滚筒端面对颗粒物料轴向混合过程影响的离散模拟

基于离散单元法模拟了仅颜色存在差异的两组分颗粒物料在轴径比0.3的窄滚筒中的轴向混合过程,滚筒的左侧端面固定,右侧端面可随侧壁旋转。结果表明,不同物料装载量和滚筒转速下,在达到完全混合状态前,黄红颗粒物料初始轴向界面处可能出现3种不同的径向结构：黄?红结构、红?黄?红结构和红?黄结构。红?黄?红结构和红?黄结构工况下,固定端面一侧还可出现更复杂的多层三明治结构。径向结构源自滚筒端面效应导致的颗粒轴向对流,颗粒轴向速度在切向截面上的分布决定了径向结构的类型。     

颗粒物料在半封闭式回转鼓内的混合特性

采用DEM离散单元法,对不同转速与倾角下半封闭式回转鼓内颗粒物料的混合过程进行了模拟。通过“颗粒接触数”定义的分离指数S,分析了不同转速和倾角对回转鼓内颗粒物料径向与轴向混合特性的影响。结果表明：转速与倾角对回转鼓内颗粒物料径向与轴向混合特性有显著的影响;倾角不变,转速分别为15r/min、30r/min、45r/min时,颗粒物料的径向与轴向混合速度随转速的增加而增加,当转速超过30r/min后,增加转速对径向与轴向混合速度的影响越来越小;转速不变,倾角分别为0°、17°、34°时,增大倾角能有效的增加轴向混合速度,但对径向混合速度没有促进作用,当倾角超过17°后,轴向混合速度的增幅随着倾角的增加而逐渐变小,而径向混合速度随着倾角的增大而减小,但增加转速可以减小径向混合速度下降幅度。     

提升管内气固两相双组分颗粒流动的离散颗粒硬球模型的模拟

基于离散颗粒(DPM)硬球模型，数值模拟提升管内双组分颗粒气固两相湍流流动行为。应用Vreman的亚格子尺度（SGS）模型模拟气体湍流，建立考虑不同颗粒加速度效应的两颗粒碰撞最小时间计算模型。数值模拟预测了大颗粒和小颗粒的速度和浓度分布。研究结果表明小颗粒具有高的轴向速度和脉动速度，而大颗粒具有低的轴向速度和脉动速度。在床中心区域，小颗粒轴向速度分布出现3个峰值，对于大颗粒轴向速度仅出现两个峰值。在壁面区域大颗粒和小颗粒速度均出现两个峰值。沿床径向方向呈现床中心颗粒浓度低、壁面区域颗粒浓度高的环核流动结果。随着表观气速的增大，颗粒浓度沿径向和床高分布趋于均匀。在床中心区域模拟计算轴向颗粒速度、颗粒浓度和RMS速度与文献实验结果相吻合。在提升管内气体湍流对小颗粒流动具有一定的影响，颗粒间碰撞作用对颗粒相流动的影响大于气相湍流的影响。     

移动床内颗粒共振行为的DEM模拟

移动床在各种过程工业中普遍存在。从颗粒尺度出发刻画移动床内颗粒物料的复杂流动行为，对于大型移动床反应器的设计、放大和优化具有重要的意义。本工作基于三维离散单元法(Discrete Element Method, DEM)数值模拟，考察了漏斗流和半整体流卸料流型下移动床内颗粒物料的运动特性，重点探讨了两种卸料流型下颗粒运动脉动特性的异同。首先通过对比流动区轮廓及其特征宽度随时间演化的模拟预测结果和实验测量结果，检验了DEM模拟结果的可靠性。DEM模拟结果表明，两种卸料流型下，流动区上部区域不同位置处颗粒平均轴向速度随时间的变化都呈现出显著的非随机波动，表现为颗粒平均轴向速度时间序列的离散傅里叶变换频谱图都出现了明显的特征峰。空间相关性分析结果表明，在流动区上部区域，不同位置处颗粒轴向速度随时间的波动呈现强烈的空间相关性，两种卸料流型下体系内都形成了局部共振。不同轴向位置处颗粒轴向速度的延迟相关性分析结果表明，这种共振行为源自于床层底部出口上方。对颗粒轴向速度和颗粒间接触力之间延迟相关性的分析结果显示，两者之间存在显著的相关性，且后者的波动先于前者的波动，说明模拟得到的共振现象可能源于自由...     

