Effect of multi-inlet flow on particle classification performance of hydro-cyclones

The effect of multi-inlet flow on particle classification performance of hydro-cyclones was examined experimentally and via a simulation study. Cut size of the type A cyclone with two inlets flow indicated smaller cut size and sharp separation performance compared to the standard cyclone. Numerical simulation indicated a nearly uniform rotational fluid velocity profile for the type A cyclone. On the other hand, the standard cyclone showed a non-uniform rotational velocity profile near the inlet part of the main flow. The type B cyclone with a small additional flow injection area, showed smaller cut sizes as the flow rate of the additional flow increased. The type B cyclone showed smaller cut size compared to the standard cyclone without the additional flow. The use of a multi-inlet cyclone is quite effective at improving particle separation performance compared to the standard one.     

基于颗粒轨迹分析的分级机切割粒径计算

为研究超细分级机的切割粒径,采用计算流体力学技术对分级机气固两相流进行了数值模拟。计算中气相采用RNG k-ε湍流模型,颗粒相采用随机轨道模型。通过分析颗粒轨迹与切割粒径的关系,揭示了颗粒在分级机内运动的物理机制;通过分析切割粒径随转子转速、风量、喂料浓度和物料密度的变化规律,阐述了各参数对切割粒径的影响。结果表明：切割粒径的理论推算中,忽略叶片厚度的影响将导致计算值偏小;低转速（450 r/min,600 r/min）时,受局部涡流的影响,切割粒径模拟值与理论计算值相差较大,最大误差为13.58%;与风量相比,转速对切割粒径的影响更为显著。模拟结果与理论计算值吻合较好,为求取分级机的切割粒径提供了一种新方法。     

Effect of inner structure of centrifugal separator on particle classification performance

This study investigated the effects of the inner structure of a centrifugal separator on particle classification performance. The typical inner structure of centrifugal separators is as follows: a blade, which consists of two orthogonal plates, is inserted into the centrifugal separator to create rigid fluid and particle rotations. The results of the present study demonstrate that centrifugal separator performance was significantly improved by attachment of a cylinder to the center of a conventional blade. Modification of the separator by attachment of the cylinder to the center of the centrifugal separator is expected to prevent the particles in the slurry from passing through the central axis of the centrifugal separator. Hence, the particles are subjected to greater centrifugal force. Both experimental and theoretical results demonstrate that particle classification performance increases as the cylinder radius increases. Thus, it is evident that attachment of a cylindrical blade to the center of the centrifugal separator allows for effective and highly efficient collection of extremely small-classified fine particles.     

Effect of ball size and powder loading on the milling efficiency of a laboratory-scale wet ball mill

Alumina powder was wet-milled by zirconia balls with varying diameter at varying rotation speed, and the resultant particle size of the milled powder was analyzed. At a given rotation speed, there exists an optimum ball size to yield minimum particle size of alumina. The optimum ball diameter decreases as the rotation speed increases. This result has been interpreted in light of the competition between the reduced kinetic energy of the smaller balls (a negative source for milling efficiency) and the increased number of contact points of the smaller balls (a positive source), which yields the optimum ball diameter at an intermediate size. As the rotation speed increases, kinetic energy of the balls increases, which, in turn, shifts the optimum ball size toward a smaller value. As the powder loading increases from 1 to 35 g at a given rotation speed and ball size, the milling efficiency decreases monotonically.     

涡流分级机撒料盘的试验研究

研究了涡流分级机撒料盘上凸棱高度对分级精度和分割粒径的影响。试验得出,随着径向凸棱高度的增加,物料分散变差,分割粒径ｄ５０减小,分级精度指数降低。在试验范围内,当撒料盘凸棱高度与内径之比为００３７时,分级精度指数最高。     

Classification performance of model coal mill classifiers with swirling and non-swirling inlets

The classification performance of model coal mill classifiers with different bottom incoming flow inlets was experimentally and numerically studied. The flow field adjacent to two neighboring impeller blades was measured using the particle image velocimetry technique. The results showed that the flow field adjacent to two neighboring blades with the swirling inlet was significantly different from that with the non-swirling inlet.With the swirling inlet, there was a vortex located between two neighboring blades, while with the nonswirling inlet, the vortex was attached to the blade tip. The vorticity of the vortex with the non-swirling inlet was much lower than that with the swirling inlet. The classifier with the non-swirling inlet demonstrated a larger cut size than that with the swirling inlet when the impeller was stationary(～0 r·min~(-1)). As the impeller rotational speed increased, the cut size of the cases with non-swirling and swirling inlets both decreased, and the one with the non-swirling inlet decreased more dramatically. The values of the cut size of the two classifiers were close to each other at a high impeller rotational speed(≥120 r·min~(-1)). The overall separation efficiency of the classifier with the non-swirling inlet was lower than that with the swirling inlet, and monotonically increased as the impeller rotational speed increased. With the swirling inlet, the overall separation efficiency first increased with the impeller rotational speed and then decreased when the rotational speed was above 120 r·min~(-1), and the variation trend of the separation efficiency was more moderate. As the initial particle concentration increased, the cut sizes of both swirling and non-swirling inlet cases decreased first and then barely changed. At a low initial particle concentration(b 0.04 kg·m~(-3)), the classifier with the swirling inlet had a larger cut size than that with the non-swirling inlet.     

