不同极间距下稀土电解槽内电-热场耦合数值模拟研究

运用数值模拟软件COMSOL建立了不同极间距下3 k A Nd电解槽的物理模型,针对包头稀土研究院的3 k A稀土电解槽进行了电-热场的耦合模拟。模拟结果验证了耦合的合理性,并发现槽电压随极间距的增大而增大,电解槽的整体温度随极间距的增大而升高。结合实际生产经验,认为极间距在75 mm时整个电解过程相对稳定,可以达到电解中槽体电压、温度分布等方面的要求。该研究结果为稀土电解槽进一步的结构优化提供了参考。     

振荡流体中离散粒子漂浮现象的初步研究

利用曲柄—连杆机构推动活塞发生对称和不对称的液体振荡,实现了密度1.179/cm³、粒度分别为0.0435、0.0401、0.0334、0.0290cm的湿离子交换树脂颗粒在水中的漂浮,及密度2.80g/cm³、粒度 0.0464cm的玻璃珠在55%甘油水溶液(粘度9.88厘泊)中的漂浮,测定了振幅为1.0、1.5、2.0、2.5cm,不对称性因子为1.0、1.2、1.4、1.6的条件下,离子交换树脂在水中的漂浮频率。在将考虑了流体对流体作用的颗粒扩散方程化简之后,推导出预测颗粒漂浮频率的含有三个待估参数的数学模型,并用水一离子交换树脂体系的实验结果对模型的参数进行了优化。结果表明,该模型能够较好地预测颗粒的漂浮频率,颗粒周围流体压力梯度的作用和流体对颗粒的曳力作用是抵销颗粒重力、导致颗粒漂浮的两个因素。计算结果还表明,在对称振荡情况下,颗粒漂浮时流体的曳力不是促其上浮而是使之下沉,而流体的压力梯度则将颗粒重力及曳力完全抵销。     

锥斗溢流管的移动床上输

在本研究以前,能否在锥斗溢流管中实现移动床上输是个有争议的问题。作者用多种物料对锥斗的几何尺寸及其他参数进行了筛选,终于实现了稳定的锥斗溢流管的移动床上输。锥斗移动床上输是一种稳定的输送状态。锥斗移动床上输的条件是,若不计壁面与固体颗粒间的摩擦力,气体对颗粒的总曳力应等于锥斗颗粒的总重力。     

无固定相分离-电感耦合等离子体质谱法在环境中痕量金属纳米颗粒分析中的应用

环境中金属纳米颗粒的分析检测不仅需要关注其浓度和化学组成,还需要对其形状、粒径和表面电荷等进行表征。此外,环境中金属纳米颗粒的分析需要解决其低赋存浓度以及复杂基质干扰的难题。无固定相分离技术与电感耦合等离子体质谱(ICP-MS)的在线联用,具有较强的颗粒分离能力和较低的元素检出限,能够快速准确地提供金属纳米颗粒的粒径分布、化学组成等信息,在金属纳米颗粒的分离检测方面表现出极大的潜能。但这一联用技术尚无法获得金属纳米颗粒物的颗粒数浓度和单个颗粒的元素信息,难以判断金属纳米颗粒涂层厚度、纯度以及颗粒的均相/异相团聚行为等。新兴的单颗粒-电感耦合等离子体质谱(SP-ICP-MS)与无固定相分离技术的在线联用,可以获得金属纳米颗粒的流体动力学粒径、元素质量计算粒径和颗粒数浓度等信息,进而弥补无固定相分离与ICP-MS在线联用技术的不足。该文介绍了流体动力色谱、毛细管电泳和场流分离3种常用无固定相分离技术的分离机制和适用检测器,着重综述了无固定相分离技术与ICP-MS/SP-ICP-MS在线联用技术的特点及其在环境金属纳米颗粒分析中的应用。关于场流分离,主要介绍了可以与ICP-MS联用的沉降场流分离和流场流分离。该文还对流体动力色谱、毛细管电泳和流场流分离与ICP-MS在线联用技术的特点进行了比较。最后,该文对无固定相分离技术与ICP-MS/SP-ICP-MS在线联用技术的发展提出了展望。     

