressure is not prominent in typical operating conditions of refrigerant gases.

The Fast Modular Reactor (FMR) is a 100-MW(thermal) gas-cooled fast reactor being developed by General Atomics Electromagnetic System with the goal of developing a FMR for flexible and dispatchable power to the U.S. electricity market in the mid-2030s. The conceptual design aims to develop and verify simplified design features. These include an inert helium gas coolant, pellet-loaded fuel rods, installations with air cooling as ultimate heat sink, and small and passive heat removal systems. The goal is to ensure the development of a safe, maintainable, cost-effective, and distributed nuclear energy-generating station. The baseline technologies selected to achieve this goal are a helium coolant that is an inert gas with no chemical reaction with structural components, not activated, single phase, enabling high-temperature operation and a high thermal efficiency Brayton cycle; conventional uranium dioxide (UO2) fuel, which is the most widely used and well-known fuel material, capable of high burnup (100 MWd/kg) and a long fuel life; and silicon carbide composite (SiGA (R)) cladding and internal structures that are chemically inert in the helium environment, exceptionally radiation tolerant, and being derisked by accident tolerant fuel technology development. The reactor was specifically designed with passive safety features, including high-temperature in-core materials and a reactor vessel cooling system consisting of cooling panels of naturally circulating water. The passive safety of the core was confirmed for the depressurized loss-of-forced cooling accident, which showed the peak cladding temperature at similar to 1600 degrees C during the transient, which is below the current design limit of 1800 degrees C. The conceptual design of the FMR has been conducted for the reactor system, vessel system, generator and turbomachine, instrumentation and control, residual heat removal system, plant service system, and containment, as well as pre-application licensing documents.

Reynolds equation has been widely used in modelling lubrication flow and load carrying performance prediction of fluid flow within gas bearings for a number of years. However, the underlying assumption of negligible fluid inertia is not a reasonable design approach philosophy as the operating speeds of such bearings become extremely high. In this paper, A 3D thermos hydrodynamic (THD) model is presented to show effects of leading-edge inlet models on pressure profile predictions of gas films for a rigid 3d bearing film, operating in high speed and highly compressible flow regimes. The model solves full 3-dimensional Navier-Stokes equations in laminar form coupled along with the gas film energy equation for gas temperature throughout the gas film. Results are also compared with a classical Reynolds equation-based 3D thermos-hydrodynamic model to show impact of choice of boundary condition treatments on performance and pressure profile predictions of such bearings, along with impacts of including inlet dead volume (IDV) region modeling on gas film properties. Further, some more results are discussed from solutions of 3-dimensional Navier-Stokes equations coupled with Peng-Robinson equation of state (EOS), to illustrate impacts on flows and pressure profiles when considering highly compressible real gas effects for typical high density refrigerant flows within such bearings.

The motivation to use air foil bearings in fuel cell compressors is driven by the demand for oil-free and high-power density system to reduce system volume and weight. The characteristics of air foil bearings that realize this demand are its independency on auxiliary system and no scheduled maintenance as well as their superb performance at high speeds. However, integration of the foil bearings to the compressor needs rigorous developmental tests for the bear-ing to withstand high g-load during vehicle maneuver and to remain stable in rotordynamics under external destabiliz-ing forces. This paper presents multi-pads foil bearing technology applicable to single stage high speed fuel cell air com-pressors.Two different multi-pad air foil bearing designs (two-pad vs three-pad) were tested using a high-speed spin test rig to identify the differences in rotordynamics responses. The two-pad bearing is superior in rotordynamics without any sub-synchronous vibration while three-pad bearing provides more uniform load capacity in all directions with less rotor-dynamics stability. Frequency-domain modal analyses verify the experimental observations. Axial foil bearings with 38mm outer diameter was designed and tested up to 140krpm with load capacity of 90N (1.4bar specific load capacity). Finally, a platform design of single stage 15kW fuel cell compressor with rated speed of 130krpm is proposed using the multi-pad foil bearings and axial foil bearings developed through this paper.

