Random particle packing with large particle size variations using reduced-dimension algorithms

We present a reduced-dimension, ballistic deposition, Monte Carlo particle packing algorithm and discuss its application to the analysis of the microstructure of hard-sphere systems with broad particle size distributions.We extend our earlier approach (the “central string”algorithm) to a reduced-dimension, quasi-3D approach. Our results for monomodal hard-sphere packs exhibit a calculated packing fraction that is slightly less than the generally accepted value for a maximally random jammed state. The pair distribution functions obtained from simulations of composite structures with large particle size differences demonstrate that the algorithm provides information heretofore not attainable with existing simulation methods, and yields detailed understanding of the microstructure of these composite systems.     

Distinct element modelling of cubic particle packing and flow

Processing of granular and powder materials is important in many engineering applications. The unpredictable behaviour of bulk solids often causes problems such as unsteady flows during handling and processing. Mixing and packing of powders are also of great importance, but experimental observation of such processes is not always convenient. Simulation is a powerful tool here. In this paper, a model for cube-shaped particles is developed using the Distinct Element Method. It introduces a model based on multi-contact principles. The cube shape was considered as an archetypal regular polyhedral shape with fundamentally different characteristics to spheres in packing and flow. Specific industrial applications are also noted. The study closely compares laboratory experiments and simulations in terms of static packing, flow pattern and flow rates for cubes and spheres. The results show that the multiple-contact model is a better solution for surface-surface contact than the single-contact version and should be used for polyhedral shapes. The simulation and experimental results are generally in close agreement demonstrating that the DEM cube model is a feasible tool. The cubes show “better” packing characteristics but “worse” flow properties as expected, however, it is noted that the system is fairly small in terms of number of particles and wall effects are quite significant here.     

Influence of particle size distribution on rheology and particle packing of silica-based suspensions

The effect of particle size, particle size distribution and milling time on the rheological behaviour and particle packing of silica suspensions was investigated using slurries containing total solids loading of 46 vol.%. Three silica powders with different average particle sizes (2.2, 6.5 and 19 μm), derived from dry milling of sand, and a colloidal fumed silica powder with 0.07 μm were used. Different proportions of colloidal fumed silica powder were added to each of the coarser silica powders and the mixtures were ball-milled for different time periods. The influence of these factors and of the particle size ratio on the rheological behaviour of the suspensions and densities of green slip cast bodies was studied.The results show that the flow properties of slips are strongly influenced by the particle size distribution. The viscosity of suspensions increases with the addition of fine particles, imposing some practical limitations in terms of volume fraction of fines that can be added. On the other hand, increasing the size ratio enhanced the shear thinning character of the suspensions, while decreasing the size ratio led to an accentuation of the shear thickening behaviour. For all mixed suspensions, green densities increased with increasing milling time, due to size reduction of silica powders and a more efficient deagglomeration of fumed silica. Increasing amounts of fumed silica led to a first increase of particle packing up to a maximum, followed by a decreasing trend for further additions. Good relationships could be observed between rheological results and packing densities.     

Performance of integration schemes in discrete element simulations of particle systems involving consecutive contacts

In this investigation, which is a follow-up study extending earlier work (Kruggel-Emden, Sturm, Wirtz, & Scherer, 2008), a realistic assessment of the performance of integration schemes in systems of moving particles and consecutive contacts is conducted. Linear contact models are applied throughout this work as they allow for an analytical solution of consecutive oblique impacts. The many-particle systems considered are the discharge of particles from a hopper and particle movement in a shaken container. Results for many-particle systems are robust with respect to the applied integration method and step size once particle interactions are resolved with a sufficient number of steps. The integration schemes are also evaluated based on consecutive particle/wall contacts. Integration of consecutive contacts in a discrete element framework implies repeatedly solving non-continuous systems of differential equations. Various termination conditions for the normal force models and adaptive time stepping for one-step integration methods are investigated. The effect of softened contacts on particle trajectories is discussed. Based on these insights, recommendations for the most accurate integration schemes are made.     

