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。     

柱状生物质颗粒与钢球颗粒在滚筒中的混合特性

采用离散单元法DEM（discrete element method）对圆柱形生物质颗粒和钢球颗粒在滚筒中的混合进行了数值模拟,分析了滚筒转速和颗粒数量比对混合质量的影响。结果表明：在本文设定的工况下,颗粒的混合模式为阶梯模式,并且颗粒在混合时可以分成3个区域,即左面的单层钢球颗粒区、中间的钢球颗粒和生物质颗粒混合区、右面的生物质颗粒堆积区。左右两边的颗粒混合效果较差,中间的颗粒混合效果较好。当滚筒转速相同时,钢球颗粒和生物质颗粒数量比为3000∶200时的颗粒混合效果比钢球颗粒和生物质颗粒数量比为3000∶100时的好,即当钢球颗粒数量远大于生物质颗粒数量时,增加生物质颗粒的数量可以提高混合效果。在钢球颗粒和生物质颗粒数量比相同的情况下,当滚筒转速在5~25r/min的范围内,滚筒转速越高,颗粒的混合质量越好,并且颗粒混合达到稳定的时间就越短。     

移动床出口设置对卸料特性影响的离散模拟

采用离散单元法(DEM)模拟研究了二维移动床出口设置对物料卸料特性的影响。为验证模拟结果的合理性,首先针对单出口移动床进行了模拟研究,模拟结果表明:出口处颗粒质量流率满足修正的Beverloo经验关系式,而且定性上物料层内部流动区宽度随出口宽度的变化规律与实验结果吻合良好。在此基础上对两出口移动床的卸料特性进行了研究,得到以下结论:当出口宽度相等时,物料层内流动区宽度等于单出口条件下流动区宽度、出口间距及出口宽度的加和,而且每个出口处颗粒质量流率与单出口条件下颗粒质量流率相当;当出口宽度不相等时,增加大出口的宽度或减小出口之间的距离有助于提高小出口处颗粒质量流率,但大出口处颗粒质量流率基本保持恒定,不受小出口影响。     

模拟颗粒流动的离散元方法及其应用

介绍了离散元 (DEM)方法的基本原理、颗粒运动控制方程和颗粒相互作用力的数学模型。综述了DEM在流化床和固定床反应器 ,以及一些单元操作如料仓卸料过程、混合过程等中的最新应用和研究结果 ,表明DEM能够反映过程的本质机理 ,可以利用基本的数据模拟复杂的颗粒流动系统。最后指出了DEM发展中亟待解决的问题     

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。     

提升管内气固流动特性的离散元模拟

采用离散单元法模型对二维提升管内气固流动特性进行了数值模拟。利用标准k-ε模型模拟气相的湍流流动,考虑了颗粒间的van der Waals力和滚动摩擦的作用。通过对颗粒和气体流动行为的分析,得到了颗粒浓度、速度、温度及气体速度等的分布,研究了表观气速和颗粒循环速率对颗粒流动的影响。结果显示:颗粒在提升管内呈现边壁浓、中心稀的环核流动及上稀下浓的流动结构;气固两相都存在一定程度的返混现象;增加表观气速,使颗粒浓度降低、速度增大,颗粒分布更均匀;增加颗粒循环速率,使颗粒浓度增大,而颗粒速度对颗粒循环速率的变化不敏感,颗粒分布的不均匀性更强。模拟结果与文献中实验定性吻合。     

内构件对移动床内颗粒物料流动特性影响的离散模拟

针对煤热解移动床反应器中需保证颗粒物料呈整体流流动状态的设计需求,采用离散单元法模拟了闭合型(圆形、三角形)和非闭合型(八字形)3种不同内构件设置对移动床内颗粒物料流动特性的影响.结果表明,闭合型内构件设置下,当系统中形成稳定颗粒流动后,流动区宽度近似等于无内构件条件下流动区宽度与内构件在流动方向上最大投影尺寸的加和;非闭合型内构件设置下,根据内构件安放位置及翅片间距,可形成3种不同的流动结构,当翅片间距等于出口尺寸时,流动区宽度最大.在所考察的参数范围内,存在最优的内构件安放位置(30d)和翅片间距(20d)(d为最大颗粒直径),能使移动床出口处颗粒质量流率显著大于无内构件情况下的颗粒质量流率.     

循环流化床生物质颗粒掺混流动特性CPFD模拟

针对循环流化床(CFB)燃煤锅炉掺混生物质颗粒对床内颗粒流动特性的影响,文中基于计算颗粒流体动力学(CPFD)方法对CFB内煤炭颗粒掺混生物质颗粒的混合流动特性进行模拟研究,考察床层压降,并分析生物质掺混比以及不同的掺混方式对颗粒流动特性的影响。结果表明：掺混质量分数15%生物质的混合颗粒最小流化速度为0.7 m/s;生物质掺混比例的增加使最小流化速度有所提升,但提升程度不大;此外,2种颗粒的堆积方式对颗粒流动特性存在影响,当生物质堆积在煤炭上方时,会阻碍床层内颗粒的正常流化,出现节涌现象。     

格栅对新型流化床流动特性影响的模拟研究

为实现水杨酸的高效生产,提出一种以新型流化床反应器(NRFB)为核心的连续化工艺系统,内构件格栅对于NRFB内实现固体颗粒与气体高效接触具有重要作用。利用CFD模拟不同数量的格栅对NRFB内流场特性以及颗粒分布特征的影响,使用等面积环面法与均匀度指数定量地考察NRFB内颗粒分布状况。结果表明：格栅的存在能使床内轴向和径向的气相分布更加均匀;不安装格栅时,颗粒在密相段易出现中心低、壁面高的“环核流”,不利于气固相间的充分接触,安装9个格栅时,颗粒的分布均匀程度能够提升17%左右,且有更多的颗粒停留在反应密相段,有效提高反应效率。研究结果可为新型流化床的中试装置搭建及流态化操作奠定理论基础。     

