Gas Foil Bearings Research Articles

Frictional analysis on stiffness and damping characteristics of bump foil gas bearings

Gas foil bearing has been used in high-speed turbomachinery for its high speed, high adaptability, and eco-friendly. The stability of the rotor bearing system is greatly enhanced due to the flexible foil support structure. In the foil supporting structure, vibration energy of the rotor bearing system can be absorbed by the Coulomb friction dissipation. In order to evaluate the frictional dissipation on bearing performance, a quasi-2-dimensional mechanical model is proposed considering the Coulomb friction. The Coulomb damping dissipation characteristics of the bearing in operating conditions are analyzed by coupling gas film. Frictional analysis is conducted on the bump foil bearing with different structural parameters, such as bump height, foil thickness, friction coefficient and number of bump foil strips. The stiffness of the foil bump is less affected by the base load. In the process of foil bearing design, the optimal friction coefficient, bump height should be properly selected in favor of either foil bearing stiffness or damping characteristics. In addition, the overall performance of bump foil bearing is strongly affected by the number of bump foil strips.

Experimental Study of the Influence of Rotor Dynamics on the Temperature Distribution of a Gas Foil Bearing

This article presents an experimental study carried out on a rotor operating on two foil bearings. One of the bearings was built in a way to measure the temperature and deformation of the top foil of the foil bearing. The dynamic performance of the two bearing journals is presented herein using the vibration trajectories and journal positions in the foil bearing sleeve. Based on these results, a method for predicting the temperature distribution on the top foil was developed. Then, the correlations between the temperature distribution and the operating parameters of the bearing journal were illustrated. It has been observed that the symmetry of the temperature distribution on the top foil depends on the eccentricity of the rotor operation. The shape and size of the vibration trajectory had no significant effect on the temperature value in the bearing, unlike the position of the journal in the bearing. The process of loss of the gas lubricating film was observed, which provides insight into the phenomena that occur during the first few seconds of rotor operation. The areas in the bearing from which fresh air had been drawn into the gap between the journal and the top foil were also determined.

Optimizing energy dissipation in gas foil bearings to eliminate bifurcations of limit cycles in unbalanced rotor systems

High-speed rotor systems mounted on gas foil bearings present bifurcations which change the quality of stability, and may compromise the operability of rotating systems, or increase noise level when amplitude of specific harmonics drastically increases. The paper identifies the dissipating work in the gas film to be the source of self-excited motions driving the rotor whirling close to bearing’s surface. The energy flow among the components of a rotor gas foil bearing system with unbalance is evaluated for various design sets of bump foil properties, rotor stiffness and unbalance magnitude. The paper presents a methodology to retain the dissipating work at positive values during the periodic limit cycle motions caused by unbalance. An optimization technique is embedded in the pseudo-arc length continuation of limit cycles, those evaluated (when exist) utilizing an orthogonal collocation method. The optimization scheme considers the bump foil stiffness and damping as the variables for which bifurcations do not appear in a certain speed range. It is found that secondary Hopf (Neimark–Sacker) bifurcations, which trigger large limit cycle motions, do not exist in the unbalanced rotors when bump foil properties follow the optimization pattern. Period-doubling (flip) bifurcations are possible to occur, without driving the rotor in high response amplitude. Different design sets of rotor stiffness and unbalance magnitude are investigated for the efficiency of the method to eliminate bifurcations. The quality of the optimization pattern allows optimization in real time, and gas foil bearing properties shift values during operation, eliminating bifurcations and allowing operation at higher speed margins. Compliant bump foil is found to enhance the stability of the system.