颗粒流体系统宏观拟颗粒模拟的并行算法

基于宏观拟颗粒模型(MaPPM), 提出了一种适合不同粒径复杂粒子系统和多相流体系统的并行算法. 利用多重网格技术, 对每个计算处理器(PE)上不同粒径级别的粒子根据其所在位置进行分区管理. 应用所设计的算法, 模拟了一个二维气-固两相流系统并测算了系统中颗粒相所受的曳力和气-固相间滑移速度随时间的变化, 以及不同时刻流场中空隙率的波动情况. 计算分析结果表明该算法具有较高的并行效率和良好的可扩展性, 也体现了MaPPM模拟复杂流体系统所具有的独特优势.     

大电流稀土电解槽阳极结构研究

针对大电流稀土电解槽阳极消耗速率快、阳极消耗变形导致电热场分布不合理的现象,对15 kA大电流稀土电解槽阳极的实际消耗问题进行了分析,并运用COMSOL多物理场耦合软件对阳极消耗过程和不同阳极倾角下的三维电热场进行了耦合模拟分析。结果表明:阳极消耗变形后,电解槽内上部电场分布比下部更加密集,高温区域向液面偏移;随着阳极斜向上倾角的增大,电解槽内电压降低,高温区域向槽底偏移,倾角为4°时电热场分布最为合理。最后结合现场试验,验证了试验结果和模拟结果的一致性。     

氧碳原子比和水煤浆质量分数对水煤浆气化影响的数值模拟

用数值方法模拟了水煤浆气化过程中氧碳原子比和水煤浆质量分数对气化过程和出口煤气成分以及碳转化率的影响规律。总结了在具有复杂化学反应的高温、高压容器中，对水煤浆气化过程的数值模拟时经常遇到的问题和解决方法。得到了气化炉内的温度场、流场、浓度场以及出口粗煤气成分，其结果与工程实际相比非常接近；并利用得到的结果分析了影响水煤浆气化过程和出口煤气成分的主要因素：氧碳原子比、水煤浆质量分数等，提出了提高出口煤气有效成分（CO+H2）的方法。     

超重力强化氯碱电解反应

将超重力场应用到氯碱电解过程中, 采用线性扫描法、计时电流法和交流阻抗法研究了超重力对析氢反应和析氯反应的影响, 并进一步考察了氯碱电解槽电压随重力系数的变化规律. 结果表明, 超重力能够强化析氢反应和析氯反应的进行; 相对于常重力条件, 气泡更容易从电解液中溢出, 溶液电阻随重力系数增加而降低; 氯碱电解槽电压也随重力系数的增加而降低, 并且电流密度越大, 槽电压降低的程度越大.     

氟化物熔盐体系稀土钕电解槽出炉过程数值模拟

以8 kA稀土电解槽的出炉装置为研究对象,根据实际生产中稀土电解出金属情况,建立模型进行数值模拟,分析得出抽吸管内流体的流动特性:金属液能够从两边缝隙以较快流速流过挡块,在挡块上方形成对称的涡流,随着金属液和电解质液不断进入管道,大量金属液集中停留在挡块上方,少量金属液和气体以气泡和絮状掺杂在上方电解质液中,达到稳定后电解质不再进入吸管;对比装置中吸管入口处锥形挡块和圆柱挡块吸取金属液过程流场的变化,得出使用锥形挡块进行稀土金属出炉时,挡块上方形成的涡流更小,金属液分布更集中;分析不同直径的锥形挡块对流场的影响,得出挡块直径与电解槽接收器内径比为19/70时金属液流动更均匀。     

用直接数值模拟方法对流化床内颗粒运动区域的研究

采用直接数值模拟技术 ,跟踪离散颗粒场中的每一个颗粒 ,对不同直径和密度的颗粒在流化床内的流化运动区域进行了探讨研究。结果表明 ,不同直径和密度的颗粒在流化床内的流化区域有显著的差异。在颗粒密度相同 ,颗粒直径取正态分布时 ,小颗粒在床内大部分集中在床层的上方 ,大颗粒的运动密集区域在床的下半部分。当颗粒直径相同而密度取正态分布时 ,轻颗粒的活动范围趋向于床层的上方 ,重颗粒的运动区域分布与轻颗粒相反。还发现 ,粒径变化对颗粒流化区域的影响大于密度变化所造成的影响。     

