Shuangtao Chen

Effects of bearing clearance and supporting stiffness on performances of rotor-bearing system with multi-decked protuberant gas foil journal bearing

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 −20 µm clearance, while unstable operation appears with 80 µm 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.

Static characteristics of six pads multilayer protuberant foil thrust bearings

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.

Comparative studies on double-layered protuberant foil bearing and Hydresil foil bearing

In this paper, the hydrodynamic performance of a new double-layered protuberant gas foil journal bearing (DPGFB) was experimentally studied in a high-speed turbo-expander and compared with the conventional Hydresil foil journal bearing (HGFB). The material properties of supporting foils are important for the bearing performance such as load capacity and frictional damping. In both DPGFBs and HGFBs, two types of materials, stainless steel (0Cr18Ni9) and beryllium bronze (QBe1.7), have been used as supporting foil materials, and their effects on the bearing performance were discussed. A stable operation of turbo-expander with rotational speed up to 70,000 rpm was achieved in the tests of both of two types of bearings. Due to the larger stiffness of the foil layer, the DPGFBs had larger lift-off speed and larger gas film friction torque than HGFBs at high speed when the same foil material was used. The comparison showed that the bearings using 0Cr18Ni9 had a larger life-off friction torque. The synchronous vibration amplitudes of the rotors during the high speed operation and the shut-down process were smaller for DPGFBs than HGFBs, which indicated that the DPGFBs had a better damping performance than HGFBs.

Heat confinement of phase-change memory using graphene

Reducing the heat loss is a key pathway for advancing phase change memory (PCM) technology because a good thermal confinement can facilitate the fast phase transition and reduce the programing power. In this study, the thermal boundary resistance between graphene and tungsten electrode heater is investigated. Density junctional theory (DFT) and atomistic Greens function (AGF) based approach is employed to explore the interfacial phonon transport across the graphene-electrode interfaces. Interfacial structure is optimized using DFT calculations, and the interaction between graphene/electrode is analyzed based on the electronic structure and cohesive energy. Phonon coupling at the graphene/electrode interface is studied through the analysis of phonon dispersion relations and phonon density of states. The AGF results show that the phonon transmission is limited to 4 THz depending on the phonon spectrum in tungsten and a thermal boundary resistance of 21.3 m2K/GW is obtained at physisorption interface between single layer graphene and tungsten. This study will provide insights to understand the thermal transport mechanism in PCM and guidelines to engineer the interfaces for better thermal confinement.

Thermal conductivity of multilayer dielectric films from molecular dynamics simulations

Reducing heat dissipation across nanometer-thick dielectrics is critically important for the self-heating behavior of nanoelectronic devices such as phase change memory. In this paper, we perform molecular dynamics simulations to study the thermal conductivity of multilayer dielectric films consisting of SiO2 and Al2O3. We show that the thermal conductivity of SiO2/Al2O3 multilayer structures can be significantly reduced as compared to that of the bulk dielectrics. The thermal conductivity calculations of crystalline and amorphous multilayer structures with different period thicknesses are presented as well as the size effects. The results show the thermal transport across the crystalline SiO2/Al2O3 multilayer structures is dominated by diffuse interface scattering between thin films, while the internal phonon–phonon scattering dominates the thermal conductivity of amorphous multilayer structures. Thickness dependence are observed for the crystalline multilayer dielectric structures but not in the amorphous structures, which can be attributed to the phonon localization by the lattice termination/deformation at interfaces between crystalline films.

Numerical analysis of the static performance of an annular aerostatic gas thrust bearing applied in the cryogenic turbo-expander of the EAST subsystem

The EAST superconducting tokamak, an advanced steady-state plasma physics experimental device, has been built at the Institute of Plasma Physics, Chinese Academy of Sciences. All the toroidal field magnets and poloidal field magnets, made of NbTi/Cu cable-in-conduit conductor, are cooled with forced flow supercritical helium at 3.8 K. The cryogenic system of EAST consists of a 2 kW/4 K helium refrigerator and a helium distribution system for the cooling of coils, structures, thermal shields, bus-lines, etc. The high-speed turbo-expander is an important refrigerating component of the EAST cryogenic system. In the turbo-expander, the axial supporting technology is critical for the smooth operation of the rotor bearing system. In this paper, hydrostatic thrust bearings are designed based on the axial load of the turbo-expander. Thereafter, a computational fluid dynamics-based numerical model of the aerostatic thrust bearing is set up to evaluate the bearing performance. Tilting effect on the pressure distribution and bearing load is analyzed for the thrust bearing. Bearing load and stiffness are compared with different static supply pressures. The net force from the thrust bearings can be calculated for different combinations of bearing clearance and supply pressure.

Thermodynamic Analysis on of Skid-Mounted Coal-bed Methane Liquefaction Device using Cryogenic Turbo-Expander

Coal-bed methane (CBM) reserves are rich in Sinkiang of China, and liquefaction is a critical step for the CBM exploration and utilization. Different from other CBM gas fields in China, CBM distribution in Sinkiang is widespread but scattered, and the pressure, flow-rate and nitrogen content of CBM feed vary significantly. The skid-mounted liquefaction device is suggested as an efficient and economical way to recover methane. Turbo-expander is one of the most important parts which generates the cooling capacity for the cryogenic liquefaction system. Using turbo-expander, more cooling capacity and higher liquefied fraction can be achieved. In this study, skid-mounted CBM liquefaction processes based on Claude cycle are established. Cryogenic turbo-expander with high expansion ratio is employed to improve the efficiency of CBM liquefaction process. The unit power consumption per liquefaction mole flow-rate for CBM feed gas is used as the object function for process optimization, compressor discharge pressure, flow ratio of feed gas to turbo-expander and nitrogen friction are analyzed, and optimum operation range of the liquefaction processes are obtained.