DEM simulations of die filling during pharmaceutical tabletting

The flow behaviour of powders from a stationary shoe into a moving die, which mimics the die filling process in a rotary tablet press, was analysed using a discrete element method (DEM), in which 2D irregular shaped particles were considered. The influence of the particle shape,size and size distribution, the number of particles used in the simulation, the initial height of powder bed in the shoe, and the filling speed on the average mass flow rate and the critical filling speed (the highest speed at which the die can be completely filled) were explored. It has been found that a maximum flow rate is obtained at the critical filling speed for all systems investigated and poly-disperse systems have higher mass flow rates and higher critical filling speeds than mono-disperse systems. In addition, the powder with particles which can tessellate generally has a lower filling rate and a lower critical filling speed.     

Modelling die filling with charged particles using DEM/CFD

The effects of electrostatic charge on powder flow behaviour during die filling in a vacuum and in air were analysed using a coupled discrete element method and computational fluid dynamics (DEM/CFD) code, in which long range electrostatic interactions were implemented. The present 2D simulations revealed that both electrostatic charge and the presence of air can affect the powder flow behaviour during die filling. It was found that the electrostatic charge inhibited the flow of powders into the die and induced a loose packing structure. At the same filling speed, increasing the electrostatic charge led to a decrease in the fill ratio which quantifies the volumetric occupancy of powder in the die. In addition, increasing the shoe speed caused a further decrease in the fill ratio, which was characterised using the concept of critical filling speed. When the electrostatic charge was low, the air/particle interaction was strong so that a lower critical filling speed was obtained for die filling in air than in a vacuum. With high electrostatic charge, the electrostatic interactions became dominant. Consequently, similar fill ratio and critical filling speed were obtained for die filling in air and in a vacuum.     

Particle flow mechanism into cable shovel dippers

Electric cable (or rope) shovels are critical equipment in the surface mining industry. An improved understanding of the factors which affect the flow of broken material into the dipper during loading can help to evaluate the performance of the excavator, define the criteria for equipment selection and develop ways to mitigate equipment damage caused by broken particles. In this paper, the mechanism of granular material flow was investigated through a series of laboratory tests by moving 1:32 and 1:20 (cube root scale) models of a 44 m³ dipper through a test bin filled with angular crushed limestone. It was found that neither dipper angle nor hoist speed has significant influence on the general flow pattern. It was also found that the general flow mechanism is independent on the size of the dipper. Quantitatively, it was determined that hoist speed and pitch angle affect the productivity of the machine with lower pitch angle resulting in higher payload, and faster hoist speed results in shorter dig time. While both lower pitch angle and faster hoist speeds also produce a higher rate of energy consumption, overall both contribute to improved machine productivity.     

Detonation in heterogeneous mixtures of liquids and particles

The mechanisms of detonation propagation in heterogeneous systems comprising closely packed particles and a liquid explosive are not fully understood. Recent experimental work has suggested the presence of two distinct modes of detonation propagation. One mode is valid for small particles (which is the regime we will address in this paper) with another mode for large particles. In this work we model numerically the detail of the wave interactions between the detonating liquid and the solid particles. The generic system of interest in our work is nitromethane and aluminium but our methodology can be applied to other liquids and particles. We have exercised our numerical models on the experiments described above. Our models can now qualitatively explain the observed variation in critical diameter with particle size. We also report some initial discrepancies in our predictions of wave speeds in nominally one dimensional experiments which can be explained by detailed modelling. We find that the complex wave interaction in the flow behind the leading shock in the detonating system of liquid and particles is characterised by at least two sonic points. The first is the standard CJ point in the reacting liquid. The second is a sonic point with respect to the sound speed in the inert material. This leads to a steady state zone in the flow behind the leading shock which is much longer than the reaction zone in the liquid alone. The width of this region scales linearly with particle size. Since the width of the subsonic region strongly influences the failure diameter we believe that this property of the flow is the origin of the observed increase in failure diameter with particle size for small inert particles. Received 3 December 1999 / Accepted 5 July 2000     