The integration of foil bearing technology into high-speed oil-free machines has been slow in progress, in part, due to the low load-carrying capacity of the foil thrust bearing. It is crucial this issue is addressed through innovative solutions without overcomplicating the bearing design because simplicity is one of the attractive features of the foil bearing. This work presents novel thrust foil bearing with taper-flat configuration and pocket grooves on the bearing top foil as a secondary pressure boosting mechanism. Parametric study of the pocket dimensions on a rigid bearing reveals that the bearing static performance is the most sensitive to the pocket angular span. Further two-dimensional fluid–structure interaction analyses on foil thrust bearing predict a reduction of power loss by 10% with increased average film thickness. Minimum film thickness also increases when the bearing is lightly loaded but it is reduced 20% at the taper-flat transition area under high loading condition. This issue can be overcome by using stiffer bump foil; however, this is not implemented in this work due to other design constraints. Test results at 90,000 rpm and 140,000 rpm show, by adding the pocket groove pattern on the top foil, the power loss is reduced by 16% compared to the traditional taper-flat configuration.

Small gas foil bearings (FBs) with shaft diameter below 25 mm can find many applications in air compressors for fuel cells, electrical turbo chargers, small unmanned air vehicles, turbo alternators, etc. These small machines are characterized by very light load to the radial FBs, and thus rotordynamics stability is more challenging than load capacity. However, a main challenge of gas foil thrust bearings (GFTBs) is how to increase the load capacity, and the challenge remains the same regardless of the size. In previous publications on experimental studies on GFTBs, the measured load capacity is well below the prediction due to challenges in testing as well as manufacturing of GFTBs. Difficulty in achieving the design load capacity often leads to increasing the bearing size in actual applications with penalty of higher power loss. This paper presents design feature of a novel GFTB with outer diameter of 38 mm and static performance up to 155 krpm under external load of 75 N using a high-speed test rig. The 38 mm GFTB presented in this paper is a three-layered structure for easy design and manufacturing, and the unique design feature allows easy scale down and scale up to different sizes. Reynolds equations for compressible gas and the two-dimensional thin plate model were adopted for fluid–structure interaction simulation to predict load capacity and power loss of the GFTB. The predicted power loss and load capacity agree well with the measurements.

The nonlinear characteristics of foil bearings often add complexities to analysis and design of foil bearings over wide size ranges. Previously, scaling laws for bearing clearance and areal stiffness of supporting foil structure for differently sized foil bearings were proposed to address these challenges. The scaling laws enable to design large foil bearing by scaling up existing small foil bearing. To verify the proposed scaling law and demonstrate the feasibility of a large foil bearing, a 200mm (in diameter) hybrid air foil bearing with potential use for 2MW generator was designed and manufactured by scaling up an existing 50mm foil bearing following the scaling laws. The 200mm hybrid foil bearing was tested for its static performance. The bearing features a three- pad offset preload with total six hydrostatic injection ports to improve the load carrying capacity at low speed. A great effort was put into designing a test rig to demonstrate the bearing static performance. The test rig is configured with a series of ball bearings-supported test rotor and the hybrid foil bearing is floating on the rotor. A pneumatic actuator is used to apply external load to the foil bearing and a torque rod measures the friction torque of the foil bearing. The test rig was designed with maximum allowable speed of 18,000rpm and the rotordynamics of the test rig was analyzed to ensure its integrity under the testing conditions. Two hybrid modes, i.e., with all injection ports open and only bottom injection ports open, were compared. The hybrid foil bearing can withstand up to 2000N at zero speed under 5 bar absolute supply pressure for both hybrid modes. Twenty start/stop cycles test at 1000rpm over extended period under 2000N in hybrid modes show the bearing temperature changes very little, demonstrating very robust bearing function. The hybrid foil bearing was also tested in hydrodynamic mode at 4400rpm under 2000N, verifying the full functionality of the bearing in hydrodynamic mode. Apreview of a 2MW permanent magnet generator featuring the 200mm hybrid foil bearing is also presented as one of the potential applications.

The main objective of this paper is to study and compare the performance characteristics of foil bearings for a typical turboblower during start/stop conditions. The test bearings feature three-segment bump foils and a full smooth top foil with a nominal diameter of 70 mm and an aspect ratio of 1. All foils are made out of Inconel 718 and have a thickness of 0.2 mm (0.008”). Three coatings, applied to the top foil, have been evaluated during the test campaign: PTFE, Molybdenum-Titanium Nitride (MoTiN) and Molybdenum-Aluminum-Titanium Nitride (MoAlTiN). MoTiN and MoAlTiN were applied using physical vapor deposition (PVD) technology. Each bearing was instrumented with sixteen thermocouples located within the bearing sleeve 1 mm away from its inner diameter. Thermocouples allow the measurement of the bearing temperature in the axial and circumferential directions. Bearing displacement was measured using eight proximity probes located at each side of the bearing (four per side). Overall vibrations of the bearing under test were measured via two accelerometers located on top of the bearing housing in the vertical and horizontal directions. In addition, a torque arm mechanism was used to measure the bearing shear force; hence deduce the friction coefficient, friction torque and total power loss. The paper discusses friction and wear results obtained after one hundred start-stop tests for each bearing. A tribological and microstructural analysis is also presented and discussed.

