Powder spreading process is to use a spreader such as blade or roller to spread powder layers for subsequent fusion in powder bed fusion additive manufacturing. In this work, the effects of various ...spreader geometries on powder spreading are examined by discrete element method (DEM). The results show that a compact region in the powder pile exists. Round and inclined surfaces of blade spreaders allow more particles in the compact region to be deposited compared with vertical blades, thus the powder layer formed is denser. However, they exert larger forces on the underlying part. Inhomogeneity of powder layers is caused by particle burst phenomenon, which is due to particle motion conflict in the compact region rather than large forces. Roller system has largest particle motion conflict thus powder layers formed are sparse and inhomogeneous with small layer gaps. Size segregation in blade systems is not as severe as roller systems.
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•Powder spreading process with different spreaders is simulated by DEM.•Round and inclined blades deposit more powders than vertical blades.•Round and inclined blades exert larger forces on the underlying part.•Large layer gap has better homogeneity and smaller force fluctuation.•Round blade system has the least size segregation.
•Bubble dynamics in gas fluidisation of ellipsoidal particles is studied by CFD-DEM.•Bubbles for ellipsoidal particles have lateral drift phenomenon.•Ellipsoids has larger bubble size than ...spheres.•Bubble shape for spheres is more circular than ellipsoids.•Bubble rising velocity for ellipsoids is lower than spheres.
Bubbles considerably influence the characteristics of gas-solid fluidized bed, hence play an important role in determining process performance. This paper presents a CFD-DEM study on the effect of particle shape on the bed microstructure and bubble properties in a pseudo-2D bubbling gas-solid fluidized bed, operated with a continuous central jet. The bubble formation process is successfully generated, where bubbles rise through the bed and burst at the top of the bed. The numerical results show that ellipsoids have slightly different flow patterns from those of spheres. However, the mechanisms of bubble splitting and coalescence are found strongly dependent on particle shape. For spheres, the bubble trajectories mainly follow the bed centreline, whereas the bubbles for ellipsoids are widely distributed on both sides of the bed centreline. This result suggests that the lateral drift of bubbles is high for ellipsoids because such particles prefer to orient their longest major axis in the direction of the fluid flow. At the lower part of the bed, both gas and particle velocity profiles are found axially similar to a Gaussian distribution. In contrast, at the upper part of the bed, their peaks become flatter and broader for ellipsoids. Additionally, the bubble equivalent diameters are higher for ellipsoids while bubbles become more circular for spheres. Both bubble frequency and bubble velocity for ellipsoids are lower than spheres. The results obtained from this study can improve the understanding of bubble dynamics in the fluidization of non-spherical particles.
Particle shape can affect the flow and thermal behavior significantly in particle–fluid flow systems. In this work, the combined approach of discrete element method (DEM) and computational fluid ...dynamics (CFD) is extended to study the heat transfer in packed and fluidized beds of ellipsoids. The aspect ratio of ellipsoids varies from 0.25 to 3.5, representing disk-type and cylinder-type particles, respectively. The conductive heat transfer models for ellipsoids are proposed first, and then the effect of aspect ratio on the bed thermal properties is investigated. It is revealed that aspect ratio affects the effective thermal conductivity of packed beds significantly due to the increased particle–particle contact number and area. The study of bed heating process indicates that compared with spheres, ellipsoids have lower convective heat transfer rate but higher conductive heat transfer rate. In fluidized beds, the convective heat transfer coefficients of prolate particles are larger than those of spheres and oblate particles. The model offers an effective method to examine the effect of particle shape on heat transfer in fluid bed reactors at a particle scale.
•CFD-DEM is extended for heat transfer of ellipsoidal particles.•Particle shape significantly affects bed effective thermal conductivity.•Ellipsoids have lower convective heat flux but higher conductive heat flux.•Prolate particles have larger heat transfer coefficient than other particle shapes.
The mixing and segregation of granular materials are essential for industrial design and applications. In this work, the superquadric equation is used to construct spherical and non-spherical ...particles, and the discrete element method is used to investigate the radial segregation characteristics of a gaussian-dispersed mixture in a horizontal rotating drum. The influences of particle shape, standard deviation of the mixture, and rotating speed on the segregation behaviors are discussed by the gyration degree and mixing index. The results reveal that small and large particles are respectively distributed in the center and periphery of the drum, while the medium-sized particles are evenly distributed in the system. Larger standard deviation, smoother particle shape, and smaller rotation speed are conducive to the segregation of granular systems. When the granular system is close to the high-speed centrifugal state, the smallest particles have the largest radial velocity and are close to the drum wall.
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•Segregation for gaussian-dispersed mixtures in rotating drums is studied by DEM.•Small and large particles accumulate respectively in the drum center and periphery.•The gyration degree of the particles first decreases and then increases as the particle diameter increases.•Spheres have more significant segregation properties than non-spherical particles due to their smoother surfaces.•Small particles are close to the wall of the drum at high rotation speed.
