Effect of screw design on hopper drawdown of spherical particles in a horizontal screw feeder

Screw feeders are used to remove material from hoppers and bins at a controlled rate. The evenness of the flow in the bin depends on the drawdown pattern, which in turn depends on the screw and hopper design, shape of the particles and wall friction effects. A key design requirement is to ensure that a progressive increase in the screws volumetric capacity is achieved along the entire length of the hopper's opening so as to produce even drawdown. If this is not achieved then compositional variations in the outgoing stream and other operational problems (such as caking) can be created. Screw designs to date have been generally based on analytical models and at times the predicted flow pattern is not achieved. In this study, the Discrete Element Method (DEM) is used to predict particle transport in a horizontal screw feeder system for a range of conventional screw designs including a variable screw pitch, variable screw flight outside diameters and variable core diameters. The influence of screw choice on the particle mass flow rate, the evenness of particle drawdown from the hopper, power consumption, screw wear and wall friction variations are all investigated. Important features captured by DEM that are not accounted for by the analytic model and which vary strongly between competing screw designs, include the particle circulation in the hopper, shearing of the particle bed in the trough just outside the screw and the spatially varying particle force along the hopper which leads to non-uniform drawdown and to the existence of large stagnant or slow moving zones. The screw design and consequent flow patterns also strongly affect the power draw with variations up to a factor of three and screw wear with large changes in their distribution and magnitude. Finally, the surface frictional properties of the screw are shown to strongly influence the rate of bed compaction within and along the screw leading to strong variation in mass flow rate, uniformity of drawdown and power draw.     

往复注射螺杆计量段三维流场分析

使用流体计算软件Polyflow，对往复注副螺杆计量段进行符合其轴向运动特点的三维流动分析。通过计算得出剪切速率场、压力场、速度场的分布结果，并对分布结果进行分析。通过不同工艺条件变化引起的流场变化，得到流场内部变化的部分规律，并建立了符合注射螺杆工作特点的模拟分析方法。     

Numerical simulation of the flow field in the mixing section of a screw extruder by the lattice Boltzmann model

The single-screw extruder is commonly used in polymer processing where the performance of the mixing section is significant in determining the quality of the final product. It is therefore of great interest to simulate the flow field in a single-screw extruder. In this paper the simulation is performed using the lattice Botlzmann model giving a fast and efficient numerical technique. The solution is compared to a finite difference and lattice gas simulation and found to give more accurate results.     

Influence of agitator design on powder flow

Design influences the flow within a powder mixer but quantitative guidance is lacking. Here the performance of mixers of different geometry was compared using positron emission particle tracking. One mixer had six long flat blades; the other carried short paddles. With the former, blade angle and number of axial compartments had little effect on agitation in the transaxial plan but axial dispersion was enhanced by longer axial compartments. A loop of circulation was found below the shaft. For the short paddle device, the transaxial agitation was more uniform, with a lower mean angular velocity and narrower ranges of velocities. The mixing elements inhibited the formation of the loop of circulation. In both cases, the axial flow had a cellular structure created by the radial supports for the blades but the short paddles mixer showed more chaotic behaviour, the axial dispersion coefficient being typically five times higher and increasing with fill rather than decreasing as seen with the six-blade device. A rationale for the design of powder mixers is thus emerging.     

Acoustic initiation of powder flow in capillaries

Second generation solid freeforming devices will have the capability to render both shape and the spatial arrangement of composition directly from a computer file. In order to build three-dimensional functional gradients in selective laser sintering it is necessary to have a computer-controlled mixing and dispensing system. We report such a system based on the horizontal acoustic vibration of vertical capillary tubes that provides both switching and flow rate control. An orchestra of such tubes can be constructed to deposit a multi-component system onto a building platform. Our concern is the initiation of flow. It requires an “attack” waveform to break the domes that provide flow arrest and release the potential energy of powder above the dome. The intensity of attack also influences the mass that flows out before a new dome forms, i.e., the response time of the valve. Neither the extension to vibration caused by ringing nor the wave amplitude account for the over-run of the valve. Much better correlation is obtained with acceleration and with calculated kinetic energy of horizontally vibrating particles.     