Fluid-solid coupled simulation on thermal management of hydrodynamic gas foil bearing with an axial cooling throughflow

The axial throughflow cooling is a realistic solution for the purpose of thermal management in the hydrodynamic gas foil bearings operating at ultra-high rotational speed and mini bearing clearance. In order to illustrate the effects of axial throughflow cooling within the fluid-solid coupled heat transfer mode on the aerodynamic and thermal behaviors of hydrodynamic gas foil journal bearing, a steady three-dimensional fluid-solid coupled numerical simulation is performed in the present study by assuming that the journal-bearing works in a steady-state condition with a fixed eccentricity ratio of 0.9. The results show that the shearing-induced circumferential flow is significantly dominant in the hydrodynamic air film layer. At both edges of the foil bearing section, local suction flow into the bearing clearance and local leakage flow out the bearing clearance happen simultaneously at different circumferential positions. The increase of axial throughflow mass flowrate plays an effective role on improving the cooling capacity. However, it should be noteworthy that the bleeding air pressure is prompted in a nearly quadratic relationship along with the increase of axial throughflow mass flowrate. Due to the non-uniformity nature of viscous-shearing heat generation inside the hydrodynamic gas foil bearing with a big eccentricity, the temperature on the shaft and top foil surfaces takes on a non-uniform distribution. The peak temperature generally has a 20 K–30 K difference with respect to the mean temperature.

Study on the static and dynamic performance of active bump-metal mesh foil bearings

The gas foil bearing (GFB) is an important component of high-speed, oil-free micro turbomachinery. However, the instability of GFBs under certain conditions hinders their widespread application. The traditional solution is to decrease the continuity of the gas film and increases the structural damping of GFBs through ‘passive’ structural improvements. Thus, the performances of improved bearings cannot be actively adjusted. This paper proposes an active bump-metal mesh foil bearing (AB-MFB) in which three metal mesh blocks (MMBs) are inserted into the active bump-type foil bearing (ABFB) to control radial preloads through piezoelectric actuators (PZTs) and increase the structural damping. A model coupling of the active-metal mesh substructures, bump foil, and top foil is presented to analyse the mechanical properties of the compliant structure of AB-MFB. Dynamic excitation tests are conducted to validate the model. This study then predicts the static and dynamic performance of the AB-MFB by combining the model and compressible gas Reynolds equation. Finally, the effects of different parameters on the dynamic performance of the AB-MFB are discussed.

Research on Linear Actuators for Active Foil Bearings.

Active foil bearings are a kind of gas foil bearing. They contain actuators which allow for modification of the bearing sleeve size and the shape of the lubrication gap. Rotor vibrations can be actively controlled by these changes. It is possible, among other things, to reduce the starting torque, control the vibration amplitude at different speeds and improve operational safety. Prototypes of active foil bearings are being developed based on different mechanical concepts. This paper provides an analysis of the linear mechanisms that are the base, and they are crucial for such developments. In the literature, there is a lack of characteristics of these actuators tested under real operating conditions of an active foil bearing. This article aims to fill this gap. New test rigs have been developed and used to precisely investigate the possibility of using actuators in active foil bearings. Since their geometry and control methods differ, the measurement systems have been adapted accordingly. The actuators studied were piezoelectrics, shape-memory alloys (SMAs) and stepper motors. Each of them was characterized by different operating characteristics. The results obtained allow for a comparison of the actuators. This approach is especially useful for the design and analysis of active foil bearings.

Performances investigation and multi‐objective optimization of gas foil bearings in hydrogen fuel cell vehicles

Performances investigation and multi‐objective optimization of gas foil bearings in hydrogen fuel cell vehicles

Thermal Characterization of a Gas Foil Bearing-A Novel Method of Experimental Identification of the Temperature Field Based on Integrated Thermocouples Measurements.

Maintenance of adequate thermal properties is critical for correct operation of a gas foil bearing. In this work, the authors present the results of the experimentally conducted thermal characterization of a prototype installation of the bearing. A novel method of temperature identification, based on integrated thermocouples readings, has been employed to determine the thermal properties of the specialized sensing top foil mounted in the tested bearing. Two measurement campaigns have been subsequently completed, applying freely-suspended and two-node support configurations, to gather complementary knowledge regarding the bearing’s operation. Apart from the rotational speed and temperature field measurements, the authors have also studied the friction torque and the shaft’s journal trajectories based on its radial displacements. The temporal courses for the above-mentioned quantities have enabled inference on the effects present during run-up, run-out and stable state operation at a constant speed. As confirmed, the applied distribution of the integrated sensors allows for temperature readings on the entire outer surface of the foil, and therefore, provides useful data for the bearing characterization. The work is concluded with presentation of the recommended directions regarding future improvements of the proposed measurement technique and more comprehensive study of the bearing’s characteristics.

