带挡板的转筒混料器的离散颗粒模拟

用离散颗粒模拟方法计算了带两块挡板的旋转混料器中所有颗粒的三维运动。该模拟法可用于单组分球形颗粒的混合。本文研究了挡板对颗粒运动和颗粒混合的影响，研究表明，表示混合程度与时间之间关系的混合曲线在数值上与过去实验结果很吻合。同该模拟法可以预言：对于颗粒混合，挡板有一个最佳长／高比，且料层的变形也受其影响。带挡板的转筒混料器的离散颗粒模拟@YoshitsuguMuguruma     

筒仓卸料时超压与变形分析

针对筒仓卸料时侧压增大对仓壁变形的影响,运用离散元法模拟煤炭颗粒在筒仓中的静储和卸料过程,导出的离散元仿真数据结合筒仓模型进行有限元分析;根据离散元分析可以得出卸料时筒仓侧压及增压比的变化规律,从有限元计算结果得到卸料时径向变形及位移比的分布。结果表明:卸料时增压比与径向位移比变化基本一致,增压比与料仓内颗粒流动速度存在一定的线性关系;静、动态时侧压峰值出现在直筒壁与漏斗壁过渡处靠近直筒壁一侧,侧压增压比在筒仓中上部达到最大值,必须强化过渡处和筒仓上部的结构设计。     

筒仓卸料的离散单元法模拟

为研究筒仓内颗粒流动的内在机理,采用颗粒离散元软件模拟贮煤筒仓的卸料过程,观察煤粒在仓筒中的流动情况及单个颗粒在流动过程中的位移和速度变化,测得静态和动态时对仓筒壁的侧压力和料斗在某一平面上的压力变化。结果表明:仓筒内颗粒流颗粒流动过程中逐渐向中心聚拢,同一水平面上的颗粒轴线上的先流出,卸料时动态侧压力大于静压力,料斗竖直方向上的动态压力小于静态压力。     

基于DEM模拟液固流化床粒度级配对颗粒流动特性的影响

采用离散单元方法(DEM)对三维液固流化床进行数值模拟。考虑润滑力的作用,对比二元混合、窄级配和宽级配3种粒度级配方式对颗粒分布、速度、温度波动和混合程度的影响。分析结果表明:大颗粒有向床层下部运动的趋势,而小颗粒则有向床层上部聚集的趋势;与窄级配相比,二元混合和宽级配的轴向速度在不同高度上差异较明显,且颗粒温度波动也较小,颗粒更加容易出现偏析;2种颗粒粒径差距越大,越容易分离,而粒径越接近,越难分离,混合程度越好。     

立式粉体干燥器内二元湿颗粒气-固逆流过程的数值模拟

【目的】为了研究二元湿颗粒在立式粉体干燥器干燥单元气-固逆流过程中的流动特性,分析不同条件下二元湿颗粒的空间分布情况,实现二元湿颗粒在气-固逆流过程中的优化设计。【方法】采用计算流体力学(computational fluid dynamics, CFD)与离散元(discrete element method, DEM)耦合液桥力模块的分析方法,分别探讨颗粒含水质量分数、粒径比和质量比对二元湿颗粒流动特性的影响及液桥力变化规律。【结果】在气-固逆流过程中,颗粒质量分数在径向和轴向分布上呈现“边壁大中心小”“上大下小”的规律;颗粒速度随着轴向高度的增加而增大,从边壁区域到中心区域的颗粒速度呈现减小趋势;随着颗粒含水质量分数和粒径比的增加,颗粒的运动由接触力和液桥力共同控制逐渐变成液桥力占主导,干燥单元顶部容易堵塞;随着质量比的增加,接触力对颗粒运动的控制进一步加强,在颗粒含水质量分数较小的情况下占主导作用。【结论】较大的质量比可以缓解干燥单元顶部堵塞,同时要选取适当的颗粒含水质量分数,才能削弱颗粒结块对流动特性研究的干扰。     

块石含量和空间分布对土石混合体抗剪强度影响的离散元分析

土石混合体的抗剪强度在很大程度上取决于其块石的含量、空间分布、形态等参数。该文采用球体单元模拟土颗粒,通过组合颗粒单元构造非规则形态的块石,对不同含石量和块石空间分布下土石混合体的直剪过程进行了离散元数值模拟。计算结果表明,在低含石量下,土石混合体的抗剪强度随含石量的增加而增加;在中含石量下,受块石空间分布的影响,其抗剪强度呈现很强的波动性;在高含石量下,其抗剪强度显著增强,波动性相对较小。抗剪强度在中高含石量下波动现象的细观机理是块石空间分布影响下的力链结构特性。当块石的空间分布使其形成较强力链结构时,抗剪强度较高;反之,块石间的力链结构不稳定,抗剪强度相对较低。以上研究有助于从细观尺度揭示土石混合体变形和剪切强度特性的内在机理。     

