快速流化床颗粒团絮的识别和表征研究

本文针对内径D=0.1m、高H=3 m的三维可视快速流化床上升管,建立了图像采集及处理系统,对平均粒径d_s=0.25 mm的石英砂(颗粒密度ρ_p=2650 kg/m~3,堆积密度ρ_(pa)=1590 kg/m~3)在颗粒循环量G_s=40～70 kg/(m~2s),流化风速U_g=4.35 m/s下形成的颗粒团絮进行了研究,提出了一套针对三维快速流化床近壁面颗粒团絮的识别和表征方法并优化了计算参数。结果表明,该方法能够准确识别不同类型("U"形、倒"U"形、条形)的颗粒团絮;颗粒团絮的尺寸随上升管高度增加和固体循环通过减小而增大,颗粒团絮数量随上升管高度增加和固体循环通量减小而减少;随固体循环通量变化,"U"形和倒"U"形的颗粒团絮下落速度的平均值均为1 m/s左右。     

透光脉动检测仪研究碱度对絮体形成、破碎及再生的影响

为了解碱度对絮体的形成、破碎及再生过程的影响,采用PDA2000型透光脉动检测仪测定不同碱度下投加硫酸铝时高岭土悬浮液的絮凝指数(FI指数),并以强度因子和再生因子评价絮体的强度和再生能力。结果表明,碱度的高低在很大程度上会影响絮体的形成、破碎及再生过程,所形成絮体的颗粒随碱度的增加而减小。混凝剂投量和碱度高低共同决定了絮体的抗破碎强度,碱度越高,混凝剂投量越大,絮体的强度越高。在电中和作用下形成的絮体在低碱度下经一次破碎后恢复程度接近100%,在网捕卷扫作用下形成的絮体,无论碱度高低,从第2次破碎起,FI指数均以大于10%的幅度逐次下降,絮体不可恢复程度显著增大。     

黏性泥沙絮凝沉降的数值研究

盐度等因素对泥沙絮凝沉降的影响是本文的研究目的.考虑了泥沙颗粒在水中的多体相互作用, 以及颗粒间的XDLVO势.分析了盐度、泥沙浓度、Hamaker常数、水合作用对泥沙絮凝沉降的影响.获得的泥沙絮凝沉降速度的拟合公式与计算和试验相符,对于工程实际有重要参考意义. 关键词： 絮凝沉降 多体作用 斯托克斯动力学 XDLVO     

粒化仓内颗粒群流动换热的数值模拟

干式离心粒化余热回收工艺处理液态高炉熔渣时,通过粒化仓和余热回收装置分别实现高温渣液的粒化初冷和二次冷却.本文建立了粒化仓内风冷作用下熔渣颗粒群的流动换热模型,通过离散相模型和Realizable k-ε模型分别追踪颗粒群轨迹和处理湍流流动.模拟得到了颗粒群的运行轨迹、温度和速度演化以及温降等.进一步讨论了颗粒初始温度和粒径的影响,结果表明,增大渣粒初始温度和减小粒径都有利于颗粒群整体温降和平均冷却速率的提高,对仓内停留时间影响较小.但增大初始温度时应同时注意出口温度,以防实际会出现出仓颗粒群的黏结.     

采用一维湍流模型对瞬态煤粉气化火焰的数值模拟

煤的气化火焰中,湍流脉动与煤颗粒气化过程间存在着强烈的相互作用。为了在全尺度范围内直接模拟这种相互作用,本文采用一维湍流模型(ODT)与煤的气化过程相耦合,在Kolrnogorov尺度下对煤气化火焰区域进行数值模拟,得到了二维平面煤气化火焰的瞬态结构。模拟结果表明,大尺度涡团能够显著改变火焰的结构,并诱发局部小尺度涡团的产生。颗粒粒径决定湍流-气化过程作用的尺度范围,粒径较小的煤颗粒容易受到气体温度和速度脉动的影响,从而改变其运动轨迹和气化反应进程。     

湍动雾化射流液雾粒径分布的数值模拟

对水在空气中湍动雾化射流的气液两相流场进行了数值模拟.其中,气体流场采用k-ε湍流模型进行模拟,给出了载气轴向速度的分布情况.对喷雾粒子的运动采用颗粒轨道法,建立粒子破碎和碰撞的数值模型,研究了液雾粒径在不同工况下沿轴向的变化趋势.数值模拟结果与实验结果在多种气液比下进行了比较,两者吻合较好.同时,分析了初始粒径和粒子总数及喷嘴尺寸对液雾粒径变化趋势的影响,讨论了有助于粒径均匀分布的条件.     