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•Cylinders and cubes with different aspect ratios and blockiness are modeled by super-quadric equation.•The mixing rate is quantified by fitting the Lacey mixing index and then ...compared.•The effects of particle shapes and rotating speeds on the mixing rate are discussed.•The translational and rotational kinetic energy of differently shaped particles during the mixing process are analyzed.
Fundamental research on the flow and mixing of non-spherical particles is critical for industrial production and design. In this paper, the Discrete Element Method (DEM) is used to study the flow and mixing of granular materials in the horizontal rotating drum, and the periodic boundary condition is employed to eliminate end wall effect. Super-quadric elements are adopted to describe spherical and non-spherical particles. The influences of rotating speed, blockiness, and aspect ratio on the mixing rate are investigated by the Lacey mixing index. The results show that the rotating speed has a primary effect on the mixing rate, whereas the effect of the particle shape on the mixing rate is a secondary factor for non-spherical granular systems. Moreover, the mixing rate of spherical and non-spherical particle systems is significantly different. The mixing rate of spheres is the lowest, and the cubes have a higher mixing rate than the cylinders. As the blockiness decreases or aspect ratio deviates from 1.0, the mixing rate decreases. Ordered face-to-face contacts and dense packing structures result in a higher mixing rate. The analysis of kinetic energy shows that particle shape affects the transfer efficiency of external energy to the granular systems. The translational kinetic energy of non-spherical particles is higher than that of spherical particles, and their rotational kinetic energy is lower than that of spheres. Meanwhile, the blockiness enhances the transfer efficiency of external energy to the non-spherical systems; in contrast, the aspect ratio reduces the energy conversion efficiency.
This paper presents a CFD-DEM study of gas-solid flow in the raceway region in an ironmaking blast furnace (BF). In the simulation, 120,000 spherical particles are packed into a full-scale 2D slot BF ...geometry. Gas is injected into the geometry via tuyeres, generating raceways in different sizes and shapes under different conditions. It is observed that the raceway characteristics are much more complex under the full-scale BF than those observed under the laboratory scale in the literature. Three kinds of raceways can be identified: anti-clockwise circulating raceway, clockwise circulating raceway, and plumelike raceway. The results are analysed in terms of solid flow patterns, and flow and force structures of particles. The simulation has a good agreement with the observed in physical experiments in terms of two contrary circulating gas vortexes located upon and below tuyeres during raceway formation. Moreover, it is also indicated that operational variables have significant effect on one of the gas vortexes, which becomes the main circulating gas flow stream and determines the gas circulating direction.
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•CFD-DEM is used for raceway formation in a large scale BF.•Three types of raceways are observed and analysed by gas streamlines.•Raceway types are affected by gas velocity and tuyere length.
Baffles can effectively improve the mixing and heat transfer in internally heated rotating drums. To explore the performance of baffles, discrete element method (DEM) is employed in this work, and ...the influence of the placement and length of baffles on the mixing and heat transfer in an internally heated drum is investigated. The simulation results show that a central cross baffle promotes the mixing and heat transfer significantly, and the optimal length varies with material properties, for example, at the length-to-diameter ratio L/D = 0.4 for monodispersed glass particles, 0.6–0.7 for binary glass-steel mixtures. The peripheral baffle generally weakens the mixing and heat transfer performance, and effects only for a sufficiently long peripheral baffle at the length-to-radius ratio L/R = 0.9. Baffles promote the heat transfer process and do not change the thermal contribution of heat transfer paths.
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•Effect of baffles on mixing and heat transfer in drums is studied by DEM.•The optimal length-to-diameter ratio of central baffles varies in a range of 0.4–0.7.•The traditional short peripheral baffle slightly deteriorates mixing and heat transfer.•The peripheral baffles perform best at high length-to-radius ratio of 0.9.•Baffles do not change the thermal contribution of heat transfer paths.
Powder bed fusion additive manufacturing has been applied to the fabrication of functionally graded materials. A new design that allows the material composition to change along the direction ...perpendicular to the powder spreading has been reported in the literature. Based on this design, this work examines the quality of the graded spread powder layer with two powders, which have a large difference of density. The results reveal that during the spreading of graded powders, the volume of particles on the heavy powder side is deposited less than that on the light powder side, indicating that heavy particles diffuse to the light powder side. This diffusion is affected by the spreading speed, but not much by the layer gap. Large spreading speed causes more significant deviation. The results also show that particle size affects diffusion, indicating that decreasing the particle size of the heavy powder may be a solution to reduce diffusion.
Particle shape can significantly affect the bubble dynamics of bubbling fluidized beds (BFB). In this paper, findings obtained from simulations using CFD-DEM are summarized to discuss the effect of ...particle shape on bubble dynamics and bubble properties such as bubble size, shape and velocity at a single orifice and uniform fluidized bed. Particles with aspect ratios at 0.5 (oblate), 1 (spherical) and 2 (prolate) are employed to represent disc-like, spherical and rod-like particles, respectively. Both single jet and uniform fluidized bed simulations demonstrate that the bubble forming/rising regions, bubble coalescence locations, and bubble splitting phenomena are significantly influenced by particle shape. The CFD-DEM results for bubble size and bubble velocity show good agreement with literature correlations.
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