Investigation of particle packing in model pharmaceutical powders using X-ray microtomography and discrete element method

This paper outlines a novel technique, based on combination of modern desktop X-ray microtomography, quantitative image processing and computer simulation using the discrete element method (DEM), to investigate randomly packed particles in an attempt to model the process of pharmaceutical tablet manufacture by powder compaction. The systems studied include glass ballotini and spheroidal micronised cellulose (Celphere), all with typical particle sizes between 180 and 300 μm. We demonstrate that X-ray microtomography (XMT) and DEM can reproduce the structure of real packing systems in three-dimensions and have the potential for further investigation of pharmaceutical processes by both modelling and experimental study. This was achieved by generating packing systems using DEM simulations that are consistent with the structural measurements made by XMT on real packed powders via the comparison of their radial distribution functions (RDFs). These results have been validated by direct volume measurements, and scanning electron microscopy (SEM) observations in terms of particle morphologies and size distribution. The result is a significant step forward for the quantitative analysis of model systems for pharmaceutical powders.     

Analysis of particle porosity distribution in fixed beds using the discrete element method

This report discusses the use of the discrete element method (DEM) to the porosity distribution of spherical particles in narrow pipes as a function of the pipe-to-particle diameter ratio. It was found that the packing structure depends mainly on the pipe-to-particle ratio and the particle friction. The numerical results with respect to the radial porosity distribution are in agreement with experimental data from the literature. Radial porosity distributions were calculated using algorithms developed by Mueller. The packing structure of the particles shows channeling for small pipe to particle diameter ratios. The simulated height averaged porosity distribution agrees with models from the literature. Moreover, DEM provides the possibility to include particle properties which reflect on the porosity distribution.     

翅片导流板填料应用于旋转填料床的特性

For an alcohol/water system and with fin baffle packing, continuous distillation experiments were carried out in a rotating packed bed (RPB) system at atmospheric pressure. The effects of the average high gravity factor (β), liquid reflux ratio (R) and feedstock flux (F) on the momentum transfer and mass transfer were investigated. The gas phase pressure drop of RPB increased with the average high gravity factor, liquid reflux ratio and feedstock flux, which was 13.55-64.37 Pa at β of 2.01-51.49, R of 1.0-2.5, and F of 8-24 L&;#8226;h－1 for a theoretical tray in the RPB with fin baffle packing. The investigation on the mass transfer in the RPB with different packings showed that the number of transfer units of RPB with a packing also increased with the average high gravity factor, reflux ratio and feedstock flux. It is found that the fin baffle packing (packing III) presents the best mass transfer performance and lowest pressure drop for the height equivalent to a theoretical plate (HETP), which is 6.59-9.84 mm.     

Effect of packing pressure on fiber orientation in injection molding of fiber‐reinforced thermoplastics

Injection molding of fiber‐reinforced polymeric composites is increasing with demands of geometrically complex products possessing superior mechanical properties of high specific strength, high specific stiffness, and high impact resistance. Complex state of fiber orientation exists in injection molding of short fiber reinforced polymers. The orientation of fibers vary significantly across the thickness of injection‐molded part and can become a key feature of the finished product. Improving the mechanical properties of molded parts by managing the orientation of fibers during the process of injection molding is the basic motivation of this study. As a first step in this direction, the present results reveal the importance of packing pressure in orienting the fibers. In this study, the effects of pressure distribution and viscosity of a compressible polymeric composite melt on the state of fiber orientation after complete filling of a cavity is considered experimentally and compared with the simulation results of Moldflow analysis. POLYM. COMPOS. 28:214–223, 2007. © 2007 Society of Plastics Engineers     

规整填料内液体分布的实验研究进展

测量规整填料内液体分布的实验方法有接液法、电导示踪法、光纤传感技术、断层成像技术和激光诱导荧光技术等。这些实验方法比较有效,但也存在一些不足。本文介绍了各种实验方法的原理、实现方式及不足之处,重点阐述了断层成像技术和激光诱导荧光技术。两种技术均实现了填料内液体分布的可视化,获得了持液量,但都存在如何有效去除背景噪声的问题。激光诱导荧光技术与断层成像技术相比,具有新型、可靠、易于实现和安全等优点。最后,探讨了激光诱导荧光技术在未来应用中的新方向,包括与粒子成像测速仪联用、与高速相机联用、对实验结果的再提取及实验填料的加工。     

新型组合式规整填料在液液萃取中的传质性能

采用质量分数30%磷酸三丁酯-煤油-醋酸-水物系,对一种新型萃取用组合式规整填料的传质性能进行了测定,考察了连续相流速和分散相流速对其传质效率的影响。实验结果表明:在相同的二相流速下,组合式规整填料的表观传质单元高度比16 mm鲍尔环平均低约54%。固定连续相流速,随着分散相流速的增加,填料的表观传质单元高度降低,传质效率提高;固定分散相流速,随着连续相流速的增加,填料的表观传质单元高度增大,传质效率降低。     