粉末材料堆积的物理模型与仿真系统

研究了粉末材料堆积过程仿真的物理模型和系统,并探讨了适合多种不同粒径颗粒混合堆积过程仿真的高性能计算方法.在该仿真系统中,考虑了重力、接触力、阻尼力、摩擦力和范德瓦耳斯力等多种作用力的影响,集成了多种接触力模型和阻尼模型,使其适用于三维大规模粉末材料堆积过程的计算机仿真.利用该系统对粉末材料领域中的两个典型应用进行了模拟研究.模拟了两种相同密度不同粒径颗粒（粒径比为10）的混合堆积过程.当小颗粒数为大颗粒数的300倍时,得到最大的堆积密度（体积分数）为0.82.另外,还模拟了两种不同密度相同粒径颗粒的混合堆积过程.当堆积结束时,出现了明显的分离（segregation）现象和团聚现象.所研究的物理模型和仿真系统既可用于粉末材料堆积过程研究,亦可用于普通的球形物体堆积过程的模拟研究. 关键词： 粉末堆积 物理模型 仿真系统 离散元法     

激光在随机分布烟尘团簇粒子中的衰减特性

基于分形理论,采用团簇—团簇凝聚模型对随机分布烟尘团簇粒子的分形结构进行了模拟.利用离散偶极子近似方法研究了不同类型的随机分布烟尘团簇粒子的单次散射特性.利用蒙特卡罗方法研究了激光信号在随机分布烟尘团簇粒子中的传输衰减特性.讨论了入射角、激光波长、烟尘粒子数密度以及组成单个烟尘团簇粒子的原始微粒粒径和数目等参量对激光衰减特性的影响.研究结果对激光在复杂随机介质中的传输衰减特性分析提供了理论依据.     

激光在随机分布烟尘团簇粒子中的衰减特性

基于分形理论,采用团簇—团簇凝聚模型对随机分布烟尘团簇粒子的分形结构进行了模拟.利用离散偶极子近似方法研究了不同类型的随机分布烟尘团簇粒子的单次散射特性.利用蒙特卡罗方法研究了激光信号在随机分布烟尘团簇粒子中的传输衰减特性.讨论了入射角、激光波长、烟尘粒子数密度以及组成单个烟尘团簇粒子的原始微粒粒径和数目等参量对激光衰减特性的影响.研究结果对激光在复杂随机介质中的传输衰减特性分析提供了理论依据.     

CdnSen（1≤n≤12）团簇结构与电子性质的第一性原理研究

采用基于密度泛函理论的第一性原理方法对CdnSen（1≤n≤12）团簇的几何结构进行优化,并对其能量、频率以及电子性质进行了模拟分析. 结果表明, 当n=1～3时,团簇的最低能量结构是平面结构, Cd-Se键长是影响硒化镉团簇总能量的重要因素;当n=4～12时,团簇的最低能量结构可以看成是由Cd2Se2和Cd3Se3团簇的最低能量结构连接而成的三维结构,而Se-Cd-Se键角是影响硒化镉团簇总能量的主要因素. 随着团簇尺寸的增大,转移的电荷逐渐增加,转移的电荷量有达到块体中电荷值的趋势. 团簇的总能量二阶有限差分,平均结合能以及最高已占据轨道和最低未占据轨道的能隙都显示团簇的幻数为n=3,6和9.     

Monitoring the Dynamics of Concentrated Suspensions by Enhanced Backward Light Scattering

The shear-induced flocculation of kaolin-polymer flocs in a stirred tank is investigated at medium to high solids concentrations (ϕ=1−10% w/w). The evolution of the average floc size is monitored by the change in intensity of laser light scattered in the 180° direction. The measurements reflect the change in particle number concentration as flocculation proceeds. As flocculation begins, coagulation dominates and the floc size increases (total particle number decreases) and then levels off at a steady state value as fragmentation becomes significant and balances coagulation. At steady state, the measurements indicate the extent of flocculation. Increasing the shear rate increases the coagulation and fragmentation rates, resulting in smaller floc sizes at steady state. Increasing the flocculant concentration increases the steady state floc size by strengthening the bonds between primary particles to resist fragmentation. At constant shear rate and flocculant concentration, increasing the solids fraction decreases the steady state floc size indicating formation of weakly bonded flocs. Flocculant mixing was the most important factor for flocculation efficiency at high solids concentrations.     