Experimental Study of Various Low-Friction Coatings for High-Temperature Gas Foil Bearings Under Cold-Start Conditions

Abstract This article presents the experimental research carried out on five different materials with which the top foils have been coated. A foil bearing with these foils was tested under different load conditions. The key operating parameters were determined, such as the moment of friction during the run-up and run-down tests as well as the temperature of the top foil. The AS20 coating, like the TiAlN coating, was intact (with the exception of several places where it was worn and where the Inconel sheet was visible) and was suitable for further use. The WC/C coating was damaged in many places, which shows that this type of coating cannot be used in combination with a journal coated with chromium oxide. The foil coated with MoS2/C had many areas where the coating was worn and also many areas where it was erased completely, so this type of coating was not durable. The use of a combination of TiAlN and MoS2/C coatings resulted in minor wear of the outer coating (made of MoS2/C) and the appearance of the inner coating, which is more resistant to abrasion. The tested materials will be used in the future to build a foil bearing resistant to operation at elevated temperatures in a gas microturbine.

A fully coupled 3D elastohydrodynamic model built with MITC element for static performance analysis of gas foil bearings

PurposeThis paper aims to present a 3D static performance analysis model for the gas foil journal bearing to provide better understanding of the gas foil journal bearing and extend the development of the calculation about the static performance.Design/methodology/approachThe foil bearing can be seen as a shell structure, and the mixed interpolation of tensorial components (MITC) element was used to build the shell model. The augmented Lagrange method was used to calculate the contact involving friction between foils and between the foil and the bearing sleeve. A displacement-controlled load scheme was used to calculate the deformation of the foils. A mapping operator was used to map the film pressure from the gas to the surface of the top foil.FindingsThis method provides high precision of calculation in the prediction of the static performance. The calculation results were compared with the experimental data, and they show good agreement. Meanwhile, the model can be applied in the prediction of the bearing performance in a broad range of working conditions.Originality/valueThis method extends the calculation of the gas foil journal bearing to a 3D scale and shows good agreement with the experimental data. Meanwhile, the present model has a good adaptability on the revolution speed and can be applied to the predictions in varied working conditions.

Influence of Ambient Pressure on Measured Stiffness and Damping of Radial Gas Foil Bearings

Abstract As gas bearings have gained popularity, they have been used in a diverse range of applications. In many cases these bearings are used in various process gases at a range of pressures. Although numerous analytical tools exist to predict the behavior of gas bearings in these environments, these tools are challenging to validate. The current work describes a test loop that is capable of measuring rotordynamic force coefficients in a hermetically sealed environment. To the author's knowledge, this capability is the first of its kind. To illustrate operation of the test rig, an industrial 3-lobe gas-foil bearing was installed in the test fixture, and operated at pressures from 0.5 to 3.5 bara at speeds ranging from 25 to 65 krpm. The tests show that dynamic coefficients increase with speed and pressure; however, a slight reduction in coefficients were observed at the highest speed under ambient conditions. In addition, subambient pressure tests showed higher than ambient stiffness, but comparably low damping in comparison to the other test cases. It is believed that this is the first publication of gas foil bearing coefficients at subambient pressure.

