空腔辐射传输的扩散-输运耦合计算

针对空腔辐射传输的特点,论述了扩散-输运耦合计算的必要性与可行性,给出了耦合计算的界面联接方法.用限流函数概念统一了几种常用限流扩散方法,给出了数值例子和相应结论。     

管内两层介质入口段扩散混合的耦合换热研究

对圆管内辐射物性不同的两层介质层流入口段,采用SIMPLEC算法与蒙特卡罗法数值模拟了二维稳态流动与扩散混合时的辐射-对流耦合换热。通过计算,分析了介质层几何参数、介质物性与流动参数对组份分布与耦合换热的影响。结果表明,介质组分的扩散混合对耦合换热存在明显的影响区域,且该影响区大于组分的扩散混合区;外层介质的吸收系数、入口截面的相对厚度对耦合换热的影响基本一致;质扩散系数对耦合换热的影响很小,入口雷诺数的增加会抑制质扩散。     

基于SIMPLE求解对流-扩散方程的一种新方法

针对SIMPLE系列算法,通过分别求解控制容积界面和节点两个位置的对流-扩散方程来提高数值计算精度,提出了一种流动与传热数值模拟的新方法。采用新的数值方法对顶盖驱动流与正方腔自然对流进行了数值模拟。数值模拟结果表明,在相同网格划分时,新的数值方法相对迎风格式、乘方格式、QUICK格式SIMLE算法的计算精度高;而在计算精度基本相同时,新方法有较高的计算效率。     

有限导流压裂定向井耦合流动模型

考虑人工裂缝、斜井筒和地层中的耦合流动,将压裂定向井的流动分为地层向裂缝渗流、裂缝内流动和斜井筒变质量管流.采用镜像反映和势叠加原理,建立地层中势分布和渗流数学模型.应用边值理论,将人工裂缝面离散为裂缝网格,并与斜井筒变质量管流模型进行耦合,建立有限导流压裂定向井耦合流动数学模型,并形成相应的迭代算法.应用分析表明:该模型与Prats图版法相比,对于垂直缝吻合度较高;裂缝导流能力和井斜角对产量及裂缝面压力分布影响较大.     

宽气压范围空气中微波击穿电场的计算公式

为了简便快捷地计算微波击穿电场,依据电子扩散模型的基本理论,结合气体放电的基本参量,应用特征扩散长度的概念,给出了适合于规则结构微波部件的击穿电场的计算方法。为避免各种气体参数的不确定性对计算准确度的影响,对等效直流电场与特征扩散长度之间的实验关系进行了拟合,并根据等效直流电场的定义,得出了一个适用于较高气压范围的击穿电场计算表达式。为了将该计算表达式扩展到更低的气压范围,综合考虑了电子扩散模型和基于二次电子发射现象的真空微放电机理,引入了一个合理形式的等效扩散长度,进一步给出了适合于更广气压范围的微波击穿电场的计算表达式,计算结果更符合A.D.Macdonald的实验结果。     

用锥光纤微球研究Er~(3+)/Yb~(3+)共掺氟氧玻璃陶瓷的发光特性

采用高温煅烧法制备了Er3+/Yb3+共掺的氟氧玻璃陶瓷新材料(41.2SiO2-29.4Al2O3-17.6Na2CO3-11.8LaF3-0.5ErF3-2.5YbF3),并制作了透明带柄微球.提出了用锥光纤微球耦合系统研究Er3+/Yb3+共掺的氟氧玻璃陶瓷材料发光特性的新方法.它具有所需激发光功率低、制备简便和便于测试的特点.用锥光纤作为耦合器将976 nm激光高效耦合入微球,并将产生的荧光和激光耦合出微球输到光谱分析仪,测量到了强的522,545和657 nm上转换荧光,也测到Er3+产生的1562 nm激光振荡光谱图.分析了Er3+/Yb3+共掺氟氧玻璃陶瓷微球中Er3+上转换发光的机理、发光效率高的机理,分析了在氟氧玻璃陶瓷微球中产生激光振荡阈值比在SiO2基质微球中高的机理.     

