A modelling and verification approach for soybean seed particles using the discrete element method

To build a discrete element method (DEM) model of soybean seed particles, the shape and size of soybean seed particles were measured and analysed. The results showed that the shape of a soybean seed particle could be approximated to an ellipsoid and that the dispersity in size could be approximated by a normal distribution. Additionally, a certain functional relationship between the primary dimension and secondary dimensions was determined. On this basis, an approach for modelling soybean seed particles based on the multi-sphere (MS) method was proposed. The soybean seed particle was simplified to an ellipsoid with the averaged size of one hundred randomly selected soybean seeds. The model of a single soybean seed particle was built by filling spheres within the ellipsoid. For modelling soybean seed assembly, the primary dimension was generated according to the normal distribution, and the other secondary dimensions were calculated based on their relationships with the primary dimension. In this way, the model of soybean seed assembly with different sizes and distributions was built. In this paper, four varieties of soybean seed were used. By comparing the simulated results and experimental results both in piling tests and “self-flow screening” tests, when the number of filling spheres was five, the simulated results were close to those obtained experimentally. Therefore, the feasibility and validity of the modelling method for soybean seed particles that we proposed were verified. Finally, an application case was employed to show how to use the soybean seed particle model and the discrete element method to analyse the discharging process of a silo.     

A study of transverse ply cracking using a discrete element method

We study the transverse cracking of the 90° ply in [0/90]_S cross-ply laminates by means of a discrete element method. To model the 90° ply a two-dimensional triangular lattice of springs is constructed where nodes of the lattice model fibers, and springs with random breaking thresholds represent the disordered matrix material in between. The spring-lattice is coupled by interface springs to two rigid bars which represent the two 0° plies in the model, which could be sublaminate as well. Molecular dynamics simulation is used to follow the time evolution of the model system. It was found that under gradual loading of the specimen, after some distributed cracking, segmentation cracks occur in the 90° ply which then develop into a saturated state where the ply cannot support additional load. The stress distribution between two neighboring segmentation cracks was determined, furthermore, the dependence of the microstructure of damage on the ply thickness was also studied. To give a quantitative characterization of stiffness degradation, the Young modulus of the system is monitored as a function of the density of segmentation cracks. The results of the simulations are in satisfactory agreement with experimental findings and with the results of analytic calculations.     

Particle conveying under microgravity in a vibrating vessel

This paper presents a numerical study on the conveying of particles in a vibrating vessel under microgravity. Such a vessel is composed of parallel plates with sawtooth wavy surfaces, which are specifically designed to convey particles using simple vibration. The numerical model was validated by good agreement between the simulated and experimental results. Then the effects of key variables, including the vessel geometry, vibration amplitude and frequency and gravity level, were systematically investigated by a series of controlled simulations. The results confirm the optimised design from the previous experiments, and numerically demonstrate that using such a system a steady conveying operation can be achieved under microgravity. The convey rate is positively affected by the vibration amplitude and frequency in a complicated way, which cannot be simply described by the commonly used vibration intensity or velocity amplitude. The gravity level also has a significant effect on the convey rate when it is over 0.001g. The convey rate can be estimated by the product of the average solid fraction and velocity. And the effects of the variables can be better understood through the analyses on these two parameters. Finally, a predictive model is proposed to estimate the convey rate under different operational conditions. The findings are useful for the design of particle conveying techniques for outer space applications.     

Experimental study and discrete element method simulation of Geldart Group A particles in a small-scale fluidized bed

Geldart Group A particles are of great importance in various chemical processes because of advantages such as ease of fluidization, large surface area, and many other unique properties. It is very challenging to model the fluidization behavior of such particles as widely reported in the literature. In this study, a pseudo-2D experimental column with a width of 5 cm, a height of 45 cm, and a depth of 0.32 cm was developed for detailed measurements of fluidized bed hydrodynamics of fine particles to facilitate the validation of computational fluid dynamic (CFD) modeling. The hydrodynamics of sieved FCC particles (Sauter mean diameter of 148 µm and density of 1300 kg/m³) and NETL-32D sorbents (Sauter mean diameter of 100 µm and density of 480 kg/m³) were investigated mainly through the visualization by a high-speed camera. Numerical simulations were then conducted by using NETL’s open source code MFIX-DEM. Both qualitative and quantitative information including bed expansion, bubble characteristics, and solid movement were compared between the numerical simulations and the experimental measurement. The cohesive van der Waals force was incorporated in the MFIX-DEM simulations and its influences on the flow hydrodynamics were studied.     