Modelling, analysis and optimization of design parameter of bump-type gas foil journal bearing

The bump-type gas foil bearing (GFB) is widely used in various high-speed oil-free turbomachinery due to its extra features such as high-speed capability without any external oil lubricant. Under the high-speed condition, the performance characteristics of GFB are majorly dependent on its design parameter. Thus, this paper briefly analyzes and optimizes the design parameters of bump-type GFB to improve its performance characteristic. The numerical simulation of bump-type GFB is performed in ANSYS software. The effect of various design parameters such as foil thickness, bump half-length, bump pitch, and bump angle on the output responses are analyzed using a response surface methodology based CCD design matrix. Here, the output responses are structural stiffness and the equivalent stress in foil bearing. To optimize it, the multi-objective GRA technique is used. These results show that the foil thickness is the most influencing and the bump angle is the least influencing design parameter. The optimal value of foil thickness, bump angle, bump half-length and bump pitch are 0.14 mm, 63.75 deg., 1.55 mm and 4.6 mm respectively. At these optimal design parameters, the deviation between the predicted regression model and the numerical results are within 3.5%. Moreover, with these optimal design parameters, the structural stiffness of foil bearings increases by 68.4% and stress distribution reduces by 44.22% compared to the general configuration of foil bearings.

Theoretical investigation of hybrid foil-magnetic bearings on operation mode and load sharing strategy

This paper presents a theoretical investigation on operation mode and load sharing strategy of hybrid foil-magnetic bearings (HFMBs). According to the inherent characteristics of gas foil bearings (GFBs) and active magnetic bearings (AMBs), six possible work scopes are discussed to understand the operation mode of HFMBs. A numerical model coupling the calculations of the film pressure in GFBs and the magnetic forces in AMBs is conducted to predict the operation performance of HFMBs. The accuracy of the model is proved by comparing the predicted and tested friction torques. Analysis on the static and dynamic performance of HFMBs within three representative cases is conducted by varying the load sharing ratio. The dynamic supporting stiffness and operation position of HFMBs can be obviously enhanced by adjusting the operating mode and load sharing strategy. Results show that the direct stiffness and dynamic damping of HFMBs adopting hybrid mode are significantly higher than those adopting AMB or GFB independent mode.

Modeling and stability characteristics of bump-type gas foil bearing rotor systems considering stick–slip friction

Frictional energy dissipation inside the foil structure is considered as a key factor in improving the stability of the gas foil bearing (GFB) rotor system. However, the effect and mechanism of friction on the stability of bump-type GFB rotor systems have remained partially addressed. This study aims to develop a novel comprehensive model to investigate the stability and frictional dissipation mechanism of the GFB rotor system. The LuGre dynamic friction model is adopted to capture accurate stick–slip states. The model order reduction technique and simultaneous solution framework are applied to improve the computational efficiency. The effect of friction on linear and nonlinear stability is investigated based on the proposed model, and an analysis of frictional dissipation is performed to explain the mechanism. The results reveal that the dissipation capacity of the foil structure is directly governed by the stick–slip states that are affected by the friction coefficient and disturbance magnitude, so the optimal friction coefficient for nonlinear stability is higher than the one for linear stability of the GFB rotor system. A further comparison between different friction models greatly highlights the necessity of the ability of the friction model to capture accurate stick–slip states.

Development and Performance Measurements of Gas Foil Polymer Bearings with a Dual-Rotor Test Rig Driven by Permanent Magnet Electric Motor

The rotordynamic stability of journal gas foil–polymer bearings (GFPBs) applied to a dual-rotor bearing system was investigated. The GFPB has a high damping structure, i.e., it has an additional polymer layer, and top/bump/bottom foil structures. Test GFPBs were fabricated with nitrile butadiene rubber polymer layers with a thickness of 2 mm. Static-load-deflection tests of test gas foil bearings (GFBs) and GFPBs were performed to estimate the geometric bearing clearance (200 µm). The dual-rotor rotordynamic test rig consisted of a motor rotor, test rotor, and beam-type coupling. Two journal test bearings were installed on both the drive-end and non-drive-end sides of the test rotor. Predicting and testing the natural mode characteristics of the dual rotors revealed that the relative error between them was less than 7%, indicating that the first and second natural frequencies were 15 Hz and 160 Hz, respectively, and the third natural frequency was 1835 Hz in the tests. Based on the API 612 standard, the upper limit of the rotating speed for the test rig was limited to be approximately 92 krpm with a separation margin of 26%. Rotordynamic tests were conducted to examine the stability performance of GFBs and GFPBs, where the adjusted bearing clearance was 150 µm. The test results indicate that GFPBs have better stability performance in terms of delaying and suppressing unstable vibrations than GFBs. Specifically, GFPBs showed stable synchronous and subsynchronous responses up to a maximum rotating speed of 80 krpm. As a result, GFPB is a reliable lubricating element that can be used for vibration dampening in machines operating at relatively low temperatures.

