气固流化床中细长颗粒数量浓度分布的数值研究

细长颗粒的循环流化在工业生产中具有非常广泛的应用背景,如生物质秸秆在循环流化床中的燃烧、烟丝在循环流化床中的干燥或加湿等。由于细长颗粒具有六自由度,因此,其运动姿态比球形颗粒复杂,其在流化床内的流化特性也不同于球形颗粒。细长颗粒的数量浓度分布是细长颗粒流化特性的重要特征之一。根据直接模拟Monte Carlo(DSMC)方法的基本思想及刚体动力学原理,建立了考虑细长颗粒间相互碰撞的三维细长颗粒流化运动数学模型,并采用此模型对某一实际流化床内的气固两相流场进行了模拟研究。在实验提升管内,所有物料中小长径比的细长颗粒最先由提升管的近壁处到达提升管出口。细长颗粒在流化过程中有明显的迁移和絮团现象。在流化状态下,细长颗粒的数量浓度分布基本不随入口风速发生变化。     

提升管内细长颗粒流化运动的气固多向耦合模型的构建

细长颗粒气固两相流研究已成为气固两相流研究的重点问题之一。而各种细长颗粒在流场中的受力、运动模型构建以及细长颗粒/流场间的耦合关系的构建是细长颗粒两相流数值模拟研究的难点之一。在已经基于刚体动力学原理构建了细长粒子气固两相流单向耦合模型的基础上,结合基于κ-ε模型的球形粒子-流场气固间耦合关联式,并改进细长颗粒间碰撞模型,从而构建了细长颗粒气固两相流动多向耦合数值模拟平台。并且采用此平台对某一实际流化床内的细长粒子气固两相流场进行了数值模拟研究。研究表明,不同时间细长颗粒在流场内的停留位置具有较强的随机性;细长颗粒在流化运动过程中伴随有较明显的取向选择性;细长颗粒的存在会导致当地流场的速度、压强均有较为明显的下降。     

搅拌釜内液-液混合溶析沉淀法制备纳米姜黄素颗粒

研究了在搅拌釜内利用溶析沉淀法制备姜黄素纳米颗粒的过程,分别探讨了搅拌釜尺寸、搅拌形式、初始浓度以及搅拌转速对制备结果的影响。通过平面激光诱导荧光技术定量测量釜内液液混合行为,揭示了流体混合环境的控制是决定溶析沉淀产品过程的关键因素。实验表明,搅拌速度过低无法保证流体混合效率,将导致颗粒黏结;搅拌速度过高带来颗粒与流场的强剪切作用,引起颗粒破碎;局部过饱和度过大,引起颗粒的生长和聚团。搅拌釜内纳米颗粒制备需提供适度强化的流体混合环境。     

三维喷动床内异径干湿颗粒混合特性数值模拟

基于计算流体力学-离散单元法,建立了三维喷动床内气固两相流数学模型,采用Fortran语言编制了并行数值模拟程序。对三维喷动床内两种不同直径的干颗粒及湿颗粒的混合特性进行了数值模拟,并从颗粒角度分析了双组分颗粒的运动机制。利用Lacey混合指数对床内整体以及特定区域的混合程度进行了定量分析,并研究了液桥体积、颗粒密度比以及表观气速对异径颗粒混合的影响。结果表明：在单孔射流喷动床内,干湿两种颗粒流动方式相似,湿颗粒无明显的聚团现象;液桥力对小直径的颗粒影响较大,使不同直径湿颗粒速度差减小;环隙区内颗粒的混合是影响整床颗粒混合的关键因素;液桥体积对颗粒混合的影响较大,对颗粒密度比以及表观气速的影响有限。     

A Novel Machine Vision Based Image Analysis Method for the Analysis of Mixing Elements in Rotary Drums

Mixing performance in continuous rotary drums has been studied. The video analysis method was developed to evaluate different configurations of straight lifters in the rotary drum. The method converts a captured video into a single image, called stack image. The color marker tracking was estimated based on the color saturation of the stack image. Coefficients of variation and mixing indices were calculated from the color saturation profiles for different straight blade lifter configurations. The video analysis method was confronted to the impulse response of acid concentrations in water solutions. The developed analysis method has been superior with viscous fluids compared to traditional tracer impulse method in mixing evaluations. Water and 1% CMC-water solution were used in this mixing study for covering broadly different viscous materials. The drum lengthwise results for one lifter configuration were obtained from a single experiment due to the block representation of the image analysis method. It enables mixing analysis of axial segments and interaction analysis of mixer configurations. Thus, the axial mixing can be studied in more detail with rotary drums. The increase of lifters, residence time, and tip speed improved axial mixing in the studied experimental setup.     