飞秒激光烧蚀石英玻璃的实验与理论研究

实验研究了800nm飞秒激光作用下石英玻璃的破坏机理和烧蚀规律，给出了破坏阈值与脉冲宽度的关系.发展了雪崩击穿模型，计算了材料的烧蚀阈值与脉冲宽度的依赖关系，烧蚀深度、烧蚀体积与脉冲能量的依赖关系，研究了导带电子的扩散对材料中激光能量的沉积、分布，以及材料的破坏阈值和烧蚀规律的影响. 关键词： 飞秒激光脉冲 破坏机理 石英玻璃 电子扩散     

局域浓度调控扩散系数的次氯酸-碘离子-丙二酸系统图灵斑图形成中的反常扩散

通过数值模拟及振幅方程解析解方法,从实空间和倒空间分析了受局域浓度扩散系数调控下次氯酸-碘离子-丙二酸反应扩散系统图灵斑图形成的扩散机理.在零扩散系数调节下,斑图形成为典型的菲克扩散;而在负向正向扩散系数调节下,斑图的形成依赖欠扩散和超扩散.图灵系统的浓度稳态振幅对随机初始条件敏感性随局域浓度扩散调控系数k的增大而增加.     

氯气/BHP液滴化学反应流动一维数值模拟

为研究横向均匀液滴发生器的机理,对Cl₂/He气体横向流过BHP液滴场的物理化学行为建立一维化学反应流动模型.其中气流场按不可压缩假设近似为一维均匀流动;发生于液滴内的化学反应由四种物质的反应扩散方程描述;气相中各种物质浓度由对流扩散方程描述.液滴是这些物质的源或汇,两个模型方程通过此源项相联系.对模型方程作数值求解.结果显示,氯气利用率、单态氧产率及效率与文献所提供的实验结果较为接近,反映了所建模型具备模拟混合气体与BHP液滴反应的基本能力.     

二极管负载电缆X射线辐照瞬态响应数值模拟

将时域传输线模型与漂移-扩散模型相结合,提出一种耦合数值计算方法,研究计算了带二极管负载的电缆在X射线辐照下的瞬态响应。该耦合数值计算方法首先利用时域传输线模型计算电缆在X射线辐照下的各节点电参数,然后将得到的终端节点的电压代入漂移-扩散模型中求解二极管的内部参数与偏置电流,最后将得到的偏置电流代入到下一时间步的传输线模型中继续进行计算。该方法能够直接反映出电缆在X射线辐照下的瞬态响应与所连接的负载状态改变之间的耦合关系,利用该方法求解X射线辐照简单电子系统的典型算例,计算结果表明,X射线辐照下,同轴电缆上各处的响应电流会随着PIN二极管状态的改变而发生明显变化,与理论分析相符合,耦合模型适用于此类问题。     

强磁场下顺磁掺杂离子的光谱扩散

采用一种简明的解析方法从纯粹频域的角度讨论强磁场下顺磁离子掺杂体系的光谱扩散过程。一般说来光谱扩散遵循[1-exp(-WT_w)]^x的形式,W是跳变速率,T_w为等待时间。在不存在冷冻核时,对于频域和时域x分别为1/2和1。考虑到冷冻核效应,光谱扩散的速率将近似地以因子(ω_eff/ω₀)²减慢,这里ω₀是描述冷冻核的特征频率。当冷冻核充分大时,频域和时域的光谱扩散的行为趋于一致x≈0.22。     

Boundary scheme for lattice Boltzmann modeling of micro-scale gas flow in organic-rich pores considering surface diffusion

We propose a boundary scheme for addressing multi-mechanism flow in a porous medium in slip and early transition flow regimes, which is frequently encountered in shale gas reservoirs. Micro-gaseous flow in organic-rich shale involves a complex flow mechanism. A self-developed boundary scheme that combines the non-equilibrium extrapolation scheme and the combined diffusive reflection and bounce-back scheme(half-way DBB) to embed the Langmuir slip boundary into the single-relaxation-time lattice Boltzmann method(SRT-LBM) enables us to describe this process, namely, the coupling effect of micro-gaseous flow and surface diffusion in organic-rich nanoscale pores. The present LBM model comes with the careful consideration of the local Knudsen number, local pressure gradient, viscosity correction model, and regularization procedure to account for the rarefied gas flows in irregular pores. Its validity and accuracy are verified by several benchmarking cases, and the calculated results by this boundary scheme accord well with our analytical solutions.This boundary scheme shows a higher accuracy than the existing studies. Additionally, a subiteration strategy is presented to tackle the coupled micro-gaseous flow and surface diffusion, which necessitates the iteration process matching of these two mechanisms. The multi-mechanism flow in the self-developed irregular pores is also numerically investigated and analyzed over a wide range of parameters. The results indicate that the present model can effectively capture the coupling effect of micro-gaseous flow and surface diffusion in a tree-like porous medium.     