Modelling dilation in an idealised asphalt mixture using discrete element modelling

This paper investigates the use of discrete element modelling (DEM) to simulate the behaviour of a highly idealised bituminous mixture under uniaxial and triaxial compressive creep tests. The idealised mixture comprises single-sized spherical (sand-sized) particles mixed with bitumen and was chosen so that the packing characteristics are known (dense random packing) and the behaviour of the mixture will be dominated by the bitumen and complex aggregate interlock effects will be minimised. In this type of approach the effect of the bitumen is represented as shear and normal contact stiffnesses. A numerical sample preparation procedure has been developed to ensure that the final specimen is isotropic and has the correct volumetrics. Elastic contact properties have been used to investigate the effect of the shear and normal contact stiffnesses on bulk material properties. The bulk modulus was found to be linearly dependent on the normal contact stiffness and independent of the shear contact stiffness. Poisson’s ratio was found to be dependent on only the ratio of the shear contact stiffness to the normal contact stiffness. An elastic contact has been assumed for the compressive normal contact stiffness and a viscoelastic contact for shear and tensile normal contact stiffness to represent the contact behaviour in idealised mixture. The idealised mixture is found to dilate when the ratio of compressive to tensile contact stiffness increases as a function of loading time. Uniaxial and triaxial viscoelastic simulations have been performed to investigate the effect of stress ratio on the rate of dilation with shear strain for the sand asphalt. The numerical results have been validated with experimental data.     

Granular temperature with discrete element method simulation in a bubbling fluidized bed

The Discrete Element Method (DEM) plays an important role in understanding and modeling the kinetic characteristics in granular systems. A soft-sphere method with a linear spring–dashpot model was used in the simulation of a bubbling fluidized bed. The time-averaged granular temperature and vertical particle velocity at different heights were numerically studied and compared to experimental measurements of Müller. The influence of a velocity-dependent coefficient of restitution and three drag models were also investigated in this work. Good agreement was found between the DEM simulation and Müller’s experiment, especially using the DiFelice drag model. The variable coefficient of restitution, with a sufficiently high yielding relative velocity, gives a granular temperature that is a little lower compared to that of a constant coefficient of restitution, while it predicts a more intense velocity fluctuation, with a lower yielding relative velocity. By comparing the granular temperature in the vertical direction and in the transverse direction, a strong anisotropy is found in the bed.     

Discrete element simulation for mixing performances and power consumption in a twin-blade planetary mixer with non-cohesive particles

The present study aims to characterize the mixing performances and power consumption of a twin-blade planetary mixer with non-cohesive particles through the discrete element method (DEM). A DEM model used for simulating the particle flow and mixing kinetics of the mixer was experimentally verified. The particle velocity and mixing mechanism are elaborated quantitatively, indicating that particle mixing is realized under the combined actions of radial, circumferential and vertical circulations, and some local collisions and mergers. Increasing the absolute speed N and the speed ratio i promotes the radial circulation, while the tangential and vertical circulations are strengthened with the increase of N and the decrease of i. The mixing time required for the homogeneous state decreases, and the power consumption increases as N increases and i decreases. Thus, increasing N and decreasing i can improve the mixing performance but require more energy to reach the homogeneous state. Also, the mixing performance shows a strong correlation with the swept volume of blades, which proves that the dominant mixing mechanism of the mixer is convection.     

DEM study of particles flow on an industrial-scale roller screen

In this work, the screening process of an industrial-scale roller screen was simulated based on the validated discrete element method (DEM). The effects of the feed rate, rotational speed of rollers, inclination angle of screen deck and the cohesion force between wet particles on the screening process were investigated. The results show that screening efficiency decreases with the increase of feed rate, which remains above 0.80 when the feed rate is less than 700 kg/s. The optimal values of rotational speed and inclination for high efficiency are 100 r/min and 6°, respectively. Working parameters have a significant effect on both the passing percentage and the average velocity of different size fractions. Hence, reasonable working parameters should be adopted to avoid the accumulation of particles on the deck. Moreover, both particle shape and cohesion force have little effect on the screening processes of the roller screen.     

Simulation of granular crystallization of cubic particles in twisting container using discrete element method

Granular compaction is a process in which the volume fraction, or density, of the granular materials increases when an excitation is applied. A recent experiment reported that twisting a large number of cubic particles in a cylindrical container leads to an ordered and dense arrangement. This structure is similar to the crystal lattice formed in solidification process. In this article, this phenomenon is repeated by using discrete element method (DEM) simulation. Two different shaped containers are used and it is found that the rectangular angles between the sidewalls and the bottom,namely wall effect, plays a key role. In addition, gravitation is also a very important parameter in this process. The higher gravitation added, the faster crystallization process is achieved. On the contrary, shear force due to friction between particles may slow down this process.     