This work presents a novel design of a hydrostatic thrust foil bearing (HSTFB) with an outer diameter of 154mm along with simulation and test results up to specific load capacity of 223kPa (32.3psi). The HSTFB incorporates a high pressure air/gas injection to the thrust foil bearing with a uniform clearance. This bearing has high load capacity, low power loss, and no friction/wear during startup and shutdown. In addition, the HSTFB allows for bidirectional operation. The paper also presents an advanced simulation model which adopts the exact locations of a tangentially arranged bumps to a cylindrical two-dimensional plate model of the top foil. This method predicts top foil deflection with better accuracy than the traditional independent elastic foundation model which distributes the bump locations over the nodal points in the cylindrical coordinates, and with less computational resource than the finite element method applied to the entire bump/top foils. The presented HSTFB, was designed for Organic Rankine Cycle (ORC) generators, but its performance was predicted and measured using air in this paper. The bearing static performance is compared analytically against the rigid counterpart, and presented at different supply pressures, speeds, and minimum film thicknesses. Experimental verification is conducted at 10, 15 and 20krpm. The measured load capacity and frictional loss agree well with the prediction. The measured film thickness also agrees with the prediction after the structural deflection of the thrust runner disc is compensated. Overall, the novel HSTFB demonstrates an excellent static performance and shows good potential for adoption to the intended ORC generators and other large oil-free turbomachines.

Air foil bearings (AFBs) are introduced as promising bearings for oil-free turbomachinery applications. AFBs provide reliable operation at high speed and high temperature with negligible power loss. Hybrid air foil bearing (HAFB) technology utilizes the radial injection of externally pressurized air into the traditional hydrodynamic AFB's film thickness through orifices attached to the top foil. Previous studies have reported enhancement in the rotordynamic stability of HAFBs compared to traditional hydrodynamic AFBs. HAFBs have several orifices distributed in the circumferential direction. In this study, the effect of the circumferential location of radial injection on the rotordynamic performance of the rotor-HAFB is studied. Analytical and experimental evaluations of the rotordynamic performance of a rotor supported by two single-pad HAFBs are presented. Parametric studies are conducted using three sets of single-pad HAFBs. The circumferential locations of orifices are different for each set. The presented simulation analyses consist of time-domain orbit simulation and frequency-domain modal analysis. Imbalance responses of rotor-HAFB were measured with various orifice locations and the results agree well with predictions. Comparison of the rotordynamic performance of HAFBs with different orifice configurations demonstrates substantial improvement in rotordynamic stability as well as enhancement in the stiffness and damping coefficients of HAFBs by choosing the best circumferential location for radial injection to control rotor eccentricity and attitude angle.

This paper details the design and performance of a high-speed (up to 190,000rpm) gas foil thrust bearing (GFTB) test rig to measure bearing load capacity. Several GFTB test rigs were reported in the literature for operating speed up to 90krpm. A few recently presented works show successful runs at 135krpm for testing gas thrust bearing with viscoelastic support and 130krpm tilting pad thrust bearing with compliant structure. However, a GFTB test rig for speed range over 100krpm has not been reported. At high speed operation, the gas film thickness of the GFTB is around a few microns which makes it difficult to achieve in testing. In many cases, the measured thrust load from experiments is well below the predicted data due to difficulty in testing and instrumentation. Difficulty in validating the actual load capacity of the bearings leads to increasing the thrust bearing size to ensure sufficient load capacity in actual applications, which results in higher power consumption. This work presents detail feature of a novel GFTB test rig and test results of 38mm GFTB. The developed test rig runs up to 190krpm and measures bearing load capacity, frictional torque and temperature across bearing ID and OD. The test rig is suitable for testing GFTB with OD from 30 mm to 40 mm. The test facility successfully tests a 38 mm GFTB to its predicted load capacity of 75N (110kPa).