Effect of boundary heat flux on solidification in a forced liquid metal flow: a phase-field simulation

A phase-field model coupling with velocity field is employed to study the effect of boundary heat flux on the microstructure formation of a Ni-40.8%Cu alloy with liquid flow during the solidification, and an anti-trapping current is introduced to suppress the solute trapping due to the larger interface width used in simulations than a real solidifying material. The effect of the flow field coupling with boundary heat extractions on the microstructure formation as well as distributions of concentration and temperature fields are analyzed and discussed. The forced liquid flow can significantly affect the heat and solute diffusions, thus influencing morphology formation, concentration and temperature distributions during the solidification. The solute segregation and concentration diffusion are changed by boundary heat extractions, and the morphology, concentration and temperature distributions are significantly influenced by increasing the heat extraction, which relatively makes the effect of liquid flow constrained. By increasing the initial velocity of liquid flow, the lopsided rate of the primary dendrite arm is enlarged and the growth manner of dendrite arms gets changed, and the transition of the microstructure from dendrite to cellular moves to the large heat extraction direction. Therefore, there exists the competition between the heat flux, temperature gradient and forced liquid flow that finally determines the microstructure formation during directional solidification.     

First-principles investigation on diffusion behaviours of H isotopes: From W(110) surface into bulk and in bulk W

The diffusion behaviours of hydrogen (H), deuterium (D), and tritium (T) from W(110) surface into bulk and in bulk W are investigated using a first-principles calculations combined with simplified models. The diffusion energy barrier is shown to be 1.87 eV from W(110) surface to the subsurface, along with a much reduced barrier of 0.06 eV for the reverse diffusion process. After H enters into the bulk, its diffusion energy barrier with quantum correction is 0.19 eV. In terms of the diffusion theory presented by Wert and Zener, the diffusion pre-exponential factor of H is calculated to be 1.57×10^-7 m²·s^-1, and it is quantitatively in agreement with experimental value of 4.1×10^-7 m²·s^-1. Subsequently, according to mass dependence (√1/m ) of H isotope effect, the diffusion pre-exponential factors of D and T are estimated to be 1.11×10^-7 m²·s^-1 and 0.91×10^-7 m²·s^-1, respectively.     

Kinetic Simulation of Fractal Aggregation on Liquid Surfaces

A modified fractal growth model based on the deposition, diffusion, and aggregation (DDA) with cluster rotation is presented to simulate two-dimensional fractal aggregation on liquid surfaces. The mobility (including diffusion and rotation) of clusters is related to its mass, which is given by D_m=D₀ s^-γ_D and θ_m=θ₀s^-γ_θ, respectively. We concentrate on revealing the details of the influence of deposition flux F, cluster diffusion factor γ_D and cluster rotation factor γ_θ on the dynamics of fractal aggregation on liquid surfaces. It is shown that the morphologies of clusters and values of cluster density and fractal dimension depend dramatically on the deposition flux and migration factors of clusters.     

Measurements of radon concentration in soil gas by CR-39 detectors

A miniature diffusion chamber with a 25 × 4 × 0.5 mm CR-39 track etch detector (Pershore Moulding Ltd.), mounted on the 1.1 m long pole has been developed for radon gas measurements at 1 meter depth in the soil. For chemically etched CR-39 (7h, 70°C NaOH) and automatic track analysis the lowest detection limit of the chamber was found to be 0.5 MBq h m⁻³ and the useful exposure range from 2 to 20 MBq h m⁻³. The typical exposure time in the soil is between 2 to 14 days. The chamber was tested against the active AlphaGUARD PQ-2000 (Genitron Instruments GmbH) probe. The test yielded consistent results for soils with typical values of permeability and which are not miniature with water. The pilot measurements of radon gas in soil conducted with the miniature diffusion chambers around 48 buildings in Kraków and Silesia regions yielded an average radon concentration of 13 kBq m⁻³. The chambers are to be applied to measure radon concentration in soil before constructing new houses in order to avoid high radon risk areas.     