涡流分级机转笼结构改进及内部流场数值模拟

引言 随着科技的进步,各行各业对粉体粒径和粒径分布的要求向细而窄的方向发展,因此对分级设备提出了高效、高精度的性能要求.涡流空气分级机是20世纪70年代末日本发明的,属于第三代动态干式分级机,具有能耗低、流场稳定等特点,广泛应用于建材、精细化工、食品、医药以及矿物加工等领域.     

卧螺离心机内盐析流场的CFD-PBM耦合计算

目前对卧螺离心机内部流场的数值模拟大多采用传统欧拉模型,考虑其内部伴有盐析等固相粒子微观行为过程的研究少有报道。本文应用计算流体力学软件Fluent,基于群体平衡模型与多相流Eulerian模型、RNG k-ε模型耦合方法,对卧螺离心机内部盐析两相流场进行三维数值模拟,通过仿真与实验相结合的方式,得到卧螺离心机内盐析流场的晶体颗粒粒径分布、组分数分布和浓度分布及其变化规律,初步揭示了卧螺离心机内伴有盐析的流场晶体颗粒分布特性。研究结果表明：卧螺离心机内盐析晶体粒径的分布随液环半径的增大而增大,螺旋叶片正壁面粒径明显较背面大,从排液端至排渣端的晶体粒径分布有整体逐渐变大的轴向粒径梯度;流道内晶体粒径随转鼓转速的增大而减小,随进口固相体积分数的增大而增大;大粒径晶体的组分数在液环外侧及螺旋叶片正面附近相对较大,而中、小粒径晶体组分数的分布规律则与之相反;盐析颗粒浓度在液环外侧较高且分布均匀,且随转鼓转速的提高而提高,PBM模型与Euler模型计算结果有一定的相似与差异。     

Effect of apex cone shape on fine particle classification of gas-cyclone

The purpose of this paper is to study the effects of apex cone shape on particle separation performance of gas-cyclones by experiment and CFD studies. It is found that the optimum apex cone angle is 70°. The minimum 50% cut size was obtained by use of this special apex cone. From the flow visualization method by use of soap foam, the upward flow and downward flow coexisted on the surface of this special apex cone. The clear interface between upward flow and downward flow was detected on the apex cone angle of 70°. The effect of the apex cone angle on particle separation performance decreases under high inlet velocity conditions, because most particles are moving in the area away from the apex cone.The particle separation performance and flow visualization results qualitatively supported the 3-dimensional CFD simulation based on the direct method.     

导风叶片对涡流空气分级机内流场的影响

利用ANSYS-Fluent 17.0软件对有、无导风叶片两种结构的涡流空气分级机内流场进行数值模拟和对比分析,研究了导风叶片对涡流空气分级机内流场的影响。数值模拟结果表明：导风叶片降低了转笼外缘处气流切向速度,从而影响转笼通道内旋涡的分布情况,使有、无导风叶片两种结构的稳定工况不同;导风叶片减小了转笼外缘处气流径向速度的波动和湍流耗散率,此处流场分布相对均匀,有利于提高分级精度;此外,导风叶片在导流过程中,改变了环形区速度场的分布,气流切向速度减小,径向速度增大,径向速度的增大使其分级粒径增大。碳酸钙物料分级实验结果表明：具有导风叶片结构的涡流空气分级机分级粒径较大,分级精度较高;导风叶片处较大的湍流耗散率有助于粉体分散,明显减弱“鱼钩效应”现象。     

涡流空气分级机转笼结构对其分级性能的影响

摘要：研究了涡流空气分级机底盘开口与封闭两种型式的转笼对分级指标的影响。重质CaCO₃物料试验表明：采用底盘开口转笼时,切割粒径小,分级精度低;随着分级转速增加,切割粒径对风速的敏感性下降。另外,用激光多普勒测速计测量了上述两种转笼结构的分级机环形区的流场特性,结果表明：转笼底盘开口,环形区气流出现旁路,进入转笼径向风速减小,造成分级物料切割粒径减小;底盘封闭的分级机环形区内靠近转笼处,切向风速突变增大,特别是轴向上湍流度的增大,有利于团聚物料的分散和分级精度的提高。     

Separation performance of sub-micron silica particles by electrical hydrocyclone

Separation performance of sub-micron particles by use of a special electrical hydrocyclone was studied. The effects of feed suspension waiting time, applied electrostatic potentials, and the feed suspension concentration, on the separation performance of the electrical hydrocyclone were investigated. An aqueous suspension of sub-micron silica particles with median diameter of about 0.2 µm was used as the test powder. A 20 mm diameter of electrical hydrocyclone operated at 20% of the underflow ratio was used. A negative center wire electrode was inserted vertically inside the conical section, and electrostatic potentials up to 100 V were applied in this electrical hydrocyclone.It was found that the 50% cut size of the electrical hydrocyclone increased with the increase of the feed suspension waiting time after a particle dispersion process by the beads mill. The classification of the sub-micron particles occurred under applied electrostatic potentials greater than about 40 V, while better classification performance was obtained with the increase of the applied electrostatic potentials. The 50% cut size decreased with the increase of the feed suspension concentration up to 1.5 wt.%, and further increasing of the concentration led to the increase of 50% cut size.A simple model based on the time of flight model, was developed in order to predict the 50% cut size of the electrical hydrocyclone. The model results qualitatively agreed with the experimental results.It was found that classification the sub-micron particles is possible by use of the special electrical hydrocyclone proposed in this study.     