WET-GRANULATION RESEARCH WITH APPLICATION TO SCALE-UP

Summary Granulation is a unit operation by which larger granules are produced from fine, powdery particles to improve appearance, flow properties and mixedness, reduce dustiness and, in general, produce engineered particles with superior attributes. Agglomeration in wet granulation is achieved by introducing a “binder” fluid onto a shearing mass of fine powders. This paper gives a general overview of the process with emphasis on a simplified granulation model based on a dimensionless parameter containing inertia and viscous dissipation energies between colliding particles: the so-celled Stokes number. The model incorporates most common features of all granulation devices (mixers) used in the pharmaceutical industry.Also described in the paper is a computer simulation that captures the movement of flowing powder in an ideal mixer-granu/ator with constant shear rate. A fraction of the total number of particles is wet (covered by binder and there-fore “sticky”) while the rest of the particles are dry. The numerical simulation depicts two distinct regimes of agglomeration found in a typical granulator: granule growth and subsequent breakup. During granule growth-simulations, final granule size and shape distributions are obtained by analyzing the size and shape of formed granules using a pattern-recognition technique. A second kind of simulation, also using rapid granular flow modeling, follows the rotation and deformation of an “agglomerate” held together by a liquid binder. Results from these simulations yield critical values of the Stokes number. Below the cdticel value, the agglomerates are stable and only rotate in response to shear while above the critical value they break into two or more pieces. At the critical value, they attain a steady elongated shape. Using values of the critical Stokes number, the model predicts the size of formed granules.The existence of the critical state in which granules attain a characteristic elongated shape is used to measure shear forces in a granulator by employing calibrated “test” particles of known strength. This knowledge is employed in granulation scale-up to determine a kinematic rule that conserves stresses in the small and the large-scale machines. It was found that conserving the magnitude of internal stresses in the moving powder yields granules with similar attributes in granulators of different size.     

用代数应力模型模化型腔充填过程中的紊流流动

针对型腔充填过程中的紊流流动，用代数应力模型研究紊流流动现象．成功地将代数应力模型引入ＰＨＯＥＮＩＣＳ软件中，完成了恒温流体充填过程三维时均速度和自由表面的计算模拟．与实验结果相比说明，本文基于ＰＨＯＥＮＩＣＳ软件开发的充填模拟具备了对复杂型腔充填过程的数值计算．     

Convective heat transfer characteristics of microencapsulated phase change material suspensions in minichannels

A comparative experimental study was conducted in order to investigate the convective heat transfer characteristics of water-based suspensions of microencapsulated phase change material (MEPCM) flowing through rectangular copper minichannels. The hydraulic diameter of the channels was 2.71 mm. MEPCM particles with an average size of 4.97 μm were used to form suspensions with mass concentrations ranging from 0 to 20%. The comparative experiments were performed for varying mass flow rates in the laminar region and varying thermal conditions. The cooling performance of the MEPCM suspensions strongly depended on the mass flow rate and the MEPCM mass concentration. The 5% suspension always showed a better cooling performance than water resulting in lower wall temperatures and enhanced heat transfer coefficients within the whole range of mass flow rates. The suspensions with higher mass concentrations, however, were more effective only at low mass flow rates. At higher mass flow rates they showed a less effective cooling performance than water.     

An analysis of critical simultaneous gas/liquid flow through a restriction and its application to flowmetering

Summary A theoretical analysis is presented of the mechanism of simultaneous flow of gas and liquid through a restriction at critical speed. The study shows that a relationship exists between the mass flows of gas and liquid, restriction size and upstream pressure. Comparison of this relationship with measurements showed a reasonable agreement. It has therefore been concluded that a restriction can be used as a gas/liquid flowmeter with reasonably high accuracy, provided that the flow is a critical one. Further increase of accuracy seems possible by using a restriction especially designed for the purpose.     

