串罐式无钟高炉炉顶炉料运动的离散元分析

建立了串罐式无钟炉顶装料系统全模型,应用离散单元法对炉料从皮带到炉喉运动的全过程进行数值计算,考察了皮带上上下料罐内的粒度偏析,对比了料罐内是否安装石盒对料罐内炉料分布、料罐装料和卸料时炉料运动及布料时料流粒度变化的影响. 结果表明,皮带上小颗粒向料层下部渗透,皮带末端料层下部平均粒度比上部小. 炉料沿上料罐周向、径向和纵向存在粒度偏析;沿下料罐径向和纵向存在粒度偏析,周向上分布较均匀,相对粒度变化的标准差为0.03. 料罐内安装石盒对周向和纵向粒度分布影响较小,石盒附近小颗粒渗透影响径向粒度分布,料面基本水平,料罐卸料呈活塞流;无石盒时料面形成堆尖,料罐卸料呈漏斗流. 布料时料流粒度变化受料罐内料流运动和炉料分布影响,料罐内不安装石盒时料流粒度变化的标准差为7.15,安装石盒时为10.42,料流粒度变化更明显.     

竖冷设备结构的离散元法模拟优化

针对国内某钢厂竖冷设备,建立了基于离散元方法的模型,研究了竖冷设备中烧结矿石的装料过程。结果表明,由于原装料结构烧结矿石下落高度大,不同粒径烧结矿石下落速度相差明显,因此,在载料车倾料过程中出现小颗粒在下、大颗粒在上的偏析形态,同时缓冲仓内缺少限制颗粒运动的结构,导致排料过程中烧结矿石的偏析不断加剧。针对原设备的问题,首先将单一缓冲仓改为上下两层缓冲仓,上层缓冲仓用于降低颗粒下落高度,减小不同粒径颗粒的速度差;其次在缓冲仓底部引入斜栅栏结构,从而阻碍大颗粒运动,减小其运动距离,且限制了角落处的大颗粒再次进入缓冲仓出料口,避免加剧缓冲仓内的颗粒偏析。模拟结果表明新结构能有效改善竖冷设备内颗粒偏析。     

复合移动床电石反应器中颗粒运动的离散单元法模拟

针对复合移动床反应器内固体颗粒运动,采用离散单元法模型（DEM）考察布料器分别为扇形开口和矩形开口时,布料器转速和开口对颗粒运动的影响,并基于文献结果论证了本文模型的准确性。模拟结果表明：①对于不同布料器,颗粒在移动床中呈现平推流和汇聚流两种流动形态。②随布料器转速及开口的增加,颗粒质量通量非线性增加。③随布料器转速的增加,下落床径向上颗粒分布更均匀;随布料器开口的增大,下落床径向上颗粒分布范围变大,颗粒分布更均匀;对下落床径向上颗粒分布,布料器扇形开口时分布呈U形、矩形开口时分布呈M形。④沿反应器轴向向下,颗粒分布有均匀化趋势;扇形开口布料器对颗粒分布的离散系数大于1,矩形开口布料器对颗粒分布的离散系数约为0.5。     

基于图像扫描的散料颗粒离散元建模及应用

在带式机输送物料的离散元数值模拟中,非规则外形物料颗粒建模一般采用球形近似体但会降低模拟精度.试验研究了对实际土石方颗粒图形进行扫描处理,先建立其简化的三维模型,再导入离散元仿真软件中进行填充,建立土石方颗粒模型的方法.通过离散元软件EDEM对颗粒模型进行试验标定,再通过实际工程现场进行验证,结果表明图像扫描法建模所测...     

基于离散单元法的丝状颗粒传热数学模型

丝状颗粒作为一类长径比较大的非球形颗粒,其传热特性及相关技术广泛应用于工农业生产的诸多领域。但目前颗粒在运动过程中传热问题的研究还很不充分,特别是对于丝状颗粒,更是缺乏有效的数学模型进行描述。从颗粒传热机理出发,提出了一种基于离散单元法的丝状颗粒传热模型,模型中综合考虑了颗粒碰撞（接触）传热、颗粒的内部导热以及颗粒与气体间的对流换热。利用该模型,对固定床中堆积丝状颗粒的热量迁移过程进行了数值模拟,着重比较了各种传热方式对传热过程的影响。研究表明,对流换热对整体传热量的贡献较大。此外,还获得了不同工况下颗粒温度随时间的变化规律。     

提升管与下行床颗粒团聚行为的离散颗粒模拟

从微观机理出发,采用计算流体力学和离散单元方法（CFD-DEM）结合的模型对二维提升管和下行床气固流动体系进行了数值模拟。模拟选用了粒径为520 μm、密度为2620 kg·m-3的球形颗粒和周期性边界条件,展示了气固并流逆重力场和顺重力场运动的颗粒聚团瞬态图像,定性或半定量地揭示了两个不同体系的颗粒微观聚集行为。提升管中颗粒聚团较为严重,且表现明显的颗粒返混现象;下行床中的颗粒聚团比较松散,且具有与宏观流动相同的流速方向,几乎无颗粒返混。通过统计分析获得宏观时均流体力学行为,包括两相的相分布和速度分布,并与文献报道的实验现象进行定性的比较。     

