分形多孔介质中流体流动的LBM模拟

基于分形理论构造了规则Sierpinski地毯结构来模拟多孔介质孔隙结构,采用Lattice Boltzmann Method(LBM)分别对不同孔隙率与进出口压差下的分形多孔介质中的流体流动进行了数值模拟.结果表明,分形多孔介质的某些流场结构也具有分形特征;体积流量和流场进出口压差呈线性关系,符合Darcy定律;流体穿过分形多孔介质的体积流量与孔隙率成指数函数关系.     

多孔介质通道的分形随机行走描述

多孔介质中的输运过程,如导热、渗流过程,关注的是热量从高温壁面穿过介质到达低温壁面、流体从多孔介质的边界沿孔隙流到另外一端的过程。此类现象可归结为载流子在多孔介质通道(基质或孔隙)中沿外部势差方向的运动过程。多孔介质通道具有分形特征,可以采用分形维数来描述其通道的通透性。本文基于现象的相似性特征,提出并发展了粒子在多孔介质中的方向随机行走模型,用粒子在基质中的方向随机行走过程来模拟真实的热流传输过程;根据分形统计规律得到粒子方向随机行走分形谱维数,并用其描述基质结构的连通性和方向性。研究结果表明,在孔隙率相同情况下,粒子在基质中的方向随机行走分形谱维数与有效导热系数大小有相同的变化趋势。     

纳米颗粒聚集形态对纳米流体导热系数的影响

纳米流体中悬浮的纳米颗粒可以增强其导热性能已经得到广泛认可,然而纳米流体颗粒增强传热的机理目前尚不清楚.研究表明,纳米颗粒的聚集是纳米流体导热系数增大的重要机制,而且纳米颗粒聚集的形态对纳米流体的导热系数有重要影响,但是目前的导热系数模型大多是建立在Maxwell有效介质理论的"静态"和"均匀分散"假设基础上.本文用平衡分子动力学模拟Cu-Ar纳米流体,采用Green-Kubo公式计算导热系数,采用Schmidt-Ott关系式计算不同聚集形态下的分形维数.对比导热系数与分形维数可以发现:在相同体积分数下,较低的分形维数会有更高的导热系数,分析了分形维数与导热系数的定量关系.此外,通过径向分布函数可以看出纳米颗粒紧密聚集与松散聚集的差异,基液分子在纳米颗粒附近的纳米薄层中处于动态平衡状态.研究结果有助于理解纳米颗粒聚集形态对导热系数的影响机理.     

多孔介质热导率的数值计算

通过数值模拟得出面积分形维数和孔隙率相同,而孔隙结构不同的分形多孔介质的导热特性是不相同的.说明仅仅依靠孔隙率和分形维数两个参数无法有效确定多孔介质的热物理性质,寻找新的表征孔隙结构的参数是必要的.同时也说明了前人所得出的关于多孔介质的热物性的解析表达式具有一定的局限性和不确定性.     

二维多孔热密封材料的有效导热系数模拟

本文对于二维多孔热密封材料的有效导热系数模拟提出一种简单有效的方法,它可以模拟介于固相连续和固相不连续之间的任何形式的孔隙分布结构,通过类比多孔渗流模型得出一种相关系数,再根据热阻串并联的关系计算四种可计算模型的有效导热系数,通过相关系数得到具体模型的有效导热系数。本文的模拟方法可以控制孔隙率、孔径、孔隙分布、温度等影响有效导热系数的重要参数,考虑了辐射的影响,而且模拟简单,计算快速,模拟结果与文献中实验的结果相比较能很好的吻合,验证了本文方法的正确性。     

多孔介质内颗粒流动特性及其对传热影响的模拟研究

研究采用CFD-DEM方法结合DEM耦合对流与导热的传热模型,对三维不规则孔隙结构多孔介质内细颗粒与孔隙流体的流动及传热特性进行数值模拟。结果显示,颗粒在流体的裹挟下由静止逐渐加速,多孔介质内部温度场、瞬时流体速度大小及方向随着细颗粒与流体的充分混合及颗粒间的弹性碰撞而发生变化。低孔隙率下颗粒易发生沉积,运动轨迹缩短。小孔隙率加速了由热颗粒引起的热传递过程,孔隙内温度上升较快。随着孔隙率增大,颗粒瞬时轴向和径向速度增大,且流动主要沿轴向进行,大孔隙率下颗粒间法向和切向接触力及颗粒拟温度较低,且对单颗粒运动轨迹有显著影响。     

抛物槽式太阳能集热器内合成油基纳米流体传热特性的数值模拟

对Al2O3-合成油纳米流体在槽式太阳能集热管内的传热特性进行流体动力学数值模拟,重点考察纳米流体导热系数模型的影响.通过与管内Nusselt数半经验模型的预测结果对比,表明使用考虑布朗运动的纳米流体导热系数模型可较好地预测集热管内传热特性.研究表明纳米颗粒与流体基液的相对运动具有促进集热管内传热的作用.最后,定量研究...     