Numerical simulation of tetrahedral particle mixing and motion in rotating drums

A regular tetrahedron is the simplest three-dimensional structure and has the largest non-sphericity. Mixing of tetrahedral particles in a thin drum mixer was studied by the soft-sphere-imbedded pseudo-hard particle model and compared with that of spherical particles. The two particle types were simulated with different rotation speeds and drum filling levels. The Lacey mixing index and Shannon information entropy were used to explore the effects of sphericity on the mixing and motion of particles. Moreover, the probability density functions and mean values and variances of motion velocities, including translational and rotational, were computed to quantify the differences between the motion features of tetrahedra and spheres. We found that the flow regime depended on the particle shape in addition to the rotation speed and filling level of the drum. The mixing of tetrahedral particles was better than that of spherical particles in the rolling and cascading regimes at a high filling level, whereas it may be poorer when the filling level was low. The Shannon information entropy is better than the Lacey mixing index to evaluate mixing because it can reflect the real change of flow regime from the cataracting to the centrifugal regime, whereas the mixing index cannot.     

In-situ experiment tests and particulate simulations on powder paving process of additive manufacturing

In this work, in-situ experimental tests are first performed to investigate the powder spreading process of additive manufacturing, where different kinds of scrapers and spreading speeds are employed. Detailed kinetic behaviours of individual powder particles are discussed by discrete element method simulations. It is found that the decrease of inclination angle of the scraper improves the powder pressure and compaction in the spreading process, leading to a denser powder flow and thus a denser powder bed. The increase of spreading speed also improves the powder pressure and compaction in the spreading process. However, the powder flow becomes looser due to the volume dilation, and thus the quality of the paved powder bed decreases. In industrial applications, if the higher powder spreading speed is employed to improve the processing efficiency, the scraper with a smaller inclination angle can be used to ensure the powder bed quality.     

振动对充填裂隙渗透率影响的实验研究

为了研究振动和充填泥沙的共同作用对裂隙岩石渗透率的影响,通过对裂隙试件在充填和振动后条件下的渗流实验,得到了频率和振幅与充填裂隙岩石渗透率的关系曲线。结果表明,充填泥沙后的相对渗透率低于充填前,最小降幅为10.19%,最大降幅为59.74%。在相同的振幅下,试件的相对渗透率随频率的增加都基本呈下降趋势;振动后的渗透率低于振动前的渗透率;充填试件在较大频率振动下,渗透率减小的更为显著。在相同的频率下,充填试件振动后的渗透率基本上都减小了,最小降幅为8.90%,最大降幅为51.23%;而未充填试件振动后的渗透率与振幅存在一定关系;相对渗透率与振幅的变化之间存在拐点,相对渗透率并没有随着振幅的增加而一直减小。     

The effect of boron nitride on the rheology and processing of polyolefins

The influence of a new processing additive (fine particles of boron nitride) on the rheology and processability of polyolefins is studied. The equipment used includes an Instron capillary rheometer equipped with capillary and special annular dies (Nokia Maillefer wire coating crosshead) and two rheometers, namely a parallel-plate and a sliding-plate rheometer. Several types of boron nitride powders, varying in average particle size and distribution and in morphology are tested at various concentration levels. The additive with the smallest average particle size and free of agglomeration was found to have the greatest influence on the processability (melt fracture performance) of the polyolefins tested. Specifically, it was found that boron nitride not only eliminates surface melt fracture but also postpones the critical shear rate for the onset of gross melt fracture to significantly higher values, depending on the additive concentration, surface energy, and morphology. A flow visualization technique was used to visualize the polymer flow at the entrance of a transparent capillary die in order to determine the mechanism by which boron nitride eliminates gross melt fracture. Received: 18 January 2000 Accepted: 15 June 2000     