粉末注射成形的离散元模拟实验研究

建立了粉末注射成形离散元颗粒模型,将粉末离散相不再简化为连续相,而是处理为符合牛顿定律、具有相互作用的颗粒模型,用平行黏结模型近似模拟黏性液桥。将蜡基黏结剂与17-4ph不锈钢粉末经过混料和注射成型得到标准拉伸样和弯曲样注射坯,并进行了单轴拉伸和三点弯曲实验。将实验结果与离散元模拟结果对比分析,发现裂纹形态、位置和力-位移曲线具有一致性,验证了离散元颗粒模型的可靠性,校核了离散元颗粒模型的微观参数。     

月壤的钻取采样离散元动态行为研究

月壤为离散介质,其力学行为不仅受外部因素影响,而且受月壤颗粒形状之间相互作用和约束因素影响,一般连续介质力学难以模拟其力学特性,而离散元法作为一种分析离散介质的数值方法,可用来模拟器具与月壤相互作用的力学行为.基于前苏联Luna 24月面钻取采样过程力学机理分析,首先对钻取运动过程进行分析,然后应用PFC3D离散元软件,构建离散元月壤模型,通过赋予颗粒接触刚度、摩擦系数、粘结系数等定义月壤本构模型,仿真钻取过程,对比钻取消耗能量和钻进阻力.计算结果表明螺距和转速对钻取消耗能量和钻进阻力影响较大,螺距越大,转速越高,钻取消耗能量和钻进阻力越大,且与螺距和转速呈线性关系.研究结果可为月表采样机构的设计与优化提供借鉴.     

碎石料应力路径大型三轴试验的离散元模拟研究

碎石料在高填方工程中得到广泛应用。由于其粒径较大,对碎石料力学特性的研究通常需要大型的试验设备。该文探索通过离散元数值模拟方法研究碎石料的应力路径相关力学特性。离散元程序可以将若干球形单元粘结成一个捆绑单元,从而模拟可破碎的碎石颗粒。该文首先简要介绍了离散元的计算原理、特点和模拟过程,并建立碎石料试件的离散元模型。通过对比一组恒定围压下的大型三轴试验,确定了各参数,对所建模型进行了标定。进而通过该模型独立预测碎石料试件在三种不同应力路径加载时的应力-应变关系及体变特性,并与相应的应力路径大型三轴试验结果进行对比,验证了离散元方法可以较为准确地计算碎石料沿不同应力路径加载时的力学特性,同时还讨论了碎石料的小应变刚度特性。     

颗粒超声层析成像的散射特征分析

用数值方法对颗粒超声层析成像进行模拟研究,对单个球形颗粒散射特征的数值解与解析解作对比,验证数值方法的准确性、可靠性,进而对管内放置有单个和3个球形颗粒的散射声场特性进行的数值模拟和分析,并由二值逻辑反投影图像重建算法对其进行空间分布的重建,分析重建图像的误差。结果表明:基于边界元方法的数值模拟和重建方法是可行和有效的。     

Dynamics of bi-dispersed settling suspension of non-colloidal particles in rotating cylinder

Non-neutrally buoyant suspension of bi-dispersed non-colloidal particles in viscous fluid rotating in a horizontal cylinder displays in-homogeneities in particle distribution with alternate bands of high and low particle concentrations along the symmetric axis of the cylinder. Experiments were carried out to characterize the axial segregation in bi-dispersed suspension at various filling fraction and rotation speed of cylinder. The mixture of same particles in absence of any suspending fluid did not show any segregation. However, in case of particles suspended in water it was observed that the rate of segregation increases with increase in filling fraction. Once the particles get segregated along the full length of the cylinder, these bands start to migrate along the tube axis finally merging to give wider bands. For a given filling fraction the rate of segregation increases with the angular speed of the rotating cylinder. When the tube is partially filled the particle segregation is observed at higher angular speed, whereas in fully filled case the segregation starts at much lower rotation speed for the same concentration of particles. The segregation pattern changes as the rotation speed is increased. At higher speed the centrifugal force dominates over gravitational and viscous drag forces and this result into completely different segregation patterns. We have also analyzed the evolution of concentration profile from the image analysis of the particles.     

