Density segregation of dry and wet granular mixtures in gas fluidized beds

The Discrete Element Method combined with Computational Fluid Dynamics was coupled to a capillary liquid bridge force model for computational studies of mixing and segregation behaviors in gas fluidized beds containing dry or wet mixtures of granular materials with different densities. The tendency for density segregation decreased with increasing fluidizing velocity, coefficient of restitution, and amount of liquid present. Due to the presence of strong capillary forces between wet particles, there was a high tendency for particles to form agglomerates during the fluidization process, resulting in lower segregation efficiency in comparison with fluidization of dry particles. Particle‐particle collision forces were on average stronger than both fluid drag forces and capillary forces. The magnitudes of drag forces and particle‐particle collision forces increased with increasing fluidizing velocity and this led to higher mixing or segregation efficiencies observed in dry particles as well as in wet particles at higher fluidizing velocities. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4069–4086, 2015     

Density segregation in vibrated granular beds with bumpy surfaces

Segregation of granular materials by virtue of density or size is a commonly encountered phenomenon in nature. Despite its widespread interest among many researchers in recent years, a complete and unified understanding of granular segregation remains elusive to date. Using molecular dynamics simulations, we report a novel technique of inducing density segregation in a binary mixture of granular materials subjected to vibrations by the use of a bumpy vibrating base. Density segregation in the vertical directions may be induced by oscillating the bumpy base composed of discrete solid particles vertically or horizontally. In both cases, lighter particles tended to rise to the top of the granular bed and form a layer above the heavier particles. We suggest that differences in granular temperature profiles arising from the two different modes of vibrations may play an important role in determining the extent of density segregation occurring in binary granular mixtures. © 2010 American Institute of Chemical Engineers AIChE J, 2010     

A continuum model for the segregation of bidisperse particles in a blade mixer

The process of blending powders using stirring blades involves complicated granular flows, particle-scale mechanisms, and blade–particle interactions, which is challenging to predict and control. This article proposes a continuum-based model for such a process by incorporating the flow rheology, isotropic particle diffusion and the percolation of granular materials. A method combining finite element method (FEM), finite difference method (FDM), and immersed boundary method (IBM) is developed to numerically implement the continuum model and applied to a cylindrical blade mixer. The model well describes the tempo-spatial distribution of small/large particles in the stirring process, such as the accumulation of small particles in the vicinity of blades. Remarkably, this model can capture the various intricate effects of blade parameters, including the blade rake angle, rotating speeds, filling level, and the friction coefficient of the mixer wall. It is therefore promising for optimizing the blade mixers in industries.     

Numerical study of mixing and thermal conduction of granular particles in rotating tumblers

A discrete element method (DEM) study is conducted to investigate the mixing and heat‐transfer characteristics of steel spherical particles under various rotation speeds and flow regimes of a rotating tumbler. The mixing degree, weighted temperature, temperature discrepancy at the mixing interface, temperature radial distribution, and information entropy are used to analyze the effect of mixing structure and evolution duration on the heat‐transfer characteristics. The results under the same revolution and the same evolution time are compared to show the effects of evolution time and mixing structure on thermal conduction. After a detailed analysis, the joint contribution of mixing degree and duration to granular heat transfer is explained, and the different approaches in static thermal conduction and dynamic mixing are shown. Moreover, a new method is proposed using the mean increase rate of temperature information entropy to determine the most effective operating condition for thermal conduction in granular particles. © 2013 American Institute of Chemical Engineers AIChE J, 59: 1906–1918, 2013     

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。     

螺旋输送器中颗粒混合过程的模拟

采用离散单元法（discrete element method,DEM）模拟了工业尺度螺旋输送器中两种不同密度与粒径的颗粒的流动状态与混合过程。采用Lacey混合指标的定量分析表明,该输送器的混合性能强烈地依赖于操作条件和结构尺寸,转速和物料的添加速率对混合效果影响最大,其次是混合段螺距和物料粒径。根据工业生产的具体要求,可参考上述发现合理地选择技术参数、提高混合效率、降低能耗。     

Simulation study of the effect of wall roughness on the dynamics of granular flows in rotating semicylindrical chutes