Linear stability of a fluid channel with a porous layer in the center简

We perform a Poiseuille flow in a channel linear stability analysis of a inserted with one porous layer in the centre, and focus mainly on the effect of porous filling ratio. The spectral collocation technique is adopted to solve the coupled linear stability problem. We investigate the effect of permeability, σ, with fixed porous filling ratio ψ = 1/3 and then the effect of change in porous filling ratio. As shown in the paper, with increasing σ, almost each eigenvalue on the upper left branch has two subbranches at ψ = 1/3. The channel flow with one porous layer inserted at its middle （ψ = 1/3） is more stable than the structure of two porous layers at upper and bottom walls with the same parameters. By decreasing the filling ratio ψ, the modes on the upper left branch are almost in pairs and move in opposite directions, especially one of the two unstable modes moves back to a stable mode, while the other becomes more instable. It is concluded that there are at most two unstable modes with decreasing filling ratio ψ. By analyzing the relation between ψ and the maximum imaginary part of the streamwise phase speed, Cimax, we find that increasing Re has a destabilizing effect and the effect is more obvious for small Re, where ψ a remarkable drop in Cimax can be observed. The most unstable mode is more sensitive at small filling ratio ψ, and decreasing ψ can not always increase the linear stability. There is a maximum value of Cimax which appears at a small porous filling ratio when Re is larger than 2 000. And the value of filling ratio 0 corresponding to the maximum value of Cimax in the most unstable state is increased with in- creasing Re. There is a critical value of porous filling ratio （= 0.24） for Re = 500; the structure will become stable as ψ grows to surpass the threshold of 0.24; When porous filling ratio ψ increases from 0.4 to 0.6, there is hardly any changes in the values of Cimax. We have also observed that the critical Reynolds number is especially sensitive for small ψ where the fastest drop is observed, and there may be a wide range in which the porous filling ratio has less effect on the stability （ψ ranges from 0.2 to 0.6 at σ = 0.002）. At larger permeability, σ, the critical Reynolds number tends to converge no matter what the value of porous filling ratio is.     

Investigation on the granulation behavior of iron ore fine in a horizontal high-shear granulator

High-shear granulation is widely used in many particulate industries for its good capability to improve the size, strength and composition uniformity of powder substances. This work conducted an experimental study to investigate the granulation behavior of iron ore fine in a horizontal high-shear granulator, such as granules size distribution, granules growth rate, and permeability of the granules bed. The results show that the granule size and permeability of packed granules bed increase gradually with increasing the granulation time, and the growth of granules can be divided into three stages: the rapid growth stage, the slow growth stage and the relatively stable stage. Both the higher rotational speed and larger number of impellers increase the kinetic energy and collision frequency of the particles, which causes the increase of average granule size, growth rate and permeability of granules packed bed. On the other hand, the shear damage effect of the impellers on the granules is also enhanced with the increase of rotational speed and impeller number, resulting in significant granule size segregation.     

惰化剂粒径对铝粉火焰传播特性影响的实验研究

为探索惰化剂粒径对可燃工业粉尘火焰传播特性的影响,通过建立竖直粉尘燃烧管道实验平台,在碳酸氢钠质量分数为30%的惰化条件下,就碳酸氢钠粒径对铝粉燃烧火焰传播特性的影响进行了实验研究。结果表明:平均粒径为30 μm的碳酸氢钠粉体对平均粒径为15 μm的铝粉的火焰传播速度具有较好的抑制作用,惰性粉体与可燃工业粉尘应存在粒度匹配效应;碳酸氢钠粉体对铝粉火焰温度的惰化抑制效果与其粒径呈反比关系;碳酸氢钠粉体会减小铝粉火焰预热区厚度,预热区厚度随碳酸氢钠粒径的增加先减小后增大。此外,分析了碳酸氢钠粒径对铝粉火焰传播特性影响的作用机理。     

Effect of particle polydispersity on particle concentration measurement by using laser Doppler anemometry

Measurement of particle concentration by laser Doppler anemometry (LDA) is studied on a vertical air jet seeded by a powder disperser with controlled particle and air flow rates. Particle arrival rate is utilized to retrieve particle number densities from conventional LDA operation. The effect of polydisperse nature of the particles is assessed. Comparisons between measured and estimated particle number densities suggest that only a certain portion of the particle population with a particle size to fringe spacing ratio around unity can be detected. Results indicate that reliable measurement of absolute particle concentration is possible for a particle population of narrow size distribution with an average diameter equivalent to fringe spacing. Present number density measurement technique which is useful for practical purposes with conventional LDA systems is found to yield physically reasonable profiles in both laminar and turbulent regimes.     