颗粒外掠含不同掺混单元平板的流动换热特性

为了提高板壳式换热器的换热性能,通过离散元法研究了平面、梯形、椭圆形、梯形+椭圆形和三角形掺混单元对颗粒流动和换热的影响。研究表明:平面的掺混率几乎为零,梯形掺混单元的掺混率最高。颗粒在绕过除平面外的掺混单元时,温度边界层被破坏,并在掺混单元下游区域重新发展。在掺混单元上游区域,掺混单元对颗粒运动有阻碍作用,阻碍作用越大接触热阻越小。颗粒在梯形掺混单元下游的特征速度最大,入口平均温度最高。梯形掺混单元的掺混效率最高。在掺混单元下游区域,梯形、椭圆形、梯形+椭圆形和三角形掺混单元的传热系数显著大于平面(平均增加41.5%、31.5%、28.9%和25.3%)。相比其他掺混单元,颗粒外掠梯形掺混单元的流动换热特性最好。     

竖直振动下颗粒物质的行为模式研究进展

竖直振动下颗粒物质行为模式的研究对化工过程中效率的提升具有重要意义,该研究近年来成为热点,并不断取得新进展。本文将已有竖直振动下颗粒物质行为模式研究归纳为:竖直振动颗粒床中颗粒的行为、竖直振动颗粒床中颗粒沿直管的爬升、竖直振动U形管中颗粒的迁移、静止颗粒床中颗粒沿竖直振动管的爬升,并对相关研究进展进行评述,发现目前对颗粒物质运动规律的认识还不深入,颗粒物质行为模式的内在机理上尚存争议。鉴于离散元方法(DEM)能够获得每一个颗粒的运动信息,从而很好地反映颗粒过程的机理和特性,对竖直振动U形管和竖直振动管中颗粒物质行为模式进行DEM模拟再现;据此提出在今后的研究中应充分发挥DEM的优势,深入研究颗粒行为模式的动力学本质和影响颗粒行为模式的因素,为颗粒输运过程的优化提供理论基础和方法指导。     

The lift on a disc immersed in a rotating granular bed

The discrete element method has been used to study the lift F_L on a stationary disc immersed coaxially in a slowly rotating cylinder containing a granular material. In a tall granular column, F_L rises with the immersion depth h, but reaches a roughly constant asymptote at large h, in agreement with previous studies. Our results indicate that the argument in some earlier studies that F_L is proportional to the static stress gradient is incorrect. Instead, our results show that the lift is caused by an asymmetry in the dilation and shear rate between the regions above and below the disc. We argue that the cause of the lift is similar to that in fluids, namely that it arises as a result of the disturbance in the velocity and density fields around the body due to its motion relative to the granular bed. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5482–5489, 2017     

Particle‐scale simulation of the flow and heat transfer behaviors in fluidized bed with immersed tube

A kind of new modified computational fluid dynamics‐discrete element method (CFD‐DEM) method was founded by combining CFD based on unstructured mesh and DEM. The turbulent dense gas–solid two phase flow and the heat transfer in the equipment with complex geometry can be simulated by the programs based on the new method when the k‐ε turbulence model and the multiway coupling heat transfer model among particles, walls and gas were employed. The new CFD‐DEM coupling method that combining k‐ε turbulence model and heat transfer model, was employed to simulate the flow and the heat transfer behaviors in the fluidized bed with an immersed tube. The microscale mechanism of heat transfer in the fluidized bed was explored by the simulation results and the critical factors that influence the heat transfer between the tube and the bed were discussed. The profiles of average solids fraction and heat transfer coefficient between gas‐tube and particle‐tube around the tube were obtained and the influences of fluidization parameters such as gas velocity and particle diameter on the transfer coefficient were explored by simulations. The computational results agree well with the experiment, which shows that the new CFD‐DEM method is feasible and accurate for the simulation of complex gas–solid flow with heat transfer. And this will improve the farther simulation study of the gas–solid two phase flow with chemical reactions in the fluidized bed. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Simulation of the granular flow of cylindrical particles in the rotating drum

This study aims at unveiling the effect of particle shape on granular flow behavior. Discrete element method is used to simulate cylindrical particles with different aspect ratios in the rotating drum operating in the rolling regime. The results demonstrate that the cylindrical particles exhibit similar general flow patterns as the spherical particles. As the aspect ratio of the cylindrical particles increases, the active‐passive interfaces become steeper, and the number fraction, solid residence time, and collision force in the active region decreases. The mechanism underlying the difference is the preferential orientation, with particles of greater aspect ratios increasingly orientating their longitudinal axes perpendicular to the drum length. Also, particle alignment in the active region is more uniform than that in the passive region. The results obtained in this work provide new insights regarding the impact of particle shape on granular flow in the rotating drum. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3835–3848, 2018     

Modeling granular material blending in a rotating drum using a finite element method and advection‐diffusion equation multiscale model

A multiscale model is presented for predicting the magnitude and rate of powder blending in a rotating drum blender. The model combines particle diffusion coefficient correlations from the literature with advective flow field information from blender finite element method simulations. The multiscale model predictions for overall mixing and local concentration variance closely match results from discrete element method (DEM) simulations for a rotating drum, but take only hours to compute as opposed to taking days of computation time for the DEM simulations. Parametric studies were performed using the multiscale model to investigate the influence of various parameters on mixing behavior. The multiscale model is expected to be more amenable to predicting mixing in complex geometries and scale more efficiently to industrial‐scale blenders than DEM simulations or analytical solutions. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3277–3292, 2018