颗粒移动床内不稳定运动的计算颗粒动力学模拟

采用考虑滚动摩擦的三方程离散单元法（DEM）模型对侧开孔的移动床中的颗粒流动进行了数值研究。结果表明,计算颗粒动力学(CGD)方法可对复杂颗粒系统内颗粒的运动行为进行准确的预测,包括时均速度场和脉动速度场。讨论了模型中颗粒摩擦参数的重要影响,并对颗粒流动表现出的间歇现象进行了分析。颗粒流动与流体流动有相似之处,都存在随机的脉动,但颗粒流的随机脉动机理与流体中的湍流机理有很大不同,颗粒流动会表现出很强的不连续性。     

滚筒端面对颗粒物料轴向流动特性影响的离散模拟研究

针对碎渣工艺中仅一个端面可随侧壁转动的短滚筒体系,采用离散单元法模拟研究了滚筒轴径比和转动速度对颗粒物料轴向流动特性的影响。模拟结果表明,系统内形成了显著的轴向对流结构：物料层顶部处颗粒物料会朝向滚筒固定端面一侧运动,而物料层趾部区域颗粒则朝向滚筒转动端面一侧运动。低转速条件下,沿物料自由表面由顶部到趾部,颗粒轴向速度呈非对称分布,顶部区域颗粒轴向速度绝对值显著小于趾部区域颗粒轴向速度绝对值;两部分区域颗粒轴向速度绝对值分别在y/R=±0.725处达到极大值,且轴向速度为0的位置并不出现在切向的中间位置。改变滚筒的轴长对这种非对称分布的影响近似可忽略,但是增大滚筒转速会增大颗粒轴向运动速度并逐步减弱这种非对称性。改变滚筒转速,对物料顶部区域颗粒的轴向流动的影响要大于对趾部区域颗粒轴向流动的影响。当滚筒轴径比达到1.2后,滚筒转动端面对物料轴向流动的影响区域不会随滚筒转速的增大而呈现显著变化。这些结果为实际滚筒碎渣工艺的结构优化提供了理论指导。     

提升管内颗粒团聚行为的离散颗粒模拟

采用计算流体力学和离散单元方法对二维提升管气固流动过程进行了数值模拟。采用大涡模拟方法模拟气体湍流流动,采用离散单元方法模拟颗粒碰撞过程。基于网格内流体能量守衡,提出了动能加权平均法,实现Euler坐标与Lagrange坐标的耦合与分解。模拟计算得到的瞬态流型揭示了颗粒聚团的合并和破碎过程及颗粒的详细运动行为,定性地揭示了提升管内气体-颗粒两相流动过程。     

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。     

Analysis of particle porosity distribution in fixed beds using the discrete element method

This report discusses the use of the discrete element method (DEM) to the porosity distribution of spherical particles in narrow pipes as a function of the pipe-to-particle diameter ratio. It was found that the packing structure depends mainly on the pipe-to-particle ratio and the particle friction. The numerical results with respect to the radial porosity distribution are in agreement with experimental data from the literature. Radial porosity distributions were calculated using algorithms developed by Mueller. The packing structure of the particles shows channeling for small pipe to particle diameter ratios. The simulated height averaged porosity distribution agrees with models from the literature. Moreover, DEM provides the possibility to include particle properties which reflect on the porosity distribution.     

基于颗粒尺度的离散颗粒传热模型

颗粒间传热在诸多工业过程中有着十分重要的作用。详细考虑颗粒间传热机理,对颗粒间各传热途径建模,包括颗粒内部导热、颗粒粗糙表面传热、颗粒表面气膜及接触颗粒间隙气膜传热,并与离散颗粒模型(DEM)耦合,建立颗粒尺度下离散颗粒传热模型。以固定床为对象,考察颗粒粒径、颗粒比热容、颗粒热导率及压缩负载对固定床有效传热系数的影响,并将本文计算值和文献的实验值及模型预测值对比,结果表明,该模型可定量预测固定床有效传热系数。本文建立的离散颗粒传热模型为合理预测颗粒体系内的传热提供了一种有效方法。     

二维导流管喷动床离散颗粒动力学模拟

采用离散单元法(DEM)-计算流体力学(CFD)双向耦合数值方法对二维导流管喷动床进行了模拟,颗粒的运动通过DEM模型描述,而气体的运动用Navier-Stokes方程进行求解,气体和固体颗粒之间的相互作用通过曳力形式传递。文中将DEM和边界元方法(BEM)结合起来解决颗粒在具有复杂边界设备内的运动。通过采用BEM+DEM-CFD相结合的方法进行模拟计算,得到了喷动床的最小喷动速度,研究了不同表观气速下床内的流型,得到了二维导流管喷动床的床层压降与表观气速的关系,统计分析了喷射区、环隙区内颗粒的运动速度和相应的空隙率,全面地描述了二维导流管喷动床内的气固流动特征。     