气固逆流下行流化床中颗粒速度的径向与轴向分布

在内径90 mm、高7 m的逆流下行床冷态实验装置中,研究了气固逆流下行床中循环锅炉灰(dp=300 mm)颗粒速度的径向分布及其沿轴向发展. 结果表明,局部颗粒速度沿径向分布是不均匀的,在完全发展区,颗粒速度中心和边壁低、在r/R=0.85附近颗粒速度最大. 由大量实验数据回归出预测充分发展段局部颗粒速度的关联式,该公式计算值与实验值的平均相对偏差小于±11%. 不同径向位置的局部颗粒速度沿轴向的变化趋势不同,边壁区域(r/R>0.622)颗粒速度沿轴向单调递增,而中心区域(0相似文献   

白钨矿连续酸浸槽内固体颗粒的流动特性实验及模型研究

以停留时间分布（RTD）为评价指标,对硫磷混酸浸出白钨矿的连续浸出槽内固相颗粒流动行为进行实验研究。同时探究了进料流量、搅拌转速、连续浸出槽中物料进出口位置组合对固相颗粒流动行为的影响。实验结果表明：随着进口流量的增大,一开始槽内的返混程度得到了增强,量纲为1化方差变大,但是继续增大进口流量,进口处物料的横向迁移速度加强,使槽内流体流动趋向平推流,导致量纲为1化方差减小;量纲为1化方差随着搅拌转速的增大而增大,但是此时在槽下部区域会逐渐形成循环死区,槽内死区体积分数随之增大;平均停留时间随着物料进出口位置的变化而发生变化,下进下出的进出料位置组合其平均停留时间最大,且最接近理论平均停留时间。最后利用非理想流动模型来表征实验过程中的停留时间分布,模型拟合的停留时间曲线与实验测量的曲线吻合程度良好。     

多粒径颗粒在圆形偏心滚筒内的运动混合

采用离散单元法(DEM),对5种粒径颗粒在滚筒内的运动混合进行数值模拟,研究了偏心距对滚筒内颗粒体系分区及区域变化、大颗粒速度变化和多粒径颗粒混合程度的影响。结果表明;非偏心滚筒内处于滚落运动模式的颗粒体系分为3个区域:平流层、活动层和涡心;偏心滚筒内不存在实际的涡心,随着滚筒转动,"移动涡心"的形状大小和位置都在周期性变化。在不同颗粒运动周期中,非偏心滚筒内的大颗粒在平流层中的匀速度基本相等;而在偏心滚筒内,大颗粒在相邻两次平流层中的两次匀速度基本都不相等,且偏心距越大的滚筒内,颗粒匀速度的变化越明显。偏心距基本不影响滚筒内多粒径颗粒的接触效果,多粒径颗粒在滚筒内的混合过程中,接触数的变化类似阻尼振动曲线变化。     

基于颗粒缩放理论的垂直螺旋输送离散模拟

利用相似理论和量纲分析进行颗粒缩放,将粒径0.5 mm的铸铁煤粉颗粒放大至2 mm,进行虚拟实验标定放大颗粒的基本接触参数,用离散单元模型模拟垂直螺旋输送中的颗粒运动,验证颗粒缩放法的准确性.结果表明,螺旋转速为500,550,600,650 r/min时,颗粒轴向速度和圆周速度增大,最大轴向速度位于中上部,处于叶片边缘,分别为0.486,0.535,0.596,0.657m/s,超过螺旋叶片半径区域的颗粒轴向速度明显减小;相同填充率下,装置出口处质量流率、颗粒轴向速度模拟值和实验值基本吻合,满足相似理论.     

工艺条件对内旋式移动床停留时间的影响

为了给热解过程中颗粒在内旋式反应器内的停留时间和混合效果提供理论指导,在内旋式移动床试验装置上,以煤颗粒作为填充物料和示踪剂,利用脉冲法研究了转轴长度、转轴转速、粒径及给料速率等工艺条件对物料停留时间的影响。结果表明:60°倾角安装的桨叶式传动叶片测得的平均停留时间分布曲线存在较大的"拖尾",曲线尾峰较多,表明反应器内存在返混;反应器内煤料流动流型介于平推流与全混流之间;转轴长度与平均停留时间呈线性关系;转轴转速与平均停留时间呈指数关系,增大转速会加剧轴向返混,同时缩短物料在反应器内的平均停留时间;随着煤料粒径的增大,反应器内平均停留时间和轴向返混程度均呈减小趋势,且粒径大的煤颗粒在输送过程中会出现一定程度的破碎,5~6 mm煤颗粒的破碎率为35.39%;给料速率主要影响煤料在反应器内的填料量,从而影响停留时间分布,平均停留时间随着给料速率的增加而降低。     