DEM simulation on particle mixing in dry and wet particles spouted bed

In this paper, the mixing characteristics of the dry and wet particles in a rectangular spouted bed are simulated using a three-dimensional discrete element method (DEM). In particular, the influence of turbulence and liquid bridge force is investigated using the standard k-ε two-equation model and the Mikami model. The Ashton mixing index is adopted to evaluate the dynamic mixing process of the particle system. The geometry of the simulated bed is the same as that of the experimental bed by Liu et al. [G. Q. Liu, S. Q. Li, X. L. Zhao, Q. Yao. Chem. Eng. Sci. 63 (2008) 1131-1141]. The effect of the spouting gas velocity on the mixing process is discussed for the mixing of dry particles (without the liquid bridge force), while the effect of the moisture content is discussed for the mixing of wet particles (with the liquid bridge force).     

柱状生物质颗粒与钢球颗粒在滚筒中的混合特性

采用离散单元法DEM（discrete element method）对圆柱形生物质颗粒和钢球颗粒在滚筒中的混合进行了数值模拟,分析了滚筒转速和颗粒数量比对混合质量的影响。结果表明：在本文设定的工况下,颗粒的混合模式为阶梯模式,并且颗粒在混合时可以分成3个区域,即左面的单层钢球颗粒区、中间的钢球颗粒和生物质颗粒混合区、右面的生物质颗粒堆积区。左右两边的颗粒混合效果较差,中间的颗粒混合效果较好。当滚筒转速相同时,钢球颗粒和生物质颗粒数量比为3000∶200时的颗粒混合效果比钢球颗粒和生物质颗粒数量比为3000∶100时的好,即当钢球颗粒数量远大于生物质颗粒数量时,增加生物质颗粒的数量可以提高混合效果。在钢球颗粒和生物质颗粒数量比相同的情况下,当滚筒转速在5~25r/min的范围内,滚筒转速越高,颗粒的混合质量越好,并且颗粒混合达到稳定的时间就越短。     

Adding active particles for overall aggregation in a mixing tank: A computational study

In order to achieve flocculation in a dense agitated solid-liquid suspension of nonaggregating particles, we explore scenarios where we add a limited amount of aggregative (ie, active) particles that can bind the nonaggregative particles. The performance of this process hinges on the competition between mixing (spreading the active particles over the flow volume) and aggregation among the active particles, with the latter reducing their effectiveness. The research has been conducted in a computational manner: direct simulations of transitional flow in a mixing tank (at an impeller-based Reynolds number of 4000) are two-way coupled with the dynamics of a collection of spherical, equally sized particles that are given specific aggregative properties. The overall solids volume fraction is 10%. A small fraction of all solid particles (5.8%) is active. Aggregation is quantified by means of the average coordination number as well as the aggregate size distribution. The way the active particles are released in the tank volume has a significant effect on the overall levels of aggregation, specifically for active particles with a strong aggregative force.     

Pressure-controlled secondary flows and mixing in sheared Platonic solid-shaped particles

Granular materials exhibit unique secondary flow behaviors upon shearing. We demonstrate, using particle dynamics simulations, that the secondary flow patterns are controlled by a pressure exerted on particle bed. A threshold pressure, at which vortex flow transitions to disturbed or chaotic flow, depends on particle shape, that influences interparticle contacts and rheological performance. Our results show that the flow patterns are essentially determined by a dimensionless term combining pressure and granular temperature for all the spherical and Platonic solid-shaped particles explored. Particle mixing is promoted by the vortex flow or disturbed flow with strong diffusion. The highest mixing rate under a specified pressure is obtained for cubic particles, due to the significant microstructural ordering near the boundaries causing a high gradient of packing density. These findings shed light on how applied pressure and particle shape affect secondary flows which is critical to the understanding and control of granular mixing.     