Numerical simulation of two-dimensional spouted bed with draft plates by discrete element method

A discrete element method (DEM)-computational fluid dynamics (CFD) two-way coupling method was employed to simulate the hydrodynamics in a two-dimensional spouted bed with draft plates. The motion of particles was modeled by the DEM and the gas flow was modeled by the Navier-Stokes equation. The interactions between gas and particles were considered using a twoway coupling method. The motion of particles in the spouted bed with complex geometry was solved by combining DEM and boundary element method (BEM). The minimal spouted velocity was obtained by the BEMDEM-CFD simulation and the variation of the flow pattern in the bed with different superficial gas velocity was studied. The relationship between the pressure drop of the spouted bed and the superficial gas velocity was achieved from the simulations. The radial profile of the averaged vertical velocities of particles and the profile of the averaged void fraction in the spout and the annulus were statistically analyzed. The flow characteristics of the gas-solid system in the two-dimensional spouted bed were clearly described by the simulation results. __________ Translated from Chemical Engineering (China), 2007, 35(6): 24–28 [译自: 化学工程]     

Using the discrete element method to develop collisional dissipation rate models that incorporate particle shape

Discrete Element Method simulations of Homogeneous Cooling Systems (HCS) are used to develop a collisional dissipation rate model for non‐spherical particle systems that can be incorporated in a two‐fluid multiphase flow framework. Two types of frictionless, elongated particle models are compared in the HCS simulations: glued‐sphere and true cylinder. Simulation results show that the ratio of translational to rotational granular temperatures is equal to one for the true cylindrical particles with particle aspect ratios (AR) greater than one and glued‐sphere particles with AR >1.5, while the temperature ratio is less than one for glued‐sphere particles with 1 < AR <1.5. The total collisional dissipation rate, which is associated with both translational and rotational granular temperature change rates, increases linearly with the particle aspect ratio. Thus, a collisional dissipation rate model for the elongated cylinders is developed by a simple modification of the existing spherical particle model. © 2017 American Institute of Chemical Engineers AIChE J, 63: 5384–5395, 2017     

Modelling of multiple intra-time step collisions in the hard-sphere discrete element method

The Discrete Element Method (DEM) is a commonly used tool for simulating particulate behaviours over time. DEM covers two fundamental bases: the soft-sphere and hard-sphere methods. Existing hard-sphere DEM applies collisions as sequential ordered binary collisions that satisfy momentum conservation and a restitution coefficient; multiple collisions occurring within a single simulation iteration are ordered such that collisions may then be applied in the sequential binary instantaneous manner. It is proposed that multiple intra-time-step collisions be instead applied by averaging the outcomes of all collisions for each particle as detected at the beginning of the iteration, reducing the computational burden of the method.This averaged hard-sphere DEM was compared to soft-sphere DEM with a linear contact-stiffness model yielding an equivalent restitution coefficient for two two-dimensional scenarios, one resulting in transient dilute material behaviour and the other in steady dense material behaviour. The algorithm applying each DEM was written such that direct comparison of the computational time costs of each could be made.The simulation results suggest that significant computational time cost savings are available for simulation of dilute phase materials when applying the averaged hard-sphere DEM, in particular where the physical particle properties require a small time step of soft-sphere DEM.     

Formulation of a physically motivated specific breakage rate parameter for ball milling via the discrete element method

A physically based specific breakage rate parameter of the population balance model for batch dry‐milling is formulated, which explicitly accounts for the impact energy distribution calculated by the discrete element method (DEM). Preliminary DEM simulations of particle impact tests were first performed, which concluded that dissipation energy should be used in contrast to collision energy to accurately define the impact energy distribution. Subsequently, DEM simulations of the motion of spheres representing silica glass beads in a ball mill were performed to determine the specific breakage rate parameter, which was in good agreement with those found experimentally. An analysis of the impact energy distribution, which was only possible within context of the physically motivated specific breakage rate parameter, emphasized the importance of accounting for a threshold impact energy. Without proper assessment of the impact energy distribution, DEM simulations may lead to an erroneous evaluation of milling experiments. © 2014 American Institute of Chemical Engineers AIChE J, 60: 2404–2415, 2014     

鼓泡流化床离散模拟中的一个局部空隙率模型

气固流化床离散颗粒模拟中通常采用面积加权平均法计算局部空隙率,不能较好地反映流化床中显著的非均匀结构特性.为了考虑非均匀结构对局部空隙率的影响,文中提出一个适用鼓泡流化床的局部空隙率模型.将流场划分为稀区(气泡区)和密区(乳相区);非均匀结构对密区局部空隙率的影响通过引入局部空隙率下限和非均匀影响系数描述;将提升管流动...