Discrete element numerical simulation of fly ash triboelectrostatic separation in a nonlinear electric field

Fly ash is solid waste produced by thermal power generation, and its carbon content is a key factor affecting its recycling. Due to the large difference in fly ash quality and insufficient tribocharging, the parallel plate electric field with constant electric field strength cannot meet the practical needs of efficient decarbonization of fly ash particles with wide charge range or small charge to mass ratio (CMR). Therefore, a nonlinear electric field structure is proposed. The separation process of fly ash particles in the nonlinear electric field is explored through the establishment of geometric model and the application of CFD-DEM coupled calculation method, and the main influencing factors of fly ash electrostatic dry separation are studied. The results show that the nonlinear electric field structure is feasible to achieve high efficiency decarbonization of fly ash. With the increase of air flow velocity, the loss on ignition of positive electrode first increases and then decreases. The loss on ignition (LOI) of positive electrode products is directly proportional to the voltage and the CMR of the input, but inversely proportional to the feed quantity. Air flow velocity of 20 m/s, voltage of 30 kV, charge-mass ratio of 1.1–1.2 nC/g and feed quantity of 5000/s are suitable conditions for efficient decarbonization of fly ash. Compared with parallel plates, hyperbolic nonlinear electric field has higher decarbonization efficiency and lower energy consumption in experiment.     

Direct numerical simulation of saturated deformable porous media using parallel hybrid Lattice–Boltzmann and finite element method

Numerical techniques for modeling saturated deformable porous media have mainly been based on mixture theory or homogenization techniques. However, these techniques rely on phenomenological relationships for the constitutive equations along with assumptions of homogeneous and isotopic material properties to obtain closure. Direct numerical simulations of the multiphasic problem for flow in deformable porous media avoid such assumptions and thus can provide significantly accurate understanding of the physics involved. They serve as a tool to investigate the constitutive relationships in complex geometries. They also allow the validation of the existing mixture theory models and determine their limitations. In this work, a parallel hybrid method using Lattice Boltzmann Method (LBM) for fluid phase and Finite Element Method (FEM) for solid phase is used for direct numerical simulation of saturated deformable porous media. The method provides a number of unique features including scalability on distributed computing necessary for such a problem. The method has been validated for modeling fluid–structure interactions in complex geometries against a number of experimental and analytical solutions. Further some challenging problems has been chosen to show the capability of the method. Copyright © 2010 John Wiley & Sons, Ltd.     

Numerical study of the interfaces of 3D‐printed concrete using discrete element method

3D concrete printing is an additive manufacturing method which reduces the time and improves the efficiency of the construction process. Structural behavior of printed elements is strongly influenced by the properties of the material and the interface surfaces. The printing process creates interface surfaces between layers in the horizontal and vertical directions. The bond strength between layers is the most critical property of printed elements. In this paper, the structural behavior of printed elements is studied using the discrete element method. The material is modelled using discrete particles with bonding between them. A new discrete model of a multilayer geometry is presented to study the behavior of the interfaces of printed concrete. The layers are made up of randomly placed particles to simulate the heterogeneous nature of concrete. The numerical model is developed to simulate the flexural behavior of multilayer specimens. A four‐point flexural test is simulated considering the interface surfaces between layers. This numerical model provides relevant results to improve the behavior of this kind of structural elements. The aim of this work is to provide a discrete element model to predict the mechanical behavior of 3D concrete printed components.     

Numerical study of particle mixing in a tilted three-dimensional tumbler and a new particle-size mixing index

A new mixing index is proposed, which is an improved Lacey index based on coordination number fraction. The differences and similarities among many mixing indices are compared, including the new mixing index, the information entropy based on coordination number fraction, the Lacey index based on local concentration, and the information entropy based on local concentration. The first two indices are microscopic since the coordination number fraction is on particle-scale, whereas the latter two are mesoscopic as the local concentration is mesoscopic scale. The newly proposed mixing evaluation indices does not include inauthentic temporal oscillations. Moreover, using mixing index, the mixing characteristics of particles in a tilted tumbler are studied by discrete element method (DEM). The tumbler’s angle of tilt α = 0°, 10°, 20°, 30°, 40°, 50°, 60° and 70°, at five rotating velocities ω = 0.175, 0.35, 0.5, 0.6, 0.7 and 1.4 rad/s corresponding to Froude number F_r = 0.0025, 0.001, 0.002, 0.003, 0.004, 0.016 respectively are simulated. It is found that both increasing the tilt angle and the rotating speed have negative effects on the particle mixing within the scope of this study.     