Interaction of charged dust particles in clouds of thermodynamically equilibrium charges

The solution of the Poisson-Boltzmann equation for a cloud of charges surrounding two charged dust particles treated as Debye atoms forming a Debye molecule is investigated numerically using Cassini coordinates. The electric force exerted on a dust particle by the other dust particle was determined by integrating the electrostatic pressure on the surface of the dust particle. It is shown that attractive forces appear when the following two conditions are satisfied. First, the Debye radius (corresponding to the electron density at half the mean distance between the dust particles) must be approximately equal to half the mean distance between the dust particles. Attraction between the dust particles emerges at a distance equal approximately to half the mean distance between the dust particles. Second, attraction takes place when like charges are concentrated predominantly on the dust particles. If the particles carry a small fraction of charge of the same polarity, repulsion between the particles takes place at all distances.     

Electrostatic force of dust deposition originating from contact between particles and photovoltaic glass

Charged photovoltaic glass produces an electrostatic field. The electrostatic field exerts an electrostatic force on dust particles, thus making more dust particles deposited on the glass. In this paper, the contact electrification between the deposited dust particles and the photovoltaic glass is studied. Meanwhile, the surface charge density model of the photovoltaic glass and the electrostatic force of charged particles are analyzed. The results show that with the increasing of the particle impact speed and the inclination angle of the photovoltaic panel, the charges on particles increase to different degrees.Under a given condition, the electrostatic forces acting on the charged particles at different positions above the glass plate form a bell-shaped distribution at a macro level, and present a maximum value in the center of the plate. As the distance between the particle and the charged glass decreases, the electrostatic force exerted on the particle increases significantly and fluctuates greatly. However, its mean value is still higher than the force caused by gravity and the adhesion force,reported by some studies. Therefore, we suggest that photovoltaic glass panels used in the severe wind-sand environment should be made of an anti-static transparent material, which can lessen the dust particles accumulated on the panels.     

驻波声场中圆柱形管道周围颗粒的运动特性

针对圆柱形管道外部的流体与颗粒介质运动问题,提出了结合圆柱周围声辐射力和声流Stokes力的研究方法。从柱体外部声流方程出发,得到影响涡流结构的无量纲参数Rem≥325.27时,外涡最大流速大于内涡最大流速。在此基础上,采用Nyborg的边界滑移速度理论,获得管道外部声流的极限滑移速度,推导得出圆柱附近的声辐射力公式。基于此公式,在理论上推导出颗粒速度为0、声辐射力和声流Stokes力平衡时,颗粒临界直径的表达式。通过对圆柱位于不同位置时,圆柱外部的颗粒运动进行仿真模拟,得到与理论公式相一致的结论:颗粒的临界直径的大小与声波频率有关,当颗粒直径小于临界直径时,声流Stokes力为主导,颗粒随声流运动,颗粒直径大于等于临界直径时,声辐射力为主导,颗粒在声辐射力作用下逐渐向声辐射力的节点聚集。理论与仿真结果表明该方法可用于分析管道外颗粒的分布状态,其研究结果有助于解决电站中换热器的管道结垢、热交换率降低等问题。     

Stable solid-state source of single photons

Non-Brownian fibers commonly flocculate in flowing suspensions. A particle level simulation technique modeling fibers as chains of rods connected by hinges is developed to probe flocculation. Simulations show that flocculation can be induced solely by interfiber friction-attractive forces between fibers are not necessary. Simulated mechanical floc characteristics are consistent with experimental observations. In contrast, simulations of flocs formed by attractive forces behave qualitatively differently.     

Pseudo-random number generation for Brownian Dynamics and Dissipative Particle Dynamics simulations on GPU devices

Brownian Dynamics (BD), also known as Langevin Dynamics, and Dissipative Particle Dynamics (DPD) are implicit solvent methods commonly used in models of soft matter and biomolecular systems. The interaction of the numerous solvent particles with larger particles is coarse-grained as a Langevin thermostat is applied to individual particles or to particle pairs. The Langevin thermostat requires a pseudo-random number generator (PRNG) to generate the stochastic force applied to each particle or pair of neighboring particles during each time step in the integration of Newton’s equations of motion. In a Single-Instruction-Multiple-Thread (SIMT) GPU parallel computing environment, small batches of random numbers must be generated over thousands of threads and millions of kernel calls. In this communication we introduce a one-PRNG-per-kernel-call-per-thread scheme, in which a micro-stream of pseudorandom numbers is generated in each thread and kernel call. These high quality, statistically robust micro-streams require no global memory for state storage, are more computationally efficient than other PRNG schemes in memory-bound kernels, and uniquely enable the DPD simulation method without requiring communication between threads.     