立式粉体干燥器中不同粒径湿颗粒流动特性研究

为了研究立式粉体干燥器内不同粒径聚甲醛颗粒流动特性，采用CFD-DEM模型耦合液桥力模块的方法，分析了含水率和颗粒粒径对聚甲醛颗粒流动特性的影响，并验证了该分析方法的正确性。研究结果表明：基于量纲分析法，结合已有实验数据拟合得出平均颗粒速度的经验关联式，计算值与实验值的最大误差为25.13%，可以较好地描述干燥单元内的平均颗粒速度变化。截面平均固含率随着颗粒直径的增大而降低，干燥单元内固含率在轴向和径向分布上分别呈现“上浓下稀”、“边壁高近流体低”的特点。当颗粒直径d_p<2 mm且含水率V_lb≥0.1%时，容易造成干燥单元入口堵塞，并且干燥单元内固含率剧减。截面平均颗粒速度随着轴向高度和颗粒直径的增大而增大，从边壁区到近流体区域的局部颗粒速度逐渐减小。当颗粒直径相同时，湿颗粒比干颗粒的颗粒速度小；当颗粒直径超过临界值2 mm时，随着含水率的增高，颗粒速度略微减小，颗粒平均停留时间和固含率总体上有所增加。     

Granular flow in a rotating drum with gaps in the side wall

This paper presents a numerical investigation of the motion of bi-sized particles in a short rotating drum by using Discrete Element Method (DEM). The side wall of the drum has equally spaced gaps whose width is just between the two particle diameters. One end wall of the drum is fixed while the other rotates with the side wall. Small particles are fed into the drum continuously at the center region in the axial direction. The effect of rotating speed on the volumetric holdup and residence time of small particle is investigated. A critical rotating speed is found, below which the decrease of rotating speed will increase the volumetric holdup and the residence time of the small particles sharply. A jump in the axial distribution of the outflow rate of the small particles is observed at the region adjacent to the fixed end wall. The flow pattern inside the drum is analyzed. In the region between the fixed end wall and the feeding point, all small particles, on average, move towards the fixed end wall. While in the region between the rotating end wall and the feeding point, the small particles curve away the rotating end wall in the upper part of the charge and return to this wall in the lower part. The particle temperature distributions at different rotating speeds are explored to understand the flow behavior observed in these simulations.     

Axial dispersion of granular material in horizontal rotating cylinders

The discrete element method is used to calculate axial dispersion coefficients for approximately monosized particles in a rotating horizontal cylinder. Axial dispersion within the cylinder is shown to follow Fick's second law, in that the mean squared deviation of axial position of a pulse of particles is proportional to time. The axial dispersion coefficient is found to depend on the particle size, gravity and drum rotation speed, allowing a dimensionless group to be formed using these four quantities. For sufficiently large cylinders, the axial dispersion coefficient is found to be independent of drum diameter. A general argument is given which suggests that axial dispersion in physical beds of approximately monosized particles should follow Fick's second law.     

Microdynamic analysis of particle flow in a horizontal rotating drum

The flow of particles in a horizontal rotating drum is studied based on the results generated by Distinct Element Method (DEM). The simulation conditions are comparable to those measured by means of Positron Emission Particle Tracking (PEPT), with a drum being 100 mm in diameter, 35% filled by spheres of 3 mm diameter, and rotating at a speed from 10 to 65 rpm. The simulation method is validated from its good agreement with the PEPT measurement in terms of the dynamic angle of repose and spatial velocity fields. The dependence of flow behaviour on rotation speed is then analysed based on the DEM results, aiming to establish the spatial and statistical distributions of microdynamic variables related to flow structure such as porosity and coordination number, and force structure such as particle interaction forces, relative collision velocity and collision frequency. An attempt has also been made to explain the effect of rotation speed on agglomeration based on the present findings.     

