Electric-magnetic-force characteristics of rare earth barium copper oxide superconductor high-field coils based on screening effect and strain sensitivity

Rare earth barium copper oxide (REBCO) is the most researched and commercialized second-generation high-temperature superconducting material. Due to the anisotropic structure, strong deformation sensitivity, and central field errors caused by screening current effects, it is still a challenge for commercialization applications. In this study, the transversely isotropic constitutive relationship is selected as the mechanical model based on the structural characteristics of REBCO tapes, and suitable microelements are selected to equate the elastic constants using their average stress-strain relationships. Then, a two-dimensional axisymmetric model for coils wound by single-layer tapes is constructed to analyze the dependence of the electric-magnetic-force distribution in the tape on the strain. Finally, the anisotropic approximation of the homogenized bulk method is used to equate large-turn high-field coils, and the electric-magnetic-force distribution characteristics of the coils with/without screening effects and mechanical strain conditions are investigated, respectively. The results reveal that the mechanical strain has a weakening effect on the electromagnetic field distribution of superconducting tapes, but causes a significant enhancement in the force field distribution. In the presence of 0.5% mechanical strain, the maximum weakening of the peak value of the current density and the critical current density inside the high-field coil can reach about 8% and 13%, respectively, with a nearly 5 times increase in the peak stress. The screening current makes the current field distribution inside the coil improve by about 10 times. The screening current induced magnetic field can reach up to 0.8 T, making the relative error of the high-field coil center up to 7.8%.     

Surface-effects-dominated thermal and mechanical responses of zinc oxide nanobelts

Molecular dynamics (MD) simulations are carried out to characterize the mechanical and thermal responses of -oriented ZnO nanobelts with lateral dimensions of 21.22 Å×18.95 Å, 31.02 Å × 29.42 Å and 40.81Å × 39.89 Å over the temperature range of 300-1000 K. The Young's modulus and thermal conductivity of the nanobelts are evaluated. Significant surface effects on properties due to the high- surface-to-volume ratios of the nanobelts are observed. For the mechanical response, surface-stress-induced internal stress plays an important role. For the thermal response, surface scattering of phonons dominates. Calculations show that the Young's modulus is higher than the corresponding value for bulk ZnO and decreases by ~ 33% as the lateral dimensions increase from 21.22 Å × 18.95 Å to 40.81 Å × 39.89 Å. The thermal conductivity is one order of magnitude lower than the corresponding value for bulk ZnO single crystal and decreases with wire size. Specifically, the conductivity of the 21.22 Å × 18.95 Å belt is approximately (31-18)% lower than that of the 40.81 Å × 39.89 Å belt over the temperature range analyzed. A significant dependence of properties on temperature is also observed, with the Young's modulus decreasing on average by 12% and the conductivity decreasing by 50% as temperature increases from 300 K to 1000 K.     

Approximate theories for wave propagation and vibrations in elastic rings and helical coils of small pitch

From the equations of linear elasticity, three levels of approximate theories are derived for in-plane deformation and motion of thin, circular rings. The accuracy of each theory is determined by comparison with harmonic wave solutions of the elasticity solution. Boundary conditions for uniqueness are established. The results may also be applied to helical coils of small pitch and to cylindrical shells when the equations are converted to plane strain.     

Convective heat transfer characteristics of high-pressure gas in heat exchanger with membrane helical coils and membrane serpentine tubes

Since the heat transfer performance of syngas cooler affects the efficiency of the power generating system with integrated coal gasification combined cycle (IGCC) directly, it is important to obtain the heat transfer characteristics of high-pressure syngas in the cooler. Heat transfer in convection cooling section of pressurized coal gasifier with the membrane helical coils and membrane serpentine tubes under high pressure is experimentally investigated. High pressure single gas (He or N₂) and their mixture (He + N₂) gas serve as the test media in the test pressure range from 0.5 MPa to 3.0 MPa. The results show that the convection heat transfer coefficient of high pressure gas is influenced by the working pressure, gas composition and symmetry of flow around the coil, of which the working pressure is the most significant factor. The average convection heat transfer coefficients for various gases in heat exchangers are systematically analyzed, and the correlations between Nu and Re for two kinds of membrane heat exchangers are obtained. The heat transfer coefficient of heat exchanger with membrane helical coils is greater than that of the membrane serpentine-tube heat exchanger under the same conditions. The heat transfer coefficient increment of the membrane helical-coil heat exchanger is greater than that of the membrane serpentine-tube heat exchanger with the increase of gas pressure and velocity.     