The effect of particle size and migration on the formation of flow-induced structures in viscoelastic suspensions

Flow-induced structures in suspensions containing spheres in viscoelastic suspending media were investigated by microscopy and rheo-optical methods. Suspensions of monodisperse polystyrene spheres with diameters ranging from 1.2 to 2.8 μm and dispersed in aqueous solutions of hydroxypropylcellulose were studied in simple shear flows. Optical microscopy observations as well as small-angle light-scattering (SALS) experiments were performed using a parallel plate geometry. In agreement with previous work, necklaces of particles aligned in the flow direction were observed when shearing faster then a critical shear rate, which was found to be independent of particle size. In contrast to earlier work, however, the role of particle migration was found to be of prime importance. Particles were shown to migrate toward the plates where the particles assembled and aligned in strings running in the flow direction. For the smallest particles (1 μm diameter), the formation of particle doublets or short strings along the vorticity direction was observed at low shear rates, which flipped to an orientation into the flow direction and grew into longer strings at higher shear rates. SALS experiments were used to quantify the degree of alignment and its dependence on particle size, shear rate, and gap. For the system under investigation, the degree of alignment was found to increase with increasing shear rate and particle size and with decreasing gap. The present results suggest that, depending on the details of the suspending medium and the size and nature of the suspended particles, the formation of aligned structures is affected by the relative magnitude of the colloidal and hydrodynamic forces and the kinetics of string formation versus the kinetics of migration.     

井下多股淹没射流水力清岩能力分析

研究石油钻井井下射流的水力清岩能力对钻井业有重要意义。本文借助于预处理方法和多块网格对接技术由计算机数值模拟了形状复杂的钻井PDC钻头井底下的多股淹没射流流场。为体现井底有岩屑时射流场对其的清洗作用。文中借助河流泥沙运动理论，分析了岩屑在井下的受力与运动以考虑固相的作用。因岩屑主要为推移运动。在井底除环空外的大部分地方可简化视为二维运动。提出了钻头井底水力清岩能力的体现参数-水力挟岩力。并通过数值模拟显示了井下流场，描述了转速，射流流量，喷嘴面积，射流雷诺数及井下过流空间等流场控制参数对井底水力清岩能力的不同影响，指出相关水力参数在射流时的优化方向。为复杂的井底流场控制和钻头的合理设计做探索基础。     

Linear and non-linear nature of the flow of polypropylene filled with ferrite particles: from low to concentrated composites

The flow behavior of a filled suspension consisting of ferrite particles suspended in a polypropylene matrix with and without the addition of a commercial dispersant (Solplus DP310) was studied. The composites were filled with 10, 20, 30, and 40 vol.%. Both capillary and parallel disk rotational flows were employed. On the one hand, dynamic results confirm general trends found for highly concentrated systems. The higher is the filler level, the lower is the linear viscoelastic domain. When adding the dispersant agent, it was shown a larger linear viscoelastic domain, lower moduli values and thus, lower viscosity. Also, the critical strain, G′ and G′′ showed a power law dependency on the volume fraction. On the other hand, the capillary results showed no dependency of the flow properties on the die. Thus, no slip of the suspension at the wall was observed. Actually, this experimental finding elucidated that the significant decrease on viscosity produced by the addition of the dispersant agent at 40 vol.% is principally due to lubricant effects and not at all to slip contributions. The results also reveal three distinct flow regimes. Low, moderate, and high shear rates lead to different microstructure under flow.     

Measurement of rheological and ultrasonic properties of food and synthetic polymer melts

A slit die viscometer has been used in conjunction with a co-rotating twin screw extruder to study the rheological behaviour of maize grits, potato powder and low density polyethylene, as a function of feed rate, screw speed and temperature. The shear viscosity of both maize and potato decreased with increasing feed rate. Increasing the temperature or screw speed at any given feed rate also reduced the viscosity. The ultrasonic velocity through the material has also been shown to be sensitive to the extruder operating conditions. Overall, the ultrasonic velocity decreased as screw speed and temperature increased and feed rate decreased.