Dynamic drag force based on iterative density mapping: A new numerical tool for three‐dimensional analysis of particle trajectories in a dielectrophoretic system

Dielectrophoresis is a widely used means of manipulating suspended particles within microfluidic systems. In order to efficiently design such systems for a desired application, various numerical methods exist that enable particle trajectory plotting in two or three dimensions based on the interplay of hydrodynamic and dielectrophoretic forces. While various models are described in the literature, few are capable of modeling interactions between particles as well as their surrounding environment as these interactions are complex, multifaceted, and computationally expensive to the point of being prohibitive when considering a large number of particles. In this paper, we present a numerical model designed to enable spatial analysis of the physical effects exerted upon particles within microfluidic systems employing dielectrophoresis. The model presents a means of approximating the effects of the presence of large numbers of particles through dynamically adjusting hydrodynamic drag force based on particle density, thereby introducing a measure of emulated particle–particle and particle–liquid interactions. This model is referred to as “dynamic drag force based on iterative density mapping.” The resultant numerical model is used to simulate and predict particle trajectory and velocity profiles within a microfluidic system incorporating curved dielectrophoretic microelectrodes. The simulated data are compared favorably with experimental data gathered using microparticle image velocimetry, and is contrasted against simulated data generated using traditional “effective moment Stokes‐drag method,” showing more accurate particle velocity profiles for areas of high particle density.     

Surface Potential of Coal Particles in Coal-Oil Mixture and Control of Settling Stability Under Gravity

This work deals with the problem of settling under gravity for coal-oil mixtures when the concentration of particles is large. The repulsive force necessary to ensure stability of coal particle is vital. The net forces acting on the particle include gravity, buoyancy, viscous drag force, and electrostatic repulsive force. Accordingly, the equation at the terminal velocity at settling is obtained along with a critical surface potential to prevent settling under gravity.     

Influence of particle-particle interactions and particles rotational motion in traveling wave dielectrophoresis

Traveling wave dielectrophoresis provides an interesting method for the controlled movement of microsized particles in suspended mixtures, and as such is a promising tool in microfluidic technology. In this case, the electrostatic force acting on the particles has two components: one due to the spatially varying magnitude of the electric field and the other due to the spatially varying phase. The actual movement of the particle is determined by the combined effect of these two forces and corresponding torques, the viscous drag exerted by the fluid on the particle, and the electrostatic and hydrodynamic particle-particle interactions. This paper presents the first numerical simulations of the motion of particles subjected to all previous forces and torques. Our technique is based on a finite-element scheme in which the particles are moved using a direct simulation scheme respecting the fundamental equations of motion for both the fluid and the solid particles. The fluid-particle motion is resolved by the method of distributed Lagrange multipliers and the electrostatic forces are computed using the point-dipole approximation. Our simulations show that the particle behavior strongly depends on the mismatch of the dielectric properties between the particles and the fluid, and that the particle-particle interaction force as well as particles rotation speeds play crucial roles in the various regimes.     

Strong magnetic field effects on solid-liquid and particle-particle interactions during the processing of a conducting liquid containing non-conducting particles

The behavior of micrometer-sized weak magnetic insulating particles migrating in a conductive liquid metal is of broad interest during strong magnetic field processing of materials. In the present paper, we develop a numerical method to investigate the solid-liquid and particle-particle interactions by using a computational fluid dynamics (CFDs) modeling. By applying a strong magnetic field, for example, 10 Tesla, the drag forces of a single spherical particle can be increased up to around 15% at a creeping flow limit. However, magnetic field effects are reduced when the Reynolds number becomes higher. For two identical particles migrating along their centerline in a conductive liquid, both the drag forces and the magnetic interaction will be influenced. Factors such as interparticle distance, Reynolds number and magnetic flux density are investigated. Shielding effects are found from the leading particle, which will subsequently induce a hydrodynamic interaction between two particles. Strong magnetic fields however do not appear to have a significant influence on the shielding effects. In addition, the magnetic interaction forces of magnetic dipole-dipole interaction and induced magneto-hydrodynamic interaction are considered. It can be found that the induced magneto-hydrodynamic interaction force highly depends on the flow field and magnetic flux density. Therefore, the interaction between insulating particles can be controlled by applying a strong magnetic field and modifying the flow field. The present research provides a better understanding of the magnetic field induced interaction during liquid metal processing, and a method of non-metallic particles manipulation for metal/ceramic based materials preparation may be proposed.     