Performance of textured foil journal bearing considering the influence of relative texture depth

Both foil structure and surface texturing have been widely used to improve bearing performance. However, there is little research on their combination, namely, textured gas foil bearing. This paper adopts the Reynolds equation as the pressure governing equation of bump-type foil journal bearing to study the influence of textures located on the top foil. The Newton-Raphson iterative method and the perturbation method are employed to obtain static and dynamic characteristics, respectively. Thereafter, based on three texture distribution types, further analysis about the effect of the relative texture depth and the textured portion is carried out. The results indicate that an appropriate arrangement of textures could improve the performance of gas foil bearing. For #1 texture distribution, the maximum increment of load capacity could exceed 10% when ω = 1.4 × 105 r/min, ε = 0.2.

A review of thermohydrodynamic aspects of gas foil bearings

In recent times, gas foil bearings have become popular for commercial use in the aircraft and space industry, turbocompressors, turbine generators and in the more complex fields of turbochargers and turboexpanders. The gain in popularity for gas foil bearings is due to their features such as contamination-free zone, wide temperature range, higher stability and higher reliability characteristics as compared to other types of bearings. However, several challenges have come across while analysing the gas foil bearing behaviour at different working conditions. The current paper presents an overview of the work done in the past few decades for developing numerical models and listing the efforts of several researchers around the world to conduct the experimental investigation for predicting and analysing thermohydrodynamic behaviour of gas foil bearings at different operating conditions. It is expected that the current paper will help readers to thoroughly understand the hydrodynamic and thermal aspects of gas foil bearings.

Experimental study on the static and dynamic performances of gas foil bearings for the centrifugal air compressor used in fuel cell vehicles

Experimental study on the static and dynamic performances of gas foil bearings for the centrifugal air compressor used in fuel cell vehicles

A methodology for performance prediction: aerodynamic analysis of axially loaded gas foil bearing

A methodology for performance prediction: aerodynamic analysis of axially loaded gas foil bearing

Design and experimental verification of the vehicle compressor rotor system with inhomogeneous foil bearings

In this paper, the design and experimental verification of the rotor system with gas foil bearings are carried out with a vehicle compressor developed in our laboratory. The designed rotating speed 100,000 rpm with 50 g/s mass flow and 1.8 pressure ratio. The journal foil bearing with inhomogeneous bump foil is designed and tested by a push-pull device to evaluate the structure stiffness of bump foil. The result shows that the stiffness curves of two bearings with the same manufacturing process are not consistent, which indicates the uncertainly in the manufacture of foil bearings and it is necessary to obtain the foil stiffness data by experiment. A multi-disc model is established to simulate the impeller in the finite element model (FEM) for the vehicle compressor is too short to ignore the impeller width. The stiffness and damping coefficients of foil bearings are used to proceed rotordynamic analysis. The vibration experiments indicate that with the operating speed enhancement, the center orbit falls smaller. When the rotating speed increases to about 60,000 rpm, two sub-synchronous frequency occur and remain at 150 and 307 Hz finally. Two radial acceleration peaks appear at 9736 and 25,828 rpm respectively, which are close to the critical speed of damped Campbell diagram. The compressor performance map shows that the pressure ratio of the compressor is slightly lower than the design value due to the eccentricity of the foil bearing, which can be solved by increasing the operating speed. This paper provides some reference value for the design and experiment of vehicle compressor supported by the foil bearings.