Validation of pressure drop prediction and bed generation of fixed-beds with complex particle shapes using discrete element method and computational fluid dynamics

Catalytic fixed-bed reactors with a low tube-to-particle diameter ratio are widely used in industrial applications. The heterogeneous packing morphology in this reactor type causes local flow phenomena that significantly affect the reactor performance. Particle-resolved computational fluid dynamics has become a predictive numerical method to analyze the flow, temperature, and species field, as well as local reaction rates spatially and may, therefore, be used as a design tool to develop new improved catalyst shapes. Most validation studies which have been presented in the past were limited to simple particle shapes. More complex catalyst shapes are supposed to increase the reactor performance. A workflow for the simulation of fixed-bed reactors filled with various industrially relevant complex particle shapes is presented and validated against experimental data in terms of bed voidage and pressure drop. Industrially relevant loading strategies are numerically replicated and their impact on particle orientation and bed voidage is investigated.     

Numerical investigation of particle shape and particle friction on limiting bulk friction in direct shear tests and comparison with experiments

In this study, the discrete element method (DEM) was used to investigate the influence of particle shape and interparticle friction on the bulk friction in a Jenike direct shear test. Spherical particle and non-spherical particles using two overlapping sphere giving particle aspect ratio of up to 2 and a full range of interparticle contact friction coefficient were studied numerically. These were compared with physical Jenike shear tests conducted on single glass beads and paired glass beads. To separate the influence of sample packing density from interparticle contact friction on the bulk shearing response, the same initial packing was used for each particle shape in the simulations. The interplay between contact friction and particle interlocking arising from geometric interaction between particles to produce the bulk granular friction in a direct shear test is explored and several key observations are reported. The results also show that particle interlocking has a greater effect than packing density on the bulk friction and for each particle shape; DEM can produce a good quantitative match of the limiting bulk friction as long as similar initial packing density is achieved.     

Comparison of multifluid and discrete particle modelling in numerical predictions of gas particle flow in circulating fluidised beds

From the perspective of wanting to evaluate the modelling of the particulate phase in multifluid computational fluid dynamics simulations, numerical predictions from a multifluid model is compared with predictions from a discrete particle model. Simulations with both one and three representative particle diameters for the particulate phase are performed. The predicted results are compared with experimental findings obtained with Laser Doppler Anemometry. The numerical predictions from the discrete particle code are found to be in better agreement with the experimental findings, but the multifluid code is by far the most efficient. To evaluate the modelling of collisions in the multifluid model, discrete particle simulations which only take into account of energy loss caused by head-on collisions are compared with discrete particle simulations, which take nonfrontal collisions into account.     

On the effect of mould temperature on the orientation and packing of particles in ceramic injection moulding

Particle packing and orientation in Ceramic Injection Moulding are caused by shear rate variations along the part, which are complicated by cooling rates and depend on material properties and process parameters. This work studies particle orientation and packing for a ceramic feedstock in relation to mould temperature, with a focus on the influence from Rapid Heat Cycle Moulding (RHCM). A sample part consisting of bars having different thicknesses (0.4 mm to 6 mm) was injection moulded using first ambient mould temperature and then a rapid heating approach; subsequently, green part microstructure was analysed to detect particle orientation and packing. Results show a more uniform orientation at thick sections using RHCM, with a 15% enlargement of a core layer characterised by randomly oriented particles, as well as a higher homogeneity of powder packing between thin and thick sections. Thus, this study demonstrates that not only can RHCM be used to enhance feature replication capability, but also has the potential to improve critical microstructural properties of green components, which highly influence sintered parts quality.     