Application of the extended RSA models in studies of particle deposition at partially covered surfaces

This paper reviews the application of the extended random sequential adsorption (RSA) approaches to the modeling of colloid-particle deposition (irreversible adsorption) on surfaces precovered with smaller particles. Hard (noninteracting) particle systems are discussed first. We report on the numerical simulations we performed to determine the available surface function, jamming coverage, and pair-correlation function of the larger particles. We demonstrate the effect of the particle size ratio and the small particle surface coverage. We found that the numerical results were in reasonable agreement with the formula stemming from the scaled-particle theory in 2D with a modification for the sphere geometry. Next, we discuss three approximate models of adsorption allowing electrostatic interaction of colloid particles at a charged interface, employing a many-body superposition approximation. We describe two approaches of the effective hard-particle approximation next. We demonstrate the application of the effective hard-particle concept to the bimodal systems and present the effect of electrolyte concentration on the effective particle size ratio. We present the numerical results obtained from the theoretical models of soft-particle adsorption at precovered surfaces. We used the effective hard-particle approximation to determine the corresponding simpler systems of particles, namely the system of hard spheres and the system of hard discs at equilibrium. We performed numerical computations to determine the effective minimum particle surface-to-surface distance, available surface function, jamming coverage, and pair-correlation function of the larger particles at various electrolyte ionic strengths and particle size ratios. The numerical results obtained in the low-surface coverage limit were in good agreement with the formula stemming from the scaled-particle theory with a modification for the sphere geometry and electrostatic interaction. We compared the results of numerical computations of the effective minimum particle surface-to-surface distance obtained using the 2D, 3D, and curvilinear trajectory model. The results obtained with the 3D and curvilinear trajectory models indicate that large-particle/substrate attractive interaction significantly reduces the kinetic barrier to large, charged-particle adsorption at a surface precovered with small, like-charged particles. The available surface function and jamming-coverage values predicted using the simplified 3D and the more sophisticated curvilinear trajectory models are similar, while the results obtained with the 2D model differ significantly. The pair-correlation function suggests different structures of monolayers obtained with the three models. Unlike the three models of the electrostatic interaction, both effective hard-particle approximations give almost identical results. Results of this research clearly suggest that the extended RSA approaches can fruitfully be exploited for numerical simulations of colloid-particle adsorption at precovered surfaces, allowing the investigation of both hard and soft-particle systems.     

Retention behavior of metal particle dispersions in aqueous and nonaqueous carriers in thermal field-flow fractionation

Until quite recently, theories on thermophoresis of particles predicted very low thermophoretic velocities of metal particles in liquids. This prediction was based on the understanding that the very high thermal conductivities of metals relative to most liquid media resulted in quite low temperature gradients across the metal particle thereby leading to low net force on the particle. In this paper, we report the retention behavior of submicrometer size metal particles of silver (Ag), gold (Au), palladium (Pd) and platinum (Pt) suspended in both aqueous and organic (specifically, acetonitrile and tetrahydrofuran) carrier liquids in thermal field-flow fractionation (ThFFF). The dependence of the metal particle retention on various factors such as particle composition, amount of added electrolyte, carrier liquid composition, field strength, channel thickness, and carrier flow-rate is evaluated and discussed. A comparison in particle retention behavior among equal-sized metal, latex and silica particles is also provided.     

