Experimental analysis of influence of double-layer bump foils on aerodynamic thrust foil bearings performance

Purpose Gas thrust foil bearings (GTFBs) are used to balance the axial load of engines. However, in some working conditions of large axial force, such as the use of single impeller air compressor, the load capacity of GTFBs is still insufficient. To solve this problem, the load capacity can be improved by increasing the stiffness of bump foil. The purpose of this paper is to explore a scheme to effectively improve the performance of thrust foil bearings. In the paper, the stiffness of bump foil is improved by increasing the thickness of bump foil and using double-layer bump foil. Design/methodology/approach The foil deformation of GTFBs supported by three different types of bump foils, the relationship between friction power consumption and external force and the difference of limited load capacity were measured by experimental method. Findings The variation of the foil deformation, bearing stiffness, friction power consumption with the external force at different speeds and limited load capacity are obtained. Based on experimental results, the selection scheme of bump foil thickness is obtained. Originality/value This paper provides a feasible method for the performance optimization of GTFBs.

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.

Effect of Corrosion Pit on the Structural Characteristics of Foil Journal Bearings

Due to corrosive environment, material loss in a localized area leads to formation of a corrosion pit. It diminishes the structural integrity and performance of product. To predict and analyze the effect of corrosion pit on the performance of gas foil bearings (GFBs), FEM analysis of bump-type GFBs is carried out. The effect of corrosion pit is investigated for three different shapes: circular, square and triangular. In this study, the FEM analysis of bump-type GFBs is performed in ANSYS software. Firstly, the influence of a corrosion pit is analyzed for various shapes under different pressure loading. These results show that the presence of a corrosion pit developed high-stress crack intensity, which can result in crack initiation in foil bearings. As pressure loading increased, the stress crack intensity in foil bearings increased. Among different pit shapes, the circular pit shape induces the maximum stress crack intensity in foil bearings, which shows that the crack initiation in foil bearings is observed for a circular pit. Then, the influence of a corrosion pit on the structural stiffness of foil bearings is evaluated for various foil materials under different friction coefficients. These results show that the influence of friction coefficient in foil bearings is increased within the presence of a corrosion pit.

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.

Tribo-corrosion Behaviour of Bare and Nano-coated Foils Used on High-speed Air Foil Bearings

Irrespective of applying lubricants between high-speed machinery’s shafts and Air-foil bearings (AFBs), wear and friction remain troublesome. Hence, the present study sought novel bare foil and equivalent nanocomposites to form a coating system that could mitigatly lower frictional wear between high-speed machinery’s shafts and Air-foil bearings (AFBs). Bare and coated foils were supplied on rectengular dimensions of 15 × 15 × 5mm3. Mechanical properties were characterized by Instron 3384 testing machine. Universal Tribometer s/n : RTEC2441, USA was utilized for tribological measurements at room temperature. Surface morphologies were examined by Scanning electron microscopy (SEM). Higher hardness and young modulus values were recorded from coated foils. On the other hand, specific wear rates were relatively low from coated foils. Furthermore, abrasive wear index was higher from the F2 coated foil. Friction coeffients were also lower from the coated foils under all engaged tribological conditions. Highly ruptured SEM morphologies were noticed from the bare foil. Nonetheless, mild and negligible aggressions were discovered from the surfaces of nano-coated foils.

Thermo-elastohydrodynamic analysis of the inhomogeneous foil bearing with misalignment

Purpose The misalignment is generally inevitable in the process of machining and assembly of rotor systems with gas foil bearings, but the exploration on this phenomenon is relatively less. Therefore, the purpose of this paper is to carry out the thermo-elastohydrodynamic analysis of the foil bearing with misalignment, especially the inhomogeneous foil bearing. Design/methodology/approach The rotor is allowed to misalign in two non-rotating directions. Then the static and dynamic performance of the inhomogeneous foil bearing is studied. The thermal-elastohydrodynamic analysis is realized by combining the Reynolds equation, foil deformation equation and energy equation. The small perturbation method is used to calculate the dynamic coefficients, then the critical whirl ratio is obtained. Findings The gas pressure, film thickness and temperature distribution distort when the misalignment appears. The rotor misalignment can improve the loading capacity but rise the gas temperature at the same time. Furthermore, the rotor misalignment can affect the critical whirl ratio which demonstrates that it is necessary to analyze the misalignment before the rotordynamic design. Originality/value The value of this paper is the exploration of the thermo-elastohydrodynamic performance of the inhomogeneous foil bearing with misalignment, the analysis procedure and the corresponding results are valuable for the design of turbo system with gas foil bearings.