催化剂装填分率对催化蒸馏干气制乙苯过程的影响

在改性的β沸石催化剂上研究了催化剂与金属填料混合装填条件下,催化剂装填分率对催化蒸馏干气制乙苯过程的影响。通过冷模试验考察了催化剂装填分率对催化剂床层流体力学性能的影响,并在小型的催化蒸馏干气制乙苯装置上考察了催化剂装填分率对反应结果的影响。试验结果表明,催化剂装填体积分率直接影响催化剂床层的流体力学和反应性能。就催化蒸馏干气制乙苯过程而言,适宜的催化剂装填体积分率为10％～30%。     

竖式移动床层中散料颗粒破碎的离散元分析

建立了竖式移动床层中颗粒破碎的炉料下降运动模型,采用离散单元法对干法熄焦炉和烧结余热回收竖罐两种竖冷装置内不同形状、尺寸及强度颗粒料的下落运动和破碎过程进行了数值计算。结果表明,颗粒在下落过程中所受压力先逐渐增大,进入出料区后又逐渐减小,颗粒破碎情况与所受压力密切相关;焦炭在干熄炉内下落到斜道区时,由于炉体直径扩张,料层所受平均压力减小,破碎速率有所减慢。由于烧结矿强度相对较小,刚进入烧结竖罐就发生破碎现象,而炉体直径扩张对破碎影响不大;受固定炉墙的影响,颗粒在靠近炉墙位置处更容易破碎。分析不同形状颗粒的破碎过程发现,正方形颗粒从某一顶点沿对角线逐渐破碎,长条形颗粒从一侧向另一侧逐渐破碎,而缺角的不规则颗粒从形状缺失一侧开始向内破碎。     

Inter-particle spacing in aqueous suspensions of nanopowders and its effects on particle packing,green body formation and fabrication of alumina

Despite the flexibility offered by a slurry-based processing of ceramics, fabrication of high-density products from nanopowders via conventional techniques is a challenge. The rheological behavior of nanopowder suspensions and the sintering processes of nanopowders have been thoroughly studied in the literature; however, the link between has commonly been overlooked. In this study, the packing behavior of alumina nanopowders and its relation to the rheological behavior of suspensions and the final sintered densities were investigated. For this reason, the availability of inter-particle spacing in aqueous nanopowder suspensions (IPS_aq) was discussed. Funk-Dinger's relation was modified by means of electric double layer and hydration layer formed around oxide powders in aqueous media. The effects of IPS_aq on the green body formation and alumina fabrication were explored in terms of densities and packing behavior. It was concluded that low viscosities or high green body densities do not necessarily lead to high-density end products. Yet, the availability of interparticle spacing in liquid media is vital to obtain efficient packing and high density in sintered products.     

Revealing structural role of ZrO2 in silicate glasses from macroscale property by rigid-unit packing fraction method

The structural role of an oxide as a former and modifier can have significant effects on the chemical durability and mechanical properties of the glass. Some oxides with high-field strength cations, for example, MgO and ZrO₂, are often labeled as a third group—intermediate, due to their either undetermined or dual structural roles dependent on the glass compositions. Based on our recent modification of the Makishima–Mackenzie (MM) model using the rigid-unit Packing Fraction (RUPF), we analyzed a series of novel zirconia-containing bioactive glasses. The RUPF-based MM-model provides better prediction of the elastic moduli of these new glasses in comparison to experimental measurements. At the same time, the structural role of zirconia can be determined by comparison with calculations by assuming various structural roles and those from experiments. We reveal that ZrO₂ acts as the network former in phosphosilicate glasses, which leading to significant increase in packing fraction and consequent increase in Young's modulus. The recent experimental and atomistic simulation results support the glass former role of zirconia in silicate glasses. This method is general and applicable to other oxides in glasses.     

Discrete thermal element modelling of heat conduction in particle systems: Pipe-network model and transient analysis

In our recent work [Y.T. Feng, K. Han, C.F. Li, D.R.J. Owen. Discrete thermal element modelling of heat conduction in particle systems: basic formulations. Journal of Computational Physics. 227: 5072-5089, 2008], a novel numerical methodology, termed the discrete thermal element method (DTEM), is proposed for the modelling of heat conduction in systems involving a large number of circular particles in 2D cases. The method cannot be easily extended to transient analysis, which causes difficulties in combining the DTEM with the conventional discrete element method for modelling thermal/mechanical coupling problems in particle systems. This paper presents a simplified version of the DTEM, termed the pipe-network model, in which each particle is replaced by a simple thermal pipe-network connecting the particle centre with each contact zone associated with the particle. The model essentially neglects the direct heat transfer between the contact zones and thus significantly simplifies the solution procedure of the original DTEM. With this feature, transient heat conduction analysis can now be performed in a straightforward manner. In addition, the entire algorithmic structure of the pipe-network model is compatible with the discrete element method, leading to an effective scheme for simulating thermal-mechanical coupling problems. Numerical experiments are conducted to establish the solution accuracy of the proposed model.