A discrete element model (DEM) is used to investigate the behavior of spherical particles flowing down a semicylindrical rotating chute. The DEM simulations are validated by comparing with particle tracking velocimetry results of spherical glass particles flowing through a smooth semicylindrical chute at different rotation rates of the chute. The DEM model predictions agree well with experimental results of surface velocity and particle bed height evolution. The validated DEM model is used to investigate the influence of chute roughness on the flow behavior of monodisperse granular particles in rotating chutes. To emulate different base roughnesses, a rough base is constructed out of a square close packing of fixed spherical particles with a diameter equal to, smaller, or larger than the flowing particles. Finally, the DEM model is used to study segregation in a binary density mixture for different degrees of roughness of the chute. © 2015 American Institute of Chemical Engineers AIChE J, 61: 2117–2135, 2015     

Mixing behaviors of wet granular materials in gas fluidized bed systems

The discrete element method combined with computational fluid dynamics was coupled with a capillary liquid bridge force model for computational studies of mixing behaviors in gas fluidized bed systems containing wet granular materials. Due to the presence of strong capillary liquid bridge forces between wet particles, relative motions between adjacent particles were hindered. There was a high tendency for wet particles to form large aggregates within which independent motions of individual particles were limited. This resulted in much lower mixing efficiencies in comparison with fluidization of dry particles. Capillary liquid bridge forces were on average stronger than both fluid drag forces and particle–particle collision forces and this accounted for the difficulty with which individual particles could be removed and transferred between aggregates. Such exchange of particles between aggregates was necessary for mixing to occur during fluidization of wet granular materials but required strong capillary liquid bridge forces to be overcome. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4058–4067, 2013     

Modeling segregation of polydisperse granular materials in developing and transient free-surface flows

Predicting segregation and mixing of polydisperse granular materials in industrial processes remains a challenging problem. Here, we extend the application of a general predictive continuum model that captures the effects of segregation, diffusion, and advection in two ways. First, we consider polydisperse segregating flow in developing steady segregation and in developing unsteady segregation. In both cases, several terms in the model that were zero in the previously examined case of fully developed streamwise-periodic steady segregation in a chute are now non-zero, which makes application of the model substantially more challenging. Second, we apply the polydisperse approach to density polydisperse materials with the same particle size. Predictions of the model agree quantitatively with experimentally validated discrete element method (DEM) simulations of both size polydisperse and density polydisperse mixtures having uniform, triangular, and log-normal distributions. © 2018 American Institute of Chemical Engineers AIChE J, 65: 882–893, 2019     

Simulation and modeling of segregating rods in quasi‐2D bounded heap flow

Many products in the chemical and agricultural industries are pelletized in the form of rod‐like particles that often have different aspect ratios. However, the flow, mixing, and segregation of non‐spherical particles such as rod‐like particles are poorly understood. Here, we use the discrete element method (DEM) utilizing super‐ellipsoid particles to simulate the flow and segregation of rod‐like particles differing in length but with the same diameter in a quasi‐2D one‐sided bounded heap. The DEM simulations accurately reproduce the segregation of size bidisperse rod‐like particles in a bounded heap based on comparison with experiments. Rod‐like particles orient themselves along the direction of flow, although bounding walls influence the orientation of the smaller aspect ratio particles. The flow kinematics and segregation of bidisperse rods having identical diameters but different lengths are similar to spherical particles. The segregation velocity of one rod species relative to the mean velocity depends linearly on the concentration of the other species, the shear rate, and a parameter based on the relative lengths of the rods. A continuum model developed for spherical particles that includes advection, diffusion, and segregation effects accurately predicts the segregation of rods in the flowing layer for a range of physical control parameters and particle species concentrations. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1550–1563, 2018     

A study of the axial and radial competition segregation in a rotating drum with internal diameter changing

A binary mixture is mixed in a rotating drum composed by 19 rings with different inner diameters. It is found that the larger particles are concentrated in the rings with smaller inner diameters. The collection of the larger particles in these rings is due to the particle dynamic angle of repose. The transition from the particle segregation core pattern at the end wall to the good radial mixing rings is through a transient turning comet segregation pattern. This transition is closely related to the distance from the ring with the smallest inner diameter to the ring with the largest inner diameter. Having a higher fraction of larger particles in a ring requires both the collection ring having a smaller inner diameter and a smoother inner diameter transition to the neighboring rings.     