大深宽比双T形微通道内液-液两相流可视化研究

通过高速摄像对水力直径0.176mm、深宽比2.4的双T形矩形微通道内的液-液两相流动进行了可视化实验研究。改变连续相（硅油）和分散相（水）的流量比,记录分析了微通道不同部位油-水两相流的流型和流型发展演化情况。实验结果表明,在微通道上游T形部位的油-水两相流型主要包括滴状流、弹状流、波平行流和平行流;在微通道的中间部位,绘制了基于水和硅油量纲为1韦伯数的流型图,并将其与相关文献进行了比较。同时,发现微通道内液塞及液滴的长度（量纲为1）与油/水流量比之间存在线性关系,液塞/液滴速度比两相混合物表观速度大,建立了能够准确描述液塞/液滴运动速度的实验拟合公式。最后,研究了液滴在微通道下游T形部位的行为,观察到断裂和不断裂两种模式并进行了分析,给出了划分断裂与否的流型图。     

Measurement of the adsorptive capacity of a powder

The purpose of this work was to recommend a method of measuring the adsorptive capacity of powdered activated carbon (PAC). The adsorptive capacity is needed in future mathematical models of dispersed phase adsorption. This measurement was difficult, because the large drag forces in powders resist any flow into the sample volume, causing big pressure gradients.

Two approaches were tested and compared; a material balance over a packed bed, and a standard volumetric method. The pressure loss across the packed bed was made small by the combination of the PAC with glass ballotini. Toluene was chosen as a sorbate because of its ease of measurement.

The results concerned pressure variations in the packed bed, and the ease with which the end-points could be found in either method. With regard to the packed bed, the time from the start of breakthrough to saturation was typically 110 min. The principal reason for this delay was channelling; the breakthrough curve could not be used to infer pore diffusion coefficients. The total pressure within the bed was kept to within 5% of its mean value. Hence, the adsorptive capacity could be plotted as a function of pressure, and fitted with a Freundlich isotherm with an exponent of n=0.2. Including the PAC, the adsorptive capacities of four kinds of carbon, at the same temperature and pressure, varied from 8 to 16%.

For the volumetric method, at least 3 h were needed for the sorbent and sorbate to reach equilibrium. A mathematical model showed that the uptake curve was controlled by heat transfer. Because the test volume was under a partial vacuum, the technician had to be careful that no air leaked into it during the test. The volumetric method was faster with 250 μm diameter carbon granules; the end-point was evident after 20 min.

In conclusion, a method involving a packed bed was developed to measure the adsorptive capacity of a powder. The end-point was determined easily; the end-point for the volumetric method was indistinct.     

Classification of ultra fine powder by a new pneumatic type classifier

Recently, there are great requirements for ultra fine powder in fields that use advanced materials, for example, electric parts in Information Technology (IT) industry. Especially, the request for the diameter control of ultra fine powder is increasing. Pneumatic type ultra fine powder separator, classifier, is one of the equipment which meet this request. But there is no clarified study on the flow condition that realizes high performance. In this study, a new pneumatic type ultra fine classifier is proposed and the relation between the performance and the flow condition is investigated by flow visualization with the tuft grid and oil dot methods and the measurement of gas velocity.Based on the results, it was known that the new classifier has a large swirling flow velocity which is able to classify ultra fine particles accurately.     

变频器调速技术在5R摆式磨粉机圆盘给料机上的应用

介绍了变频器技术在磨粉机圆盘给料机上的应用,对使用效果和存在问题进行了分析,并提出了改进建议,实现圆盘给料机连续喂料,实现磨粉纯度的稳定,提高磨粉纯度合格率.     

A new technique for the visualization of the concentration boundary layer in an electrodialysis cell

A new method for the visualization of the concentration boundary layer is described. The technique involves the use of a dilute solution of an indicator which reacts with H⁺ formed on the membrane surface to form a coloured trace when the electrodialysis cell is operating above the limiting current density. The thickness of the concentration boundary layer determined by the visualization method agrees well with results obtained from limiting current density measurements and theoretical predictions. The visualization method proposed in this work can be used for understanding the transport taking place between a solid wall and a liquid in steady and unsteady flow processes.     