甲烷/煤尘复合爆炸火焰的传播特性

为揭示甲烷/煤尘复合爆炸火焰的传播机理,利用气粉两相混合爆炸实验系统,在低于甲烷爆炸下限条件下,采用高速摄影机记录火焰传播图像,通过热电偶采集火焰温度,研究了煤尘种类以及甲烷体积分数对甲烷/煤尘复合火焰传播特性的影响。结果表明:挥发分是衡量煤尘燃烧特性的主导因素;随着煤尘挥发分的升高,燃烧反应增强,火焰传播速度升高,火焰温度升高;挥发分含量差异较小时,水分含量越低,燃烧反应越剧烈;在相同条件下,焦煤的燃烧反应强度最高,其次为长焰煤,最后为褐煤;随着甲烷体积分数的增加,煤尘颗粒的燃烧可由释放挥发分的扩散燃烧转变为气相预混燃烧,燃烧反应增强,火焰传播速度和火焰温度显著升高;热辐射和热对流作用促进煤尘颗粒热解,释放挥发分进行燃烧反应,维持复合火焰的持续传播;随着混合体系中甲烷体积分数的增加,混合爆炸机制由粉尘驱动型爆炸转为气体驱动型爆炸,燃烧反应增强;甲烷/煤尘复合爆炸火焰可由未燃区、预热区、气相燃烧区、多相燃烧区和焦炭燃烧区5部分组成,湍流扰动导致燃烧介质空间分布存在差异,使得燃烧区无规则交错分布。     

复合材料—土建结构的力学特性与加固技术

简述了复合材料在土建行业中的两种主要应用；结构构件和加固介质，介绍了国内外的一些研究情况，指出了亟待解决的问题等。     

Dispersion characteristics of wave propagation in layered piezoelectric structures: Exact and simplified models

This article presents a study of the dispersion characteristics of wave propagation in layered piezoelectric structures under plane strain and open-loop conditions. The exact dispersion relation is first determined based on an electro-elastodynamic analysis. The dispersion equation is complicated and can be solved only by numerical methods. Since the piezoelectric layer is very thin and can be modeled as an electro-elastic film, a simplified model of the piezoelectric layer reduces this complex problem to a non-trivial solution of a series of quadratic equations of wave numbers. The model is simple, yet captures the main phenomena of wave propagation. This model determines the dispersion curves of PZT4-Aluminum layered structures and identifies the two lowest modes of waves: the generalized longitudinal mode and the generalized Rayleigh mode. The model is validated by comparing with exact solutions, indicating that the results are accurate when the thickness of the layer is smaller or comparable to the typical wavelength. The effect of the piezoelectricity is examined, showing a significant influence on the generalized longitudinal wave but a very limited effect on the generalized Rayleigh wave. Typical examples are provided to illustrate the wave modes and the effects of layer thickness in the simplified model and the effects of the material combinations.     

Local crack tip strain behavior during fatigue crack initiation and propagation in polycarbonate

To investigate whether the two fatigue processes of crack initiation and propagation can be combined, the change of local notch root strain and its history, as well as the change of local crack tip strain and the local strain history, of a fatigued element ahead of the propagating crack tip up to failure in a polycarbonate subjected to low-cycle fatigue tests are measured by the fine grid method. As a result, the existence of a unified local strain field in which the two fatigue processes can be substantially combined is experimentally confirmed. Therefore, the local crack tip strain may be examined by a simpler, one-parameter approach for fatigue life estimation.     

Investigation of the special characteristics of the propagation and reflection of pressure waves in a porous medium

In view of the wide use of porous materials in technology there arises the need to investigate the dynamic processes taking place in them. The main difference between a porous substance and a solid condensed material is the fact that the condensed phase occupies only part of the volume of the porous medium, which leads to a lowered volumetric density and to a large degree of compressibility. There is particular interest in polymeric media with a small density on the order of 20 kg/m³, in which up to 98% of the volume is occupied by the gas phase. Such a density is achieved if the medium has a cellular structure of the foam, for example, in polyurethane foam plastics. At the present time, only the elastic properties of polyurethane foam plastics under the action of cyclic [1] and impact [2, 3] loads are known. Questions of the formation of pressure waves in such a medium, with the refraction in it of a shock wave from the gas, of the structure of the wave propagating over the foam plastic, as well as the special characteristics of its reflection from the interface, remain unclear. In the experiments described below, an investigation was made of pressure waves with intensities up to 20 bars in elastic polyurethane foams (PUF) with a porosity of 0.98 and the special characteristics of the reflection of such waves from a rigid wall were also determined.     

Global modelling of heat release during initiation and propagation of detonation and deflagration waves in methane-air-particle systems

In the present paper the direct initiation of a self supporting detonation and propagation of a low-speed combustion in methane-air-coal particles mixtures are solved. For particles, a heterogeneous regime of combustion is used, for methane one overall chemical reaction is taken into account: CH + 2O = CO + 2HO. The heat release rate is assumed to be defined as a delay time based on the well-known thermal theory of Frank-Kamenetsky (1967). The proposed model allows one to investigate the influence inert particles or coal dust on the explosion limits of methane-air mixtures. It is shown that the addition of a limited quantity of particles leads to detonation stability. In low speed combustion problems this method allows one to get a good correlation between theoretical and experimental velocities of steady flame propagation in carbon-hydrogen gaseous mixtures. Coal dust influence on gasdynamics of a methane-air mixture combustion is investigated in an unsteady problem by using of the global modelling. It is shown that limited coal dust concentration increases the flame wave intensity in lean methane-air mixtures in contrast to inert particles. In stoichiometric gas mixtures, sand and coal dusts decrease a flame velocity. Far from the ignition point flame, the velocity is largely defined by the dust mass concentration and not by the size of particles. Received 5 July 1997 / Accepted 13 July 1998     