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。     

Diffusion,mixing, and segregation in confined granular flows

Discrete element method simulations of confined bidisperse granular shear flows elucidate the balance between diffusion and segregation that can lead to either mixed or segregated states, depending on confining pressure. Results indicate that the collisional diffusion is essentially independent of overburden pressure. Because the rate of segregation diminishes with overburden pressure, the tendency for particles to segregate weakens relative to the remixing of particles due to collisional diffusion as the overburden pressure increases. Using a continuum approach that includes a pressure-dependent segregation velocity and a pressure-independent diffusion coefficient, the interplay between diffusion and segregation is accurately predicted for both size and density bidisperse mixtures over a wide range of flow conditions when compared to simulation results. Additional simulations with initially segregated conditions demonstrate that applying a high enough overburden pressure can suppress segregation to the point that collisional diffusion mixes the segregated particles. © 2018 American Institute of Chemical Engineers AIChE J, 65: 875–881, 2019     

Designing minimally segregating granular mixtures for gravity-driven surface flows

In dense flowing bidisperse particle mixtures varying in size or density alone, smaller particles sink (percolation-driven) and lighter particles rise (buoyancy-driven). But when particle species differ from each other in both size and density, percolation and buoyancy can either enhance (large/light and small/heavy) or oppose (large/heavy and small/light) each other. In the latter case, a local equilibrium can exist in which the two mechanisms balance and particles remain mixed: this allows the design of minimally segregating mixtures by specifying particle size ratio, density ratio, and mixture concentration. Using DEM simulations, we show that mixtures specified by the design methodology remain relatively well-mixed in heap and tumbler flows. Furthermore, minimally segregating mixtures prepared in a fully segregated state in a tumbler mix over time and eventually reach a nearly uniform concentration. Tumbler experiments with large steel and small glass particles validate the DEM simulations and the potential for designing minimally segregating mixtures.     

Screw-generated forces in granular media: Experimental,computational, and analytical comparison

This study presents an experimental, computational, and analytical comparison of a submerged, double-helix Archimedes screw generating propulsive force against a bed of glass beads. Three screws of different pitch lengths were studied. Each screw was tested at six speeds in approximately 10 trials for a total of 180 experimental trials. These experiments were then replicated in EDEM, a discrete element method (DEM) software program. DEM simulation results for thrust forces in the 30–120 rpm regime had a 5%–20% inflation of forces compared to experimental results. These simulations were then compared with resistive force theory (RFT) plate approximation of the screw geometries. We analyze a superposition-based partition approach to the full-length screws as well as force generation in shortened, one- and two-blade screws. We find that the force generation is dependent on the flow patterns and cannot be reduced to partitioned approximations as with simple intruders. © 2018 American Institute of Chemical Engineers AIChE J, 65: 894–903, 2019     

Granular flow in a wedge-shaped heap: Velocity field,kinematic scalings,and segregation

Free surface granular flows in bounded axisymmetric geometries are poorly understood. Here, we consider the kinematics and segregation of size-bidisperse flow in a rising conical heap by characterizing the flow of particles in a wedge-shaped silo with frictional sidewalls using experiments and discrete-element-method simulations. We find that the streamwise velocity is largest at the wedge centerline and decreases near the sidewalls, and that velocity profiles in the depthwise and spanwise directions are self-similar. For segregating size bidisperse mixtures, the boundary between small and large particles deposited on the heap is significantly further upstream at the sidewalls than at the centerline, indicating that measurements taken at transparent sidewalls of quasi-2D or wedge-shaped heaps are unrepresentative of an axisymmetric heap. The streamwise velocity and flowing layer depth locally satisfy the scaling relation of Jop et al (J Fluid Mech. 2005;541:167-192) when modified to account for the wedge geometry, highlighting the influence of wall friction on the flow.     

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。     

Segregation of granular particles by mass,radius, and density in a horizontal rotating drum

The impact of particle properties on segregation and mixing of bidisperse granular beds in a rotating horizontal drum have been studied by discrete element method (DEM) simulations. Bidispersities in radius, density, and mass have pronounced influences on the stationary mixing pattern, although they hardly affect the granules' flow regime. At 50% fill level, all beds mix well for a Froude number of ～0.56, corresponding to a flow regime intermediate to cascading and cataracting, while segregation occurs both at lower (rolling and cascading regime) and higher (cataracting/centrifuging regime) Froude numbers. These observations are explained qualitatively by noticing that the angular drum velocity dictates the flow regime, which in turn determines the effectiveness and direction of four competing (de)mixing mechanisms: random collisions, buoyancy, percolation, and inertia. A further dozen particle properties have been varied, including the friction coefficients and elastic modulus, but these proved inconsequential to the steady‐state degree of mixing. © 2013 American Institute of Chemical Engineers AIChE J, 60: 50–59, 2014     

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