随机双指数记忆耗散系统的非马尔可夫扩散

针对具有双指数耗散记忆核函数的两自由度耦合系统, 本文利用Laplace变换导出了热宽带噪声激励下该系统响应二阶矩的解析表达式. 并观察到位移二阶矩不同于单自由度情形下的反常扩散:<x²(t)> ∝ t^α (0<α<2, α≠1), 而是随时间及噪声等参数变化呈现普遍的振荡扩散现象.分析可得, 阻尼耦合因子B使粒子远离简谐势场的束缚, <x²(t)>随B的增大扩散加剧而摩擦系数增大却使其趋于平稳状态.进一步, 若两热噪声互关联时, 较小的互关联时间对二阶矩的影响较大, 反之作用较小. 伴随互关联强度递增, 位移二阶矩的扩散加剧, 位移间的相关性加强, 与物理直观相符. 关键词： 热噪声 非马尔可夫扩散 广义朗之万方程 关联性     

Observation of diffusion through ion irradiated Makrofol KG

The features of the latent track in polycarbonate can be analysed by measuring the diffusion constant under ultra high vacuum conditions. Stacks of 30 μm Makrofol KG foils were irradiated with uranium ions of 11.4 MeV/u at the GSI Darmstadt, Germany. We used different fluences from 3·10¹⁰ to 5·10¹¹ ions/cm². The diffusion constant was determined by the time-lag-method [1]. A quadrupole mass filter was used to observe the diffusion of the gas. We measured the diffusion of argon through different foils of each stack at room temperature. In all cases also unirradiated foils were measured. The dependence of the permeability and the diffusion constant on the ion fluence and the energy loss of the ions will be given and indications on the size of latent ion tracks concerning gas diffusion will be discussed.     

A method for detection of spectral spin diffusion from minor peaks, and its application to F MAS NMR of antimony(III)-doped fluorapatite

A new technique for detecting spectral spin diffusion in solids under MAS NMR conditions that is particularly well suited for accurately measuring cross-relaxation from minor spectral components is presented. The pulse sequence, SINK (Saturation Inter-Nuclear Kinetics), selectively saturates the magnetization of a minor spectral component with a series of rotor-synchronized DANTE pulse trains and monitors spin diffusion to other peaks with a non-selective 90° pulse. We have used SINK with ¹⁹F MAS NMR on samples of calcium fluorapatite doped with Sb³⁺ to measure spin diffusion between a weak peak at 68.6 ppm due to fluoride ions associated with Sb³⁺ and other peaks in the spectrum. The SINK experiment clearly demonstrates that spin diffusion from the former peak to the main resonance of fluorapatite at 64.0 ppm is faster than spin diffusion to a second antimony-related peak at 65.6 ppm. These results strengthen our previous conclusion that antimony(III) occupies a phosphate site in the apatite lattice, with an SbO₃³⁻ group replacing a PO₄³⁻ group. The SINK experiment also enables the detection of a “hidden” peak at approximately 62.9 ppm that is otherwise obscured by the intense main peak at 64.0 ppm.     

Boron diffusion in bcc-Fe studied by first-principles calculations

The diffusion mechanism of boron in bcc-Fe has been studied by first-principles calculations. The diffusion coefficients of the interstitial mechanism, the B–monovacancy complex mechanism, and the B–divacancy complex mechanism have been calculated. The calculated diffusion coefficient of the interstitial mechanism is D_0= 1.05 ×10~(-7)exp(-0.75 e V/k T) m~2· s~(-1), while the diffusion coefficients of the B–monovacancy and the B–divacancy complex mechanisms are D_1= 1.22 × 10~(-6)f1exp(-2.27 e V/k T) m~2· s~(-1)and D_2≈ 8.36 × 10~(-6)exp(-4.81 e V/k T) m~2· s~(-1), respectively. The results indicate that the dominant diffusion mechanism in bcc-Fe is the interstitial mechanism through an octahedral interstitial site instead of the complex mechanism. The calculated diffusion coefficient is in accordance with the reported experiment results measured in Fe–3%Si–B alloy(bcc structure). Since the non-equilibrium segregation of boron is based on the diffusion of the complexes as suggested by the theory, our calculation reasonably explains why the non-equilibrium segregation of boron is not observed in bcc-Fe in experiments.