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。     

Validating granular segregation rate models

One significant hindrance to the development of granular segregation rate models is the inherent difficulty of performing the dynamic experiments required for validation. Here, we seek to overcome this experimental hurdle by establishing an “equilibrium” between segregation and flow perturbation in free surface granular flows and use steady‐state—rather than dynamic—measurements for validation. That is, we combine the segregation rate expressions to be tested with a segregation control framework such that the perturbation rate enables us to infer the segregation rate by measuring simply the steady state extent of segregation. We use periodic flow inversions via an axially located baffle in a tumbler‐type mixer to provide the perturbations that ultimately alter the steady‐state distribution of particles. This work examines the efficacy of existing models for binary segregation driven by either size or density differences. For completeness, we test our model validation framework both computationally and experimentally. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3756–3763, 2017     

Application of inductive heating to granular media: Modelling of electrical phenomena

A model is examined in order to predict the behaviour of a granular bed made with conductive particles subjected to an inductive electromagnetic field. The model shows how heat generated in the bed can be described by its electric impedance for the high-frequency generator required for inductive heating. Once the relationship between electrical characteristics and power dissipation has been established, comparisons between experimental and theoretical results are presented and the validity of the model is discussed.     

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     

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     

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。     

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。     

Texturing unsaturated granular media submitted to compaction and kneading processes

The aim of this study is to characterise the mass and volume of wet unsaturated granular media for an equilibrium state obtained after a compaction or kneading process. For two mixing processes for powder and water, under constant operating conditions, the dry density is correlated to water content by a characteristic relation. As observed in the dynamic compaction of soils, this phenomenon exists also for the kneading and static compaction of different raw materials. An analytical expression is suggested for modelling the relation between these two parameters.     

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     

Numerical investigation of monodisperse granular flow through an inclined rotating chute

A discrete element model of spherical glass particles flowing down a rotating chute is validated against high quality experimental data. The simulations are performed in a corotating frame of reference, taking into account Coriolis and centrifugal forces. In view of future extensions aimed at segregation studies of polydisperse granular flows, several validation steps are required. In particular, the influence of the interstitial gas, a sensitivity study of the collision parameters, and the effect of system rotation on particle flow is investigated. Shirsath et al. have provided the benchmark laboratory measurements of bed height and surface velocities of monodisperse granular flow down an inclined rotating chute. With a proper choice of the friction coefficients, the simulations show very good agreement with our experimental results. The effect of interstitial gas on the flow behavior is found to be relatively small for 3‐mm granular particles. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3424–3441, 2014     

Three dimensional discrete element modeling of granular media under cyclic constant volume loading: A micromechanical perspective

Discrete element modeling is employed to investigate the micromechanics of two granular assemblies subjected to constant-volume cyclic loading. For this purpose, two assemblies of spherical particles are modeled at the same confining pressure but with two different void ratios. The cyclic behaviors of the assemblies are inspected and the micromechanical parameters and their variations during cyclic loading are carefully observed and analyzed. The evolution of contact force networks with the progression of the loading cycles confirms that the contact force networks are hysteretic and their formation depends on the previous strain conditions of the assemblies. The distributions of the contact normals and their normal forces are also investigated to obtain a quantitative insight of the changes in the contact force networks. The probability distributions of the normal and tangential forces during cyclic loading are similar to the results of previous experimental studies that were conducted on two-dimensional specimens of granular materials. In addition, variations of the fabric tensors, which were calculated for strong contacts, are studied to trace the changes of the structural anisotropy of the specimens. The results suggest that the structural anisotropy of the specimens increases dramatically when they approach the state of liquefaction and that the degree of anisotropy is more profound in the strong contacts. Finally, the displacements of the particles during specific loading cycles are calculated to determine the relation between the movements of the particles and the changes in the macro-scale behavior of the two assemblies. The results of this study elaborate the origin of liquefaction phenomena with respect to the microstructure of the granular soils, showing the role of different mode of contacts failure in micro-scale (sliding and rolling) on the overall observed behavior of granular soils with two different relative densities, moreover the importance of strong and weak contacts in cyclic constant-volume loading of the media. It also emphasizes on the variation of structural anisotropy in undrained cyclic loading of granular media and its relationship with common soil behavior in macro-scale during liquefaction failure.     

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。     

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     

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.     

Quantitative comparison of hydrodynamic and elastoplastic approaches for modeling granular flow in silo

Hydrodynamic and elastoplastic theories are two commonly used continuum approaches for modeling granular materials. Here, the elastoplastic approach is extended to incorporate the rheology of dense granular flow, which therefore allows a quantitative comparison with the hydrodynamic approach under the same setting of rheological laws, material parameters, and numerical method. The flow patterns yielded by two approaches are apparently similar and the discharge rates are close. Yet the elastoplastic approach creates a narrow dome-like flow zone in contrast to the wide cone-like flow zone generated by the hydrodynamic approach. The shear localization is also less prominent in elastoplastic modeling owing to the existence of elastic deformation. The stresses predicted by two approaches match well in flow zones but show significant differences in stagnant zones. The proposed elastoplastic approach incorporating flow rheology can be used generally in both solid and fluid states of granular materials. © 2019 American Institute of Chemical Engineers AIChE J, 65: e16533, 2019     

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     

Dynamics and scaling of explosion cratering in granular media

Granular cratering is a ubiquitous phenomenon occurring in various natural and industrial contexts. Although impact‐induced granular cratering has been extensively studied, fewer experiments have been conducted on granular cratering via low‐energy explosions. Here, we study the dynamics and scaling of explosion granular cratering by injecting short pulses of pressurized air in quasi‐two‐dimensional granular media. Through an analysis of the dynamics of explosion processes at different explosion pressures, explosion durations, and burial depths, we identify two regimes, the bubbling and the eruption regimes, in explosion granular cratering. Our experiments explore the distinctive dynamics and crater morphologies of these regimes and show the energy scaling of the size of explosion craters. We compare high‐energy and low‐energy explosion cratering as well as explosion and impact cratering in terms of their energy scalings. Our work illustrates complex granular flows in explosion cratering and provides new insights into the general scaling of granular cratering processes. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2972–2981, 2018     

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     

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。