Discrete element method modeling of non-spherical granular flow in rectangular hopper

Discrete element method (DEM) was developed to simulate the corn-shaped particles flow in the hopper. The corn-shaped particle was described by four overlapping spheres. Contact force and gravity force were considered when establishing the model. In addition, the velocity distribution and voidage variance of corn-shaped and spherical particles were investigated. The results show that the vertical velocity difference between centre and side wall and the horizontal velocity of corn-shaped particles are relatively larger than that of spherical particles. The mean voidage for corn-shaped particles is smaller than for spherical particles in any hopper. And the mean voidage values decrease with the increase of the ratio of width and length (D/L) and the ratio of height and width (H/D) for both corn-shaped and spherical particles. The local voidage profiles in hoppers with different D/L were also studied. It demonstrates that the wall effect on the voidage of spherical particles is more remarkable than that of the corn-shaped particles. The voidage fluctuations of corn-shaped and spherical particles decrease obviously with increasing D/L when the particles are far away from the wall. And when the particles are discharging, the wall effect on the spherical particles is more remarkable than the condition of packing naturally.     

Preparation of griseofulvin microparticles by supercritical fluid expansion depressurization process

The supercritical fluid expansion depressurization (SFED) process is a novel technique proposed recently to prepare microparticles with narrow size distribution. The process has shown a very promising potential in pharmaceutical micronization. An SFED experimental apparatus was set up and griseofulvin (GF) microparticles were prepared successfully with the solvent of the mixture of acetone and ethanol. The influences of the operation parameters, including the pressure and temperature in the mixing vessel, the solution concentration and the solution feeding rate, on the particle morphology, size and size distribution were investigated in detail. The results show that approximately spherical particles with size less than 1.5 μm can be prepared successfully by SFED process. The pressure in the mixing vessel and the solution feeding rate are two most effective parameters while the solution concentration is the next, and the temperature in the mixing vessel has little effect on the GF particles. The optimal operation condition for preparing GF micro-particles in the range of this work is: the pressure of 8 MPa and the temperature of 60 °C in the mixing vessel, the solution feeding rate of 9 ml/min and the solution concentration of 15 mg/ml.     

Adhesion forces of charged particles

Adhesion forces of toner and polymer particles to aluminum substrates were measured by the centrifugal, detachment field and microelectrode detachment field methods, and factors affecting the adhesion forces are discussed. The adhesion forces of toner particles increased with an increase in either particle size or particle charge. The adhesion force of an irregularly shaped toner was larger than that of a spherical toner. The mean adhesion force of polymer particles to aluminum substrates decreased with an increase in surface roughness of the substrates. The CF₄ plasma treatment of the polymer particles shifted their adhesion force distribution in a smaller direction. It was confirmed that the results by the centrifugal and the detachment field methods were in good agreement with each other. The contribution of van der Waals, electrostatic and water bridging forces to the adhesion forces of toner particles are also discussed.     

DEM investigation on the breakage of spherical aggregate under multi-contact loadings

Particle crushing is commonly encountered during the storage, transportation, and handling of granular assemblies. This work is aimed at ascertaining the applicability of four particle breakage criteria frequently mentioned in literature. Discrete element method (DEM) simulations were conducted to investigate the crushing of spherical aggregates under multicontact loadings. Mean and major principal stress criteria, octahedral shear stress criterion, and maximal contact force (MCF) criterion were then evaluated based on the obtained DEM results. It is found that the first three parameters all vary with the number of loading contacts, demonstrating they cannot predict the crushing of particles under arbitrary loading configuration. Simulation results indicate that the MCF at crushing is related to the number and the spatial arrangement of loading contacts. Thus, strictly speaking, this parameter cannot uniquely define particle breakage either. The influences of the microstructural heterogeneity on the breakage strength of particle and also on the applicability of MCF criterion are discussed.     

Numerical simulation of turbulent solid–liquid two-phase flow and orientation of slender particles in a stirred tank

In this paper, turbulent solid–liquid two-phase flow involving slender particles in a tank stirred by standard Rushton turbines is simulated with two-fluid model using the improved inner–outer iterative method. Standard k–ε model is used to deal with turbulent flow. By comparison with the case of equivalent spherical particles, it is found that the flow field of slender particles is similar to that of spherical particles. The evolution of particle orientation as it follows the liquid flow in a stirred tank is modeled directly from the rigid slender rods revolution equation. Experiments about solid–liquid two-phase flow are also performed in a baffled tank using DPIV (digital particle image velocimetry). All simulation results are compared with experiments. The comparison between simulation and experiments confirms that the results are reliable. The good agreements between simulation and experiments verify the reliability of the methods employed in this paper. The influences of impeller speed on flow field and orientations are also investigated.     