简单流体迁移性质分子动力学模拟研究

通过平衡分子动力学方法和Lennard-Jones势能模型,本文对简单流体氖、氩、氪和氙的迁移性质进行了模拟研究,得到了其随温度变化的关系图.并将模拟结果与NIST推荐的数据进行了比较,除氖以及氙高温度区外,得到的黏度和导热系数的相对偏差皆在±15%之内.同时分析了几种影响简单流体迁移性质分子动力学模拟结果的主要因素.     

基于分形理论的多孔介质导热系数研究

本文根据分形理论,对多孔介质几何结构进行了描述,计算出了不同孔隙率多孔介质简化模型的剖面面积分布分形维数d,并利用分形维数结合土壤导热模型推出了土壤的有效导热系数表达式,计算结果表明,该方法是符合客观实际的．     

环路热管毛细芯有效导热系数的实验研究

通过建立相关测试平台,对一系列环路热管用烧结毛细芯的有效导热系数进行了实验研究,分析了组元配比,孔隙率和浸润工质等参数对有效导热系数的影响。研究发现烧结镍铜毛细芯有效导热系数显著低于单一组元毛细芯,并随着孔隙率的增加而降低,饱和了水的毛细芯导热系数相比干态毛细芯的导热系数有所增加。实验结果与文献中的有效导热系数模型比较,Alexander模型与实验数据拟合最为良好。     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Measurement of nonlinear coefficient of optical fiber based on small chirped soliton transmission

We measure the waveform and phase curves of short optical pulses before and after transmission over different lengths of fibers by use of the pulse analyzer with the frequency-resolved optical gating (FROG),and numerically simulate pulse evolution under the experimental conditions.The nonlinear coefficient of the fiber is given by comparing the experimental results with the numerical ones.Difference between the experiment and numerical simulation is analyzed.     

Resonance scattering of canonical elastic shells in absorbing fluid medium

Resonance scattering of elastic spherical shell and cylindrical shell while the surrounding fluid medium has absorption is studied. The normal mode solution derived using exact elastic theory and the separation of variables is still applicable. However, the scattering form function has to be modified for the absorbing medium, otherwise the unreasonable result would be obtained. The backscattering form function in the absorbing medium is redefined, and the form function of elastic spherical and cylindrical shell with vacuum or solid matter filled is calculated in various absorption conditions. The results show that the absorption of surrounding fluid leads to notable attenuation of the coincidence resonances in the mid-frequency, but it has a little influence on the low-frequency resonance scattering induced by the filler inside the shell.     

Single-beam self-referenced phase-sensitive surface plasmon resonance sensor with high detection resolution

A versatile and low-cost single-beam self-referenced phase-sensitive surface plasmon resonance(SPR)sensing system with ultra-high resolution performance is presented.The system exhibits a root-mean-square phase fluctuation of ±0.0028.over a period of 45 min.i.e.a resolution of±5.2×10-9 refractive index units.The enhanced performance has been achieved through the incorporation of three design elements:a true single-beam configuration enabling complete self-referencing so that only the phase change associated with SPR gets detected,a differential measurement scheme to eliminate spurious signals not related to the sensor response,and the elimination of retardation drifts by incorporating temperature stabilization in the liquid crystal phase modulato .Our design should bring the detection sensitivity of non-labeling SPR biosensing closer to that achievable by conventional fluorescence-based techniques.     

Influences of SiO2 protective layers and annealing on the laser-induced damage threshold of Ta2O5 films

Ta2O5 films are prepared on BK7 substrates with conventional electron beam evaporation deposition.The effects of SiO2 protective layers and annealing on the laser-induced damage threshold (LIDT) of the films are investigated.The results show that SiO2 protective layers exert little influence on the electric field intensity(EFI)distribution,microstructure and microdefect density but increase the absorption slightly.Annealing iS effective on decreasing the microdefect density and the absorption of the films.Both SiO2 protective layers and annealing are beneficial to the damage resistance of the films and the latter is more effective to improve the LIDT.Moreover,the maximal LIDT of Ta2O5 films is achieved by the combination of SiO2 protective layers and annealing.