Particle screening phenomena in an oblique multi-level tumbling reservoir: a numerical study using discrete element simulation

Due to their wide usage in industrial and technological processes, granular materials have captured great interest in recent research. The related studies are often based on numerical simulations and it is challenging to investigate computational phenomena of granular systems. Particle screening is an essential technology of particle separation in many industrial fields. This paper presents a numerical model for studying the particle screening process using the discrete element method that considers the motion of each particle individually. Dynamical quantities like particle positions, velocities and orientations are tracked at each time step of the simulation. The particular problem of interest is the separation of round shape particles of different sizes using a rotating tumbling vertical cylinder while the particulate material is continuously fed into its interior. This rotating cylinder can be designed as a uniform or stepped multi level obliqued vertical vessel and is considered as a big reservoir for the mixture of particulate material. The finer particles usually fall through the sieve openings while the oversized particles are rebounded and ejected through outlets located around the machine body. Particle–particle and particle–boundary collisions will appear under the tumbling motion of the rotating structure. A penalty method, which employs spring-damper models, will be applied to calculate the normal and frictional forces. As a result of collisions, the particles will dissipate kinetic energy due to the normal and frictional contact losses. The particle distribution, sifting rate of the separated particles and the efficiency of the segregation process have been studied. It is recognized that the screening phenomenon is very sensitive to the machines geometrical parameters, i.e. plate inclinations, shaft eccentricities and aperture sizes in the sieving plates at different levels of the structure. The rotational speed of the machine and the feeding rate of the particles flow have also a great influence on the transportation and segregation rates of the particles. In an attempt to better understand the mechanism of the particle transport between the different layers of the sifting system, different computational studies for achieving optimal operation have been performed.     

Segregation behavior of particles with Gaussian distributions in the rotating drum with rolling regime

The mixing and segregation of granular materials are essential to provide valuable insights and references for practical industrial production. In this paper, the segregation behaviors of particles with Gaussian distributions and 40% filling level in the rotating drum with rolling regime were numerically studied by the discrete element method. The effects of rotation speed and particle size parameter λ (size ratio of the largest versus smallest particles) on the segregation behavior (mixing index, segregation rate), flow characteristics (particle velocity and trajectory, gyration degree and radius, particle size distributions) and the microscopic properties (collision, contact force, axial diffusion, and kinetic energy) of granular systems were systematically investigated. The results show that the segregation rate and degree of particles with Gaussian distributions gradually increase with the increase of the rotation speed and particle size parameter λ. The radial and axial segregation patterns become more obvious with the increase of λ. And the variation of the flow characteristics of particles with different sizes in the same system is also inconsistent. The microscopic properties of Gaussian-dispersed particles change with the rotation speed and λ. The rapid radial segregation depends on the larger pores existed in the granular system, which leads to a gradual increase of the axial dispersion coefficient of large particles and a gradual decrease of the axial dispersion coefficient of small particles.     

Insights from simulations into mechanisms for density segregation of granular mixtures in rotating cylinders

The Discrete Element Method (DEM) is used to study the segregation of a binary mixture of differing density (but same size) granular material in an axially rotating cylinder. The rotation rates used produce a flow that is on the borderline between the avalanching and rolling regimes. The simulations replicate the experimental data well at both qualitative and quantitative levels. Both wall-induced and radial segregation are observed. The simulations show segregation is delineated into two main time regimes. At early times segregation is rapid (when the dense core is being established) and slows down appreciably thereafter. The final asymptotic state is found to be independent of the initial segregation state of the particles. We compare these results with previous theoretical models and relate these two distinct time regimes to the underlying segregation mechanisms. These comparisons suggest segregation varies as a function of two fundamental quantities (a) density ratio of particles and (b) angular speed of the rotating cylinder. It is shown that maximal segregation occurs for specific ranges of these quantities.     

The effect of liquids on radial segregation of granular mixtures in rotating drums

The presence of liquids can significantly affect the dynamics of granular flow. This paper investigates the effect of liquids on radial segregation of granular mixture in a rotating drum using the discrete element method. The wet granular mixture, due to differences in particle size and density, segregates in a similar way to that of dry particles: lighter/larger particles move to the periphery of the bed while heavier/smaller particles stay in the centre. An index based on the variance of local concentration of one type of particles was proposed to measure the degree of segregation. While the liquid induced capillary force slows down the segregation process, its effect on the final state is more complicated: small cohesion shows no or even positive effect on segregation while high cohesion significantly reduces particle segregation. The effect can be explained by the change of flow regimes and the competing effects of mixing and segregation (un-mixing) in particle flow which are both reduced by the interparticle cohesion. A diagram is generated to describe the combined effect of particle size and density on segregation of wet particles. A theory is adopted to predict the segregation of particles under different density/size ratios.     