Numerical calculation of the electrophoretic mobility of concentrated suspensions of soft particles

The electrophoretic mobility of spherical soft particles in concentrated colloidal suspensions is numerically calculated. The particle is modeled as a hard core coated with an ion-penetrable membrane bearing a uniform distribution of fixed charges, while the high particle concentration is taken into account by means of a cell model. The network simulation method used makes it possible to solve the problem without any restrictions on the values of the parameters such as particle concentration, membrane thickness, fixed charge density in the membrane, viscous drag in the membrane, number and valence of ionic species, electrolyte concentration, etc. The theoretical model used is similar to the one presented by Ohshima [H. Ohshima, J. Colloid Interface Sci. 225 (2000) 233], except for the use of the Shilov-Zharkikh, rather than the Levine-Neale, boundary condition for the electric potential, and the inclusion in the force balance equation of an additional term corresponding to the force exerted by the liquid on the core of the moving particle [J.J. López-García, C. Grosse, J. Horno, J. Colloid Interface Sci. 265 (2003) 327]. The obtained results only coincide with existing analytical expressions for low particle concentrations, low particle charge, and when the electrolyte concentration is high, the membrane is thick, and its resistance to the fluid flow is high. This suggests that most interpretations of the electrophoretic mobility of soft particles in concentrated suspensions require numerical calculations.     

Effect of electrostatic, hydrodynamic, and Brownian forces on particle trajectories and sieving in normal flow filtration

Particle deposition and fouling are critical factors governing the performance of microfiltration and ultrafiltration systems. Particle trajectories were evaluated by numerical integration of the Langevin equation, accounting for the combined effects of electrostatic repulsion, enhanced hydrodynamic drag, and Brownian diffusion. In the absence of Brownian forces, particles are unable to enter the membrane pores unless the drag associated with the filtration velocity can overcome the electrostatic repulsion. Brownian forces significantly alter this behavior, allowing some particles to enter the pore even at low filtration velocities. The average particle transmission, evaluated from the probability of having a particle enter the pore, increases with increasing filtration velocity due to the greater hydrodynamic drag force on the particle. These results provide important insights into particle behavior in membrane systems.     

Effects of pre-oxidation temperature on coal secondary spontaneous combustion

The hydrodynamic force (drag) on spherical and irregularly shaped particles significantly increases when the particles move close to solid and permeable boundaries. The overall effect of the increased hydrodynamic drag is to hinder the particle movement in the vicinity of boundaries and this includes the Brownian movement and electrophoresis. The Monte Carlo simulation method is used to model the Brownian movement, the resulting diffusion, and the electrophoresis of spherical particles in narrow, cylindrical pores, filled with Newtonian fluids. It is observed that the effect of the pore walls is a significant reduction of the space-averaged electrophoretic velocity of the particles, which implies reduced particle flux through the pores. The hindered electrophoresis is primarily a geometric phenomenon, caused by the increased drag and depends on the size of the particles and the pore-to-particle diameter ratio. The temperature of the fluid slightly affects the hindered electrophoresis through its effect on the viscosity, which is a determinant of the Brownian force, the diffusivity and the electrophoretic velocity. The hindered electrophoresis is almost independent of the other fluid and particle properties, such as density. Based on the simulation results a non-linear correlation for the flux of particles is derived, valid in the ranges 5?<?R/α?<?120, 5 nm?<?α?<?100 nm and 273 K?<?T?<?355 K.

     

稀土掺杂的纳米TiO2分散性及光学性能研究

采用溶胶凝胶法在同一条件下制备REa+:Ti4+=0.02和0.10的稀土掺杂的纳米TiO2材料,其中REn+为Nd3+,Ce4+,La3+,Er3+四种稀土元素.通过TG-DTG,XRD,TEM,UV-Vis等测试方法对所制备的纳米TiO2进行焙烧温度、晶胞尺寸、晶粒形貌、谱学特性等方面的研究.研究表明,稀土掺杂的纳米TiO2径粒尺寸均在20 nm左右,少量稀土掺杂不但有利于TiO2颗粒的分散,而且有利于TiO2向金红石相转变.同时,由于表面修饰作用和晶场效应的影响,掺杂使纳米材料在可见光范围内有不同程度的"红移"现象,Ce4+掺杂的纳米TiO2在可见光的"红移"程度最强.稀土掺杂的纳米TiO2在农用膜和节能材料上可广泛运用.