Nanoparticle electrostatic loss within corona needle charger during particle-charging process

A numerical investigation has been carried out to examine the electrostatic loss of nanoparticles in a corona needle charger. Two-dimensional flow field, electric field, particle charge, and particle trajectory were simulated to obtain the electrostatic deposition loss at different conditions. Simulation of particle trajectories shows that the number of charges per particle during the charging process depends on the particle diameter, radial position from the symmetry axis, applied voltage, Reynolds number, and axial distance along the charger. The numerical results of nanoparticle electrostatic loss agreed fairly well with available experimental data. The results reveal that the electrostatic loss of nanoparticles increases with increasing applied voltage and electrical mobility of particles; and with decreasing particle diameter and Reynolds number. A regression equation closely fitted the obtained numerical results for different conditions. The equation is useful for directly calculating the electrostatic loss of nanoparticles in the corona needle charger during particle-charging process.     

湿颗粒堆力学特性的离散元法模拟研究

利用基于线性黏聚接触模型的离散元法对不同颗粒系统的堆积过程进行了数值模拟研究,分析了颗粒形状和湿颗粒间液桥力对颗粒堆积形态的影响机理,获得了球形和块状湿颗粒堆基底表面所受的法向力以及堆中颗粒间的法向力和切向力"中心凹陷"式的分布规律,讨论了颗粒形状和黏聚能量密度对基底表面作用力和颗粒间作用力的影响.研究结果表明,颗粒形状和液桥力对颗粒堆的堆积形态具有显著的影响.堆积角随着黏聚能量密度的增加而增大,并且相同条件下的块状颗粒堆积角大于球形颗粒.颗粒形状和黏聚能量密度对基底表面所受作用力和堆中颗粒间的作用力变化及最大幅值均有影响作用.当黏聚能量密度值逐渐增大时,颗粒堆的作用力最大幅值均逐渐增大,并且块状颗粒堆的作用力最大幅值大于球形颗粒堆.当黏聚能量密度值过大时,颗粒堆力学特性更加复杂,液桥力对颗粒堆积特性的影响作用大于颗粒形状的影响.     

Analysis of the forces acting on the saltating particles in the coupled wind-sand-electricity fields

Based on the theoretical model describing the saltation of sand particles in the coupled wind-sand-electricity fields, the numerical simulations of the forces acting on saltating particles, such as the aerodynamic drag force, Magnus effect, Saffman force and electrostatic force, are analyzed in comparison to the gravity force of the particles in the steady windblown sand movement. Furthermore, the laws of the above forces vary with the friction velocity, the diameter of the sand particle, the initial angular velocity and the lift-off velocity are discussed. Supported by the National Natural Science Foundation of China (Grant Nos. 10772075 and 10772074), the Key Project of the National Natural Science Foundation of China (Grant No.10532040), and the Program for New Century Excellent Talents in University (Grant No. NCET-04-0979)     

An Extended Entropic Model for Cohesive Sediment Flocculation in a Piecewise Varied Shear Environment

In this study, an extended model for describing the temporal evolution of a characteristic floc size of cohesive sediment particles when the flocculation system is subject to a piecewise varied turbulent shear rate was derived by the probability methods based on the Shannon entropy theory following Zhu (2018). This model only contained three important parameters: initial and steady-state values of floc size, and a parameter characterizing the maximum capacity for floc size increase (or decay), and it can be adopted to capture well a monotonic pattern in which floc size increases (or decays) with flocculation time. Comparison with 13 literature experimental data sets regarding floc size variation to a varied shear rate showed the validity of the entropic model with a high correlation coefficient and few errors. Furthermore, for the case of tapered shear flocculation, it was found that there was a power decay of the capacity parameter with the shear rate, which is similar to the dependence of the steady-state floc size on the shear rate. The entropic model was further parameterized by introducing these two empirical relations into it, and the finally obtained model was found to be more sensitive to two empirical coefficients that have been incorporated into the capacity parameter than those in the steady-state floc size. The proposed entropic model could have the potential, as an addition to existing flocculation models, to be coupled into present mature hydrodynamic models to model the cohesive sediment transport in estuarine and coastal regions.