Investigating the flow of rod-like particles in a horizontal rotating drum using DEM simulation

Understanding the flow and mixing of rod-like particles is fundamental because of the widespread use of rods in the process industry. In this paper, discrete element method is used to investigate the flow and mixing of rod-like particles in a horizontal rotating drum, with rod-like particles being modeled by super-ellipsoids. The influence of the aspect ratio of the rod and the rotation speed of the drum on the flow of rod-like particles is studied. The investigation of spherical particles is also included in this paper to reveal the differences between rod-like and spherical particles. The simulation results show that the flow of rods is more intermittent than that of spheres and that there is more intermittent flow for rod-like particles with larger aspect ratios. Both the aspect ratio of the rod and the rotation speed of the drum considerably influence particle mixing. The mixing rate, as quantified by the slope of the variation in the mixing index with respect to drum revolution, increases as rotation speed and aspect ratio decrease. The study of particle orientation indicates that rod-like particles have a preferred orientation during rotation of the drum: the major axis of the rod inclines to be parallel to the end plate of the drum.     

Tangential velocity profiles of granular material within horizontal rotating cylinders modelled using the DEM

Flow regimes of granular materials in horizontal rotating cylinders are industrially important since they have a strong influence on the rates of heat and mass transfer within these systems. The tangential velocity profile, which describes how the average particle velocity in the direction parallel to the surface of the bed varies along a radius perpendicular to the surface of the bed, has been examined for many experimental and simulated systems. This paper is concerned with tangential velocity profiles within rotating cylinders simulated using the discrete element method. For high fill levels good agreement is found between the simulated velocity profiles and the equation proposed by Nakagawa et al. (Exp Fluids 16:54–60, 1993) based on magnetic resonance measurements. At lower fill levels slip is observed between the cylinder wall and the particles in contact with it and also between the outer layer of particles and the bulk of the bed. It is demonstrated that this slip occurs when the particles in contact with the wall are able to rotate and that it may be prevented either by using non-spherical particles or by attaching “lifters” to the cylinder wall.     

Finite plane strain deformations of nonlinear viscoelastic rubber-covered rolls

Finite deformations of a nonlinear viscoelastic rubber layer bonded to a uniformly rotating rigid cylinder and indented by another rigid cylinder are studied by the finite element method. The problem formulation includes both material and geometric nonlinearities. The effect of the change in the thickness of the layer, its speed of rotation and its material properties on the pressure distribution at the contact surface, and other aspects of the deformation of the layer, are studied.     

Rheological behaviour and fluidity of SiCp+AZ91HP magnesium composites

Abstract

Rotating cylinder viscosity and spiral fluidity experiments were carried out to investigate the effects of SiC particle fraction and size and casting temperature on the viscosity and the fluidity of Mg–8.2Al–0.7Zn (wt-%)alloy containing up to 10 vol.-%SiC particles. The rotating cylinder viscosity (process based viscosity) was deduced from the data measured by a power measurement system during the actual composite manufacturing by the rotating cylinder method, and the fluidity was measured using a specially designed spiral fluidity mould. The spiral fluidity, which is the reciprocal of viscosity, decreased with increasing SiC particle fraction (for a given size) and with decreasing SiC particle size (for a given volume fraction). Although the decrease in the fluidity of the composite slurry was observed due to the expected increase in the viscosity by the presence of SiC particles, the results showed that the fluidity of the composite slurry with 10 vol.-% 50 µm SiC particles was adequate to make a variety of castings at 680°C.     

Steady‐State Creep Analysis of Functionally Graded Rotating Cylinder

Axi‐symmetric component like cylinder etc. has to operate under severe thermo‐mechanical loads, which cause significant creep. It, thus, reduces its service life. The present study investigates the steady‐state creep in a functionally graded rotating cylinder at constant angular speed. The cylinder is made up of aluminium matrix and reinforced with silicon carbide particles. The thermal gradient in the functionally graded rotating cylinder is estimated by performing finite element analysis on ANSYS software (ANSYS Inc., Canonsburg, USA). The creep behaviour of the cylinder has been described by threshold stress‐based creep law. The creep parameters are obtained by conducting regression analysis. The mathematical models have been developed to describe steady‐state creep in the cylinder. The study reveals that the radial, tangential, axial and effective stresses in the cylinder are significantly affected by the presence of particle gradient alone as well as with the presence of particle & thermal gradient both. It has been found that the creep rates have been reduced significantly by imposition of particle and thermal gradients together and thus increases the service life of cylinder.