Measurement of the local velocity of the solid phase and the local solid hold-up in a three-phase flow by X-ray based particle tracking velocimetry (XPTV)

The measurement of the local solid velocity and the local solid hold-up in three-phase flows (gas, liquid, solid) is of great interest with regard to the design of three-phase reactors. Moreover, such measurements are necessary for the validation of flow simulations of three-phase flows. The optical methods usually applied for velocity measurements such as particle image velocimetry do not work in three-phase flows. This is due to the opacity of the solid phase or/and because of the reflections and refractions that occur on phase boundaries. Other measurement methods are intrusive and very time consuming. The measurement of the local solid hold-up is even more difficult. The new X-ray based particle tracking velocimetry (XPTV), described in this paper, measures the solid velocity and the solid content simultaneously. This fast working and non-intrusive technique has already been successfully applied in a bubble column to measure the liquid velocity. XPTV is a three-dimensional three-component method. It works independently from void fraction and solid hold-up.     

用可视化方法实验研究规整填料的局部传质(英文)

A chromochemical reactive mass transfer technique has been employed to study local mass transfer characteristics of structured packing. This technology adopted by experiment is an Ammonia Adsorption Method (AAM) that yields the surface distribution of transferred mass by analyzing the color distribution on a filter paper with the results of the color chemical reaction. A digital image processing technology is applied for data visualiza-tion. The three-dimensional plot of the local mass transfer coefficients shows that there exist three peak values on different positions of a unit cell of structured packing. In order to improve mass transfer efficiency of the structured packing, one piece of baffle is added between packing sheets. As a result, the average mass transfer coefficient in-creases by (10-20)% and the pressure drop decreases by (15-55)%.     

Setting the bar for powder flow properties in successful high speed tableting

An inadequate powder flow leads to problems in tablet manufacturing. The knowledge of minimum flow properties required for successful tableting on a high speed press is critically important to the efficient development of pharmaceutical tablets. This may be achieved by identifying a powder that exhibits minimally acceptable flow properties on a high speed tablet press. A grade of a common tablet excipient, microcrystalline cellulose (Avicel PH102) lies near the borderline between acceptable and poor flow regions during high speed tableting. It can therefore serve as a reference material for judging adequacy of flow properties of prototype formulations by the way of comparison. A powder exhibiting poorer flow properties than Avicel PH102 likely exhibits flow problems and should be avoided. An implementation of this simple approach in formulation development can minimize potential flow problems during large scale tablet manufacture.     

Visualization of the shear region in a cohesive powder by scanning with a polarized neutron beam

The flow of cohesive powder occurs by the formation of shear planes or zones. How these form and how particles microscopically behave in a shear zone is fundamental for understanding powder flow. In this work Neutron Depolarization has been used to study in-line particles in a powder sample. The Neutron Depolarization technique gives a unique insight in the particle rotations and width of the shear zone. It has been shown that rotation of particles during a normal consolidation becomes less when the sample is more compacted. Shear displacement experiments showed that particles rotate in a preferred direction. The width of the region in which the preferred rotation takes place is found to be at least 1000 to 2000 particle diameters.     

Electrostatic charging during the flow of grains from a cylinder

Electrostatic charging is ubiquitous in granular processing, leading to problems of safety, jamming and unwanted material segregation. To better understand the mechanics of granular charging, we focus here on flow through a metal cylinder, where we can isolate charging regions near the cylinder walls from noncharging regions further away. We confirm that monodisperse grains charge in proportion to the area of contact between grains and the cylinder walls, and so in large cylinders, most particles remain almost uncharged. Those particles that do charge reach a plateau charge density after filling the cylinder and flowing past the walls a distance of less than one and a half centimeters. For bidisperse granular blends, the net charge produced by the mixture is dominated by the component that comes into contact with the walls of the apparatus. This is found to be caused by segregation effects as well as the coating of the larger particles by the smaller ones. We make use of these results to predict the charge generated in mass flow hoppers, and we test these predictions. Finally we examine the effect of grounding the experimental apparatus, and we find that paradoxically, grounding does not prevent charge accumulation.     