Characterization of Mixing and Size Segregation in a Rotating Drum by a Particle Tracking Method

The mechanisms of segregation in solids mixing, even in simple rotating drums, are not clearly understood. Although most past studies have focused on binary mixtures, this work investigates the effect of polydispersity on granular flow, mixing, and segregation in a rotating drum operated in rolling regime through particle trajectories obtained from the radioactive particle tracking technique. Velocity profiles, radial segregation, and axial dispersion coefficients for monodisperse and polydisperse systems of glass beads are analyzed with respect to rotational speed and particle size. A model is introduced to predict the residence times along streamlines and evaluate the rate at which the material renews at the free surface and within the inner layers of the bed. Our results reveal similar velocity profiles and residence times for monodisperse and polydisperse systems. They also indicate that the particles distribute along the radial direction of the drum, although not necessarily in a core/shell configuration. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1894–1905, 2013     

基于离散相模型的旋转填充床内的流场分析

利用CFD软件Fluent初步模拟了旋转填充床(Rotating Packed Bed,RPB)内的流体流动.计算中气相采用RNGk-ε湍流模型,液相采用拉格朗日离散相模型(DPM).通过一定简化后建立了旋转填充床二维模型,考察了液体颗粒在填充床内的速度分布,以及转速对液体颗粒速度的影响.结果显示液滴速度随转速的增加而增加,转速对液滴径向速度的影响不大.此外还针对转速、气体进口速度对干床压降的影响做了一定的研究,发现压降随转速的增加而增加,但其影响没有气体进口速度对压降的影响明显.填料的内缘处存在剧烈的端效应,模拟结果表明,端效应区的湍动强度明显大于其他区域.     

卧螺离心机用于盐泥固液分离时转鼓转速对流场特性的影响

对矿井采卤制盐工艺产生的盐泥使用卧螺离心机进行固液分离,应用计算流体力学软件Fluent,基于多相流Eulerian模型、RNG k-ε湍流模型及多重参考系（MRF）方法,对卧螺离心机流体域进行三维数值模拟,采用仿真与实验相结合的方式,研究了转鼓转速与分离特性的关系,结果表明：卧螺离心机在考虑螺旋叶片情况下比忽略螺旋叶片时切向速度滞后系数有明显提升;提出修正离心液压的概念,通过该理论数值证明分离液静压值与理论值的误差源于切向速度的滞后;当转鼓转速达3000r/min以上继续提高时,沉渣固相质量分数不会随之继续增大;分离液中固相微粒的沉降速度随转速增大会小幅均匀增大;实验表明卧螺离心机用于高浓度卤水高速离心时,将在径向产生一定程度的NaCl浓度梯度,溶质浓度在分离液外层较高,内层较低。     

Experiments on floating bed rotating drums using magnetic particle tracking

Magnetic particle tracking (MPT) was employed to study a rotating drum filled with cork particles, using both air and water as interstitial medium. This noninvasive monitoring technique allows for the tracking of both particle translation and rotation in dry granular and liquid–solid systems. Measurements on the dry and floating bed rotating drum were compared and detailed analysis of the bed shape and velocity profiles was performed. It was found that the change of particle–wall and particle–particle interaction caused by the presence of water significantly affects the bed behavior. The decreased friction leads to slipping of the particles with respect to the wall, rendering the circulation rate largely insensitive to increased drum speed. It was also found that the liquid–particle interaction is determining for the behavior of the flowing layer. The well-defined experiments and in-depth characterization performed in this study provide an excellent validation case for multiphase flow models.     

The movement of solids through flighted rotating drums. Part II solids-gas interaction and model validation

A residence time model for a rotating drum with lifting flights which includes axial displacement of airborne particles due to the drag of the gas stream has been developed in Part I of this study (Sherritt et al., 1993). Because the particles fall in curtains, the equations for single particle drag overestimated the effect of the gas stream on the displacement of the falling particles. In this paper, large scale wind tunnel experiments, which involved pouring a curtain of particles into a gas stream, indicated that the shielding effect of the curtain could be represented by a lower-than-average gas velocity within the curtain. The magnitude of the curtain shielding effect is related to the flight discharge rate, the fall distance and the flight length. Incorporating the results of the experiments into the residence time model provides a significant improvement on the effect of the gas velocity, and hence the best model available, especially for industrial-scale equipment.     

AN RTD STUDY FOR THE FLOW OF LIGNITE PARTICLES THROUGH A PILOT ROTARY DRYER PART I; BARE DRUM CASE

In Part I of the present work a pilot rotating cylindrical drum, without an internal lifting flight system (bare) has been employed for the study of lignite motion through it, at ambient temperature. Tracer pulse stimulus - response experiments have been carried out io deduce residence time distribution ( RTD) data and relate them to the operating conditions ( slope, speed of revolution, etc.). Mean residence time, space-time and solids hold-up have been correlated with the drum operating conditions. Experimental data of mean axial velocity of solids have been compared with theoretical predictions and found to deviate within a ± 15% margin. A size segregation of particles during their motion through the kiln under a variety of operating conditions has been confirmed and quantified. An average maximum divergence of 20% between the residence time of the smallest and that of the largest nominal particle sizes has been assessed.