Investigating the upper limit for applying the coarse grain model in a discrete element method examining mixing processes in a rolling drum

Mixing is an essential manufacturing process in various industries. The processing procedure and final product quality depend on the homogeneity of mixing. Because it is difficult to evaluate mixing systems experimentally, the discrete element method is commonly employed. However, as the number of particles increases, this approach incurs huge computational costs. The coarse grain model offers a potential solution, but its applicability has not been widely demonstrated; this study aimed to elucidate the upper limit for applying the coarse grain model. To determine the appropriate simulation parameters, calibrations were performed by comparing the powder bed in experiments versus simulations. Various mixing processes were numerically evaluated, and the mixing characteristics were qualitatively consistent among all coarse-grained ratios. These mixing systems were also evaluated quantitatively based on Lacey’s mixing index, which indicated that the upper limit of the coarse-grained ratio was five times. It is therefore important to secure a sufficient number of particles in each cell and to use an appropriate number of cells. This study clarified the upper application limit and criteria for the coarse grain model and verified the maximum coarse-grained ratio (five times). This approach can be used to determine the coarse-grained ratio and reduce computational costs.     

Finite element simulation of internal flows with heat transfer using a velocity correction approach

This paper enumerates finite-element based prediction of internal flow problems, with heat transfer. The present numerical simulations employ a velocity correction algorithm, with a Galerkin weighted residual formulation. Two problems each in laminar and turbulent flow regimes are investigated, by solving full Navier-Stokes equations. Flow over a backward-facing step is studied with extensive validations. The robustness of the algorithm is demonstrated by solving a very complex problem viz. a disk and doughnut baffled heat exchanger, which has several obstructions in its flow path. The effect of wall conductivity in turbulent heat transfer is also studied by performing a conjugate analysis. Temporal evolution of flow in a channel due to circular, square and elliptic obstructions is investigated, to simulate the vortex dynamics. Flow past an in-line tube bank of a heat exchanger shell is numerically studied. Resulting heat and fluid flow patterns are analysed. Important design parameters of interest such as the Nusselt number, Strouhal number, skin friction coefficient, pressure drop etc. are obtained. It is successfully demonstrated that the velocity correction approach with a Galerkin weighted residual formulation is able to effectively simulate a wide range of fluid flow features.     

Mixing assessment of non-cohesive particles in a paddle mixer through experiments and discrete element method (DEM)

In this study the mixing kinetics and flow patterns of non-cohesive, monodisperse, spherical particles in a horizontal paddle blender were investigated using experiments, statistical analysis and discrete element method (DEM). EDEM 2.7 commercial software was used as the DEM solver. The experiment and simulation results were found to be in a good agreement. The calibrated DEM model was then utilized to examine the effects of the impeller rotational speed, vessel fill level and particle loading arrangement on the overall mixing quality quantified by the relative standard deviation (RSD) mixing index. The simulation results revealed as the impeller rotational speed was increased from 10?RPM to 40?RPM, generally a better degree of mixing was reached for all particle loading arrangements and vessel fill levels. As the impeller rotational speed was increased further from 40?RPM to 70?RPM the mixing quality was affected, for a vessel fill level of 60% and irrespective of the particle loading arrangement. Increasing the vessel fill level from 40% to 60% enhanced the mixing performance when impeller rotational speed of 40?RPM and 70?RPM were used. However, the mixing quality was independent of vessel fill level for almost all simulation cases when 10?RPM was applied, regardless of the particle loading arrangement. Furthermore, it was concluded that the particle loading arrangement did not have a considerable effect on the mixing index. ANOVA showed that impeller rotational speed had the strongest influence on the mixing quality, followed by the quadratic effect of impeller rotational speed, and lastly the vessel fill level. The granular temperature data indicated that increasing the impeller rotational speed from 10?RPM to 70?RPM resulted in higher granular temperature values. By evaluating the diffusivity coefficient and Peclet number, it was concluded that the dominant mixing mechanism in the current mixing system was diffusion.     

Effect of fill level in continuous twin-screw granulator: A combined experimental and simulation study

Twin-screw granulators for continuous wet granulation have attracted interest in the pharmaceutical industry. The physical properties of granules and tablets prepared through the twin granulation process depend on several factors, such as screw and barrel geometries, operating conditions, and formulations of raw materials. The fill level has been reported to be one of the most important factors in determining granule properties. However, the mechanism of the fill level effect on granule properties has not yet been fully clarified. In this study, the effects of the fill level in the twin-screw granulator on granule and tablet properties were investigated through a combination of experimental and simulation studies. In the experiment, temporal changes in the fill level were directly measured. It was found that a high fill level increased granule strength, resulting in large tablet hardness. The discrete element method simulations demonstrated that the compressive force on the particles in the granulator strongly depends on the fill level. In addition, by combining the experimental and simulation results, it was quantitatively revealed that a high fill level increased the interparticle adhesion, leading to the high hardness of tablets prepared with the granules.