Slow and intermediate flow of a frictional bulk powder in the Couette geometry

Most current research in the field of dry, non-aerated powder flows is directed toward rapid granular flows of large particles. Slow, frictional, dense flows of powders in the so-called quasi-static regime were also studied extensively using Soil Mechanics principles. The present paper describes the rheological behavior of powders in the “intermediate” regime lying between the slow and rapid flow regimes. Flows in this regime have direct industrial relevance. Such flows occur when powders move relative to solid walls in hoppers, bins and around inserts or are mixed in high and low shear mixers using moving paddles. A simple geometry that of a Couette device is used as a benchmark of more complicated flows.The constitutive equations derived by Schaeffer [J. Differ. Equ. 66 (1987) 19] for slow, incompressible powder flows were used in a new approach proposed by Savage [J. Fluid Mech. 377 (1998) 1] to describe flows in the intermediate regime. The theory is based on the assumption that both stress and strain-rate fluctuations are present in the powder. Using Savage's approach, we derive an expression for the average stress that reduces to the quasi-static flow limit when fluctuations go to zero while, in the limit of large fluctuations, a “liquid-like”, “viscous” character is manifested by the bulk powder.An analytical solution of the averaged equations for the specific geometry of the Couette device is presented. We calculate both the velocity profile in the powder and the shear stress in the sheared layer and compare these results to experimental data. We show that normal stresses in the sheared layer depend linearly on depth (somewhat like in a fluid) and that the shear stress in the powder is shear rate dependent. We also find that the velocity of the powder in the vicinity of a rough, moving boundary, decays exponentially so that the flow is restricted to a small area adjacent to the wall. The width of this area is of the order of 10-13 particle diameters. In the limit of very small particles, this is tantamount to a shear band-type behavior near the wall.     

Structure of powder flow in a planetary mixer during wet-mass granulation

Wet massing granulation, a widely used industrial process, is difficult to monitor and control and the structure of the flow is poorly understood. Flow patterns in a planetary mixer were investigated using positron emission particle tracking. Both dry and wet powders of a model pharmaceutical formulation were studied to develop understanding of the influence of moisture content on the flow structure during granulation. The flow structure was characterised using the distributions of the velocity components in different cross-sections of the mixer. Fourier analysis showed that the dry system is essentially dissipative and disordered whereas the wet system, being more inertial, shows signs of being more ordered with a periodic recirculation within the bowl. In both systems, radial and axial displacements are strongly correlated. For the dry system, within a central radial core region, the behaviour of the particle was determined by the rapid movement of the agitator, forming a single toroidal recycling cell. The radial and axial velocities of the tracer were up to two orders of magnitude lower than the tangential component. However, in the regions close to the wall, the particle was found to exhibit small movements dictated by the planetary rotation. For wet systems these two main regions were again observed. However, velocity field and velocity distribution showed the presence of two toroidal circulation loops, one above the other. In the wall region, the small movements governed by the planetary motion were again found, but with the amplitude of the displacements reduced by an order of magnitude.     

Study of the effects of feed frames on powder blend properties during the filling of tablet press dies

This paper studies systematically the effect of a feed frames, a device used in rotary tablet presses to drive the powders into compression dies, on the properties of the powders entering the tablet press dies. The work focused on the effect of blend composition, feed frame parameters (blade speed, residence time), and rotary die disc parameters (die disc speed, die diameter) on the flow pattern, uniformity of die filling, applied shear, and the flow properties of pharmaceutical blends. The flow pattern suggests a stratified filling of the dies and therefore, non-uniform properties of the tablets. The amount of powder entering the dies depended on blend flow properties, feed frame speed, and dies disc speed. In addition, blend properties changed significantly as the powder flowed through the feed frame. The flowability of lubricated blends improved significantly as the number of feed frame blade passes increased, decreasing in turn the RSD of the die filling weight.     

Viscosity characterization and flow simulation and visualization of polytetrafluoroethylene paste extrusion using a green and biofriendly lubricant

Polytetrafluoroethylene (PTFE) and expanded PTFE (ePTFE) are ideal for various applications. Because PTFE does not flow, even when heated above its melting point, PTFE components are fabricated using a process called paste extrusion. This process entails blending PTFE powder particles with a lubricant to form PTFE paste, which is subsequently preformed, extruded, expanded (in the case of ePTFE), and sintered. In this study, ethanol was proposed as an alternative green lubricant for PTFE processing. Not only is ethanol benign and biofriendly, it provides excellent wettability and processing benefits. Using ethanol as a lubricant, the shear viscosity of PTFE paste and its flow behavior during paste extrusion were investigated. Frequency sweeps using a parallel-plate rheometer were performed on PTFE paste samples and various grits of sandpaper were used to reduce wall slip of PTFE paste. A viscosity model was generated and a multiphysics software was used to simulate PTFE paste extrusion. The simulated extrusion pressure was compared to experimental data of actual paste extrusion. Flow visualization experiments using colored PTFE layers were conducted to reveal the flow profile of the PTFE paste. The morphology of the expanded ePTFE tubes was examined using scanning electron microscopy and the effect of expansion ratio on ePTFE morphology was quantified.