Dynamic fracture surface energy values and branching instabilities during rapid crack propagation in rubber toughened PMMA

An experimental device based on strip band geometry has been designed to explore the brittle behaviour of polymers during rapid crack propagation. The macroscopic crack speed is found to be quasi-constant along an entire RT-PMMA specimen, even in the case of crack branching and until arrest, if any. At the macroscopic branching velocity, the experimental fracture surface energy and the fracture surface roughness have no single values in RT-PMMA. In fact, the macroscopic fracture surface energy increases with the number of instabilities or frustrated micro-branches.     

岩石结构面直剪力学特征的颗粒流宏细观分析

目前,由于颗粒尺寸和位置随机分布而产生的粗糙度,导致在模拟过程中出现应力集中现象,从而无法利用PFC2D准确模拟岩石结构面的剪切力学行为。为了避免以上弊端,采用结构面两侧颗粒组接触判别法分别对倾角为20°和30°的三角形单锯齿结构面进行不同法向应力下的直剪模拟。模拟结果与物理试验结果基本吻合,表明此模拟方法可以真实模拟岩石结构面剪切力学行为。为了进一步验证此模拟方法在天然结构面模拟试验中的可行性,利用PFC2D对Barton标准十条节理轮廓线模型进行不同法向应力下的直剪试验,模拟结果与理论值吻合,表明模拟方法具有可行性,为剪切破坏过程和机理分析提供可靠的理论依据,为研究粗糙结构面强度特征提供了一种思路。     

Dependence of the critical stresses on the loading time parameters during spall in copper,aluminum, and steel

Journal of Applied Mechanics and Technical Physics -     

On hyperbolic heat conduction in solids: minimum mean free path of energy carriers and a method of estimating thermal diffusivity and heat propagation characteristics

The paper gives the analytical solution to the one dimensional hyperbolic heat conduction equation in an insulated slab-shaped sample that is heated uniformly on the front face with δ or laser impulse. The solution results in a formula that enables to estimate the minimum mean free path of energy carriers in the sample to detect the second sound (i.e. the thermal wave) at the sample rear face. A method of experimental data evaluation at the second sound effect is proposed, which gives the thermal diffusivity of the sample and the parameters of heat propagation.     

Bubble and liquid flow characteristics in a cylindrical bath during swirl motion of bubbling jet

Gas injection into a cylindrical bath through a centric bottom nozzle causes a swirl motion like rotary sloshing. Conditions indicating the initiation and cessation of the swirl motion have been made clear by many researchers. So far, the effect of the swirl motion on transport phenomena in the bath is not clear yet. The present study was made to clarify the bubble characteristics (void fraction, bubble frequency) and liquid flow characteristics (mean velocity, turbulence intensity, Reynolds shear stress) during swirl motion of bubbling jet. These two characteristics were investigated using an electro-resistivity probe and a two-dimensional LDV, respectively.     

Chemical characteristics of PM2.5 during dust storms and air pollution events in Chengdu,China

Daily fine particulate (PM_2.5) samples were collected in Chengdu from April 2009 to February 2010 to investigate their chemical profiles during dust storms (DSs) and several types of pollution events, including haze (HDs), biomass burning (BBs), and fireworks displays (FDs). The highest PM_2.5 mass concentrations were found during DSs (283.3 μg/m³), followed by FDs (212.7 μg/m³), HDs (187.3 μg/m³), and BBs (130.1 μg/m³). The concentrations of most elements were elevated during DSs and pollution events, except for BBs. Secondary inorganic ions (NO₃^?, SO₄^2?, and NH₄⁺) were enriched during HDs, while PM_2.5 from BBs showed high K⁺ but low SO₄^2?. FDs caused increases in K⁺ and enrichment in SO₄^2?. Ca²⁺ was abundant in DS samples. Ion-balance calculations indicated that PM_2.5 from HDs and FDs was more acidic than on normal days, but DS and BB particles were alkaline. The highest organic carbon (OC) concentration was 26.1 μg/m³ during FDs, followed by BBs (23.6 μg/m³), HDs (19.6 μg/m³), and DSs (18.8 μg/m³). In contrast, elemental carbon (EC) concentration was more abundant during HDs (10.6 μg/m³) and FDs (9.5 μg/m³) than during BBs (6.2 μg/m³) and DSs (6.0 μg/m³). The highest OC/EC ratios were obtained during BBs, with the lowest during HDs. SO₄^2?/K⁺ and TCA/SO₄^2? ratios proved to be effective indicators for differentiating pollution events. Mass balance showed that organic matter, SO₄^2?, and NO₃^? were the dominant chemical components during pollution events, while soil dust was dominant during DSs.