The multilayer protuberant foil bearing, as a new type of compliant surface foil bearing, shows a great and wide application promise. Six pads multilayer protuberant foil thrust bearings with different configurations were designed and fabricated in this study. The static characteristics of these bearings and the effects of their key configuration parameters including the thickness of top foil, the thickness of protuberant foil, and the layers of protuberant foil are investigated. The experimental results reveal that the bearings show nonconstant structural stiffness, and the stiffness mainly depends on both the load force and the configuration of the bearings. In the airborne regime, the torque of the bearing is mainly dependent on the load force rather than the rotational speed, which can be interpreted by the proportional relationship between the bearing clearance and the rotational speed. Furthermore, the experimental results also show that the maximum load capacities of the bearing are also greatly affected by the bearing configuration. With more layers of the protuberant foils and thinner top foil, the bearing shows larger maximum load capacity. The work provides some insights about the relationships between the characteristics and the configuration of the bearings.
The supporting stiffness and coulomb damping in a bearing play significant roles in the smooth operation of rotor-bearing system. The performance of multi-decked protuberant gas foil journal bearing is evaluated experimentally in a high-speed turboexpander. The effect of radial clearance on the bearing performance is analyzed based on the relationship between rotor speed and supply pressure in the speed-up and speed-down processes. The maximal speed of the 25 mm diameter rotor reached as high as 100 kr/min, and subsynchronous vibrations are suppressed in the tests. For the bearings with 0.05 mm protuberant foils, there will be thermal runaway problem with À20 mm clearance, while unstable operation appears with 80 mm clearance. For the bearing with 0.07 mm protuberant foil, the vibration amplitude is constrained within smaller amplitude due to stiffer supporting structure. The test results indicate that the bearing can operate stably under different gas film thickness and supporting stiffness, and that this kind of foil bearing can be applied in high-speed turbomachinery due to its stability and adaptability.
Sphericity and wall thickness uniformity are some of the hardest specifications to fulfill, as required by inertial confined fusion (ICF) research for polymer shells prepared by the microencapsulation technique. Driven by the need to control the deformation of compound droplets, the effects of the molecular weight of poly(vinyl alcohol) (PVA) on the formation and stability of the droplets, as well as the sphericity and wall thickness uniformity of the resulting shells, were investigated. On increasing the molecular weight of the PVA, the densities of the external water phases (W2) are almost the same, but the viscosity of the W2 phase increases more quickly than the interfacial tension. This makes the detaching force increase more quickly than the upward one, causing the formation of compound droplets and detachment from the oil tube. On the other hand, the increase in interfacial tension makes the maximum pressures (
P
max) in the O phase (O) of the compound droplets increase, causing them to rupture easily and decreasing their stability. However, for PVA with the same molecular weight, the viscous shear force in the flowing field reduces the role of gravity and makes the inner water droplet move towards the center of the compound droplet, decreasing its
P
max in the flowing field and improving its stability. Moreover, during the solidifying process, the viscous shear force increases more quickly than the interfacial tension force due to the quicker increase in viscosity with an increase in the molecular weight of the PVA. The increase in the viscous shear force can make the droplets deform, resulting in a decrease in their sphericity. However, the appropriate viscous shear force can also center the compound droplet—although they become decentered when the viscous shear force is too large, leading to the wall thickness uniformity increasing at first before decreasing quickly. The results presented in this work provide a more in-depth understanding of the formation, stability and deformation of compound droplets, to the benefit of preparing polymer shells with the high sphericity and uniform wall thickness needed in ICF experiments.
A type of foil journal bearings with double-layer protuberant foils as elastic support was studied by numerical analysis and experiments. Two kinds of material (beryllium bronze and stainless steel) with different elasticity were used for the protuberant foils of the bearings. The analyzing model couples the hydrodynamics pressure of the gas film to the elastic deflection of the top foil and the underlying protuberant foils. The hydrodynamics of gas film is described by the Reynolds equation. The top foil and the protuberant foils are modeled as thin plates and the protuberances are treated as rigid support. For calculating the deflection of the top foil, the deflection of the protuberant layer beneath the top foil is taken into account. With a given load, the gas film thickness and pressure are obtained by using finite element method and finite difference method. The key static and dynamic parameters are presented. In experiments, both of the two foil bearings run well in a turbo-expander. The new takeoff and shutoff pressure and speed are proposed for practical operation of the hydrodynamic lubricated high speed turbo-expander. In addition, the synchronous and subsynchronous rotor motions have been tested and analyzed. The experimental tests of the two bearings in a high-speed turbo-expander suggest that the bearing using stainless steel as the foil material shows higher stiffness, which agrees well with the numerical predictions.
In this paper, the pressure distribution of aerostatic thrust bearings with vacuum pre-loading was investigated by solving the full Navier–Stokes equations based on the computational fluid dynamics method. The influences of the supply pressure, vacuum pressure, orifice diameter, and gas film thickness on the absolute pressure ratio were investigated. The finite difference method was used to study the effects of the vacuum chamber area, orifice diameter, orifice number, supply pressure, and vacuum pressure on the bearing stiffness. It is confirmed that the orifice diameter and film thickness had a great influence on the absolute pressure ratio, which increased with the reduction in the gas film thickness and the rise in the orifice diameter. The bearing stiffness can be improved by increasing the supply pressure, orifice number or vacuum chamber area or decreasing the orifice diameter or vacuum pressure, which provides useful guidance for the optimization design of aerostatic thrust bearings with vacuum preloading.
Multi-layer protuberant foil bearings uses multi-layers of protuberant foils as the elastic support structure to support the smooth top foil. A numerical model with a computational method is developed in this paper to predict the characteristics of the multi-layer protuberant foil bearing. According to this model, the deflection of each foil has been calculated separately with the consideration of the interactions between contacting layers of foils, and the profile of foil deflections, film pressure and film thickness also have been computed. The results show that these profiles of multi-layer protuberant foil bearings are very different from those of bump foil bearings. And the flexibility of the multi-layer protuberant foil bearing is improved as the layer number of protuberant foils increases. Moreover, the effects of the bearing number and the eccentricity on the static and dynamic characteristics of multi-layer protuberant foil bearings have been analyzed. It is found that the bearing always operates with a small attitude angle when the bearing number is large. In addition, the bearing has quite different dynamic characteristics when operating at the critical condition from those at the synchronous condition, which suggests that dynamic computation at the both conditions is required for dynamic analysis of the foil bearing.
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