Effect of blade angle and particle size on powder mixing performance in a rectangular box

This study focuses on the understanding of flow over a single blade and its impact on powder mixing. The Discrete or Distinct Element Method (DEM) is used and the flow of a single blade through a bed of a binary particle mixture is studied. Mixing performance with respect to a blade-rake angle and particle size is investigated using the Modified Generalized Mean Mixing Index (MGMMI) and the maximum mean instantaneous velocities. A wide range of angles and different loading scenarios of the binary particle mixture were studied. Velocity profiles for all these cases were computed, as well as the forces on particles and the blade. The results showed an inverse relation between the interparticle force and blade-rake angle. Systems with a higher number of larger particles experienced a higher interparticle force. Similar results were obtained for the blade force. The results for mixing efficiency showed that if the smaller particles are placed at the top this leads to a higher mixing performance. The mixing performance was highest for blade-rake angles that offered a maximal surface area or maximal resistance to the flow of particles, which occurred for blade-rake angles from 70° to 90°.     

转鼓式生物反应器中液相体系的流动性能

采用粒子示踪测速法在18 L转鼓式生物反应器中考察了转速和挡板数、挡板类型和粘度对转鼓流场的影响. 结果表明,在低粘的自来水介质中,转速为4~12 r/min时,Y向平均分速度由0.029 m/s增加到0.053和0.064 m/s时,挡板重力提升能力不断增强,提高转鼓转速可促进全局混合. 直挡板数由4增加到8时,挡板提升能力增强32.5%,且能获得更均匀的流场分布;与自来水体系相比,高粘度8 g/L黄原胶溶液体系中流场分布更均匀,剪切更温和.     

物料无桨混合过程数值模拟

目前物料混合主要采用有桨混合方式,该方法容易使物料受到较大摩擦作用。为了解决这一问题,本文对无桨滚筒混合工艺过程进行探索研究。采用离散元软件EDEM模拟无桨滚筒对两种物料的混合过程,分析混合过程中物料的运动特征,通过定义颗粒混合均匀性指数来定量表征混合均匀程度,分析了不同填料（10000~20000个颗粒）和转速（7~22r/min）对混料均匀性的影响规律,并对物料受力情况进行了定量分析。研究结果表明：在无桨滚筒混料过程中,可以实现对两种物料的混合,且随着转速提高,物料能较快达到均匀。在混合过程中物料受力呈波动式变化,其中最大受力小于0.6N,稳定在0.1~0.35N;物料受力平均值小于0.015N,稳定在0.005~0.01N;最大受力是平均受力值的20~30倍;且随着填料和滚筒转速增加,物料的受力略有增加。     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

水力空化混合强化超临界流体辅助雾化制备罗红霉素超细微粒

分析了超临界流体辅助雾化（SAA）过程,发现饱和器内超临界二氧化碳与溶液的混合是SAA成功的关键因素之一,由此引入了水力空化混合器以强化饱和器内两相间的传质。在自行组建的引入水力空化混合器的超临界流体辅助雾化（SAA-HCM）装置上,以罗红霉素为模型药物,考察了混合器压力、沉淀器温度、溶剂、进料中CO₂与液体溶液流量比（R）和溶液浓度对微粒形态和粒径的影响。结果表明,水力空化混合器能有效地强化两相间的传质,SAA-HCM工艺可制备出罗红霉素超细微粒,大部分微粒形态呈球形,通过改变操作参数可制得粒径在1～3 μm的适于吸入式给药的气溶胶药物微粒和粒径小于1 μm的超细微粒。     

Discrete element simulation study of a Freeman powder rheometer

The powder rheometer is a commonly used device for assessing the bulk flow performance of pharmaceutical powders. Discrete element simulations were performed to determine the effects of particle properties such as size, shape, size distribution and friction on the force and torque on the impeller blade in a powder rheometer. DEM simulations are well suited for such a study as they can isolate the impact of each particle property on the bulk powder behavior. The results can then be used to guide the ‘particle engineering’ of pharmaceutical powders to meet specific performance targets. The operation of the FT4 Freeman powder rheometer system was simulated using the discrete element method (EDEM? from DEM Solutions). The effects of various particle properties (size, shape, friction, etc.) were examined using the force and torque on the impeller blade as the key performance indicators. The effect of particle size (mean, distribution) on the mixing torque/force was small and the use of a pre-conditioning step also had minimal impact. As the particle aspect ratio was increased from 1.0 (perfect spheres) to 2.0, both the torque and force values also increased (max increase of ～40%). Increasing the rolling friction of spherical particles produced similar results as the large aspect ratio particles. Increased particle–particle friction caused a larger increase in the measurements (max increase of ～60%) in comparison with increased particle–vessel and particle–impeller friction (max increase of ～20%). Experiments with glass beads were also performed and were used to validate the simulations.