Effects of the impeller blade geometry on the performance of a turbo pneumatic separator

Abstract

This paper mainly presents an analysis of the effects of the impeller blade geometry on the performance of a turbo pneumatic separator based on the CFD simulations by using Fluent 14.5.0. The simulation results indicate that the air vortex can be eliminated by using the positive bending arc blades with a remarkably small impacting angle. Furthermore, the optimum installation angle range for the positive bending arc blades was studied. The material classification experiment results show that for the positive bending arc blades with the optimum installation angles, the fishhook depth is decreased, the classification accuracy is increased, and the particle size distribution of fine powders is narrowed, especially under the design condition, compared to the straight blades. This suggestion of impeller blade geometry for the turbo pneumatic separator is the important basis for designing this type of separators and improving its classification performances.     

偏心搅拌槽内宏观不稳定性的分离涡模拟

为研究偏心搅拌槽内的宏观不稳定性,基于分离涡模型建立了偏心搅拌数值模拟方法。采用滑移网格法模拟搅拌桨的旋转,实现了偏心搅拌流场的非稳态模拟。对不同工况时的宏观不稳定性进行了频谱分析,讨论了偏心率、雷诺数和桨径比对宏观不稳定频率的影响,并进行了PIV实验验证。研究表明,偏心搅拌槽内存在明显的宏观不稳定现象,其变化周期约为桨叶通过周期的10倍;PIV实验结果与分离涡模拟结果吻合较好,都表明偏心搅拌槽内的宏观不稳定频率随雷诺数的升高而增大,随偏心率的增大而减小,随桨径比的增大而增大。     

Particle Classification by Multishaft Disk Separator

Particle classification by sieving filtration using rotating disk filters mounted on single and double hollow shafts was investigated. The shaft and disks were enclosed in a stainless steel housing. Parameters such as particle concentration, feed flow rate and rotation speed of the filter disk were investigated to study their influence on the filtrate flux rate and particle classification. The filtrate flux rate was not affected by increasing the particle concentration. The analyzed particle size distributions were found to be narrow when the particle concentration increased. Increasing the rotation speed of the shaft produced a narrow particle size distribution. Increasing the feed flow rate (enhanced filtration pressure difference) resulted in a wider particle size distribution, but beyond a certain point a cake will form.     

接枝基底胶乳对冰箱板材用ABS树脂性能的影响因素

讨论了接枝基底胶乳包括凝胶含量、粒径大小及分布对冰箱板材用ABS树脂力学性能和加工性能的影响,为特殊品级的ABS树脂开发提供研究基础。     

颗粒在涡轮式分级机分级轮中的运动轨迹

通过引入一个简化的单颗粒动力学模型,对颗粒的运动轨迹进行了模拟.该模型假设颗粒在分级轮中具有二力平衡状态,即颗粒的运动状态主要取决于流体黏滞力与离心力的作用.为了便于求解,将方程转化为等速旋转坐标系中的方程,通过求解方程并绘制图形,求得颗粒在叶片间的运动轨迹.对不同转速、风量、叶片间距及不同叶片倾斜角度下的颗粒运动轨迹进行了详细研究,计算结果表明：转速、风量、叶片间距和叶片角度是影响颗粒运动轨迹的主要因素;转速的增加和风量的减小均可以显著减小分级粒径的大小;叶片间距的减小使颗粒与叶片的碰撞次数增多;在相同条件下,负角度叶片的分级粒径小于径向叶片,径向叶片的分级粒径小于正角度叶片.     

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.     

Small Electrocyclone Performance

Cyclone Separators and Electrostatic Precipitators (ESPs) are both effective particle separators. The former are more efficient at removing the larger particles, while the latter more suited to removing the smaller size classes. We explore the performance of an “Electrocyclone”, constructed by simply retrofitting an electrode coaxially to a small existing “Whitby” cyclone. Tests were performed with respect to particle size, resitivity, loading and various other operating parameters. Non‐electrical separation efficiencies ranged from 71 to 75 % and with the application of additional electrical forces the increase in separation efficiency was between 17 and 21 % at a cyclone Reynolds number of 19000, with the most conductive particle most easily separated. Further parametric testing correlated the effects of dust loading, electrocyclone Reynolds Number and particle cut upon separation efficiency. In particular we show that the separation of the smallest size cuts (D < 38 μm) of the dust sample almost doubled upon application of the corona. We conclude, based on this initial study of small devices, the range of use of cyclones may be extended significantly by the application of additional electrophoretic separation.     

Ultra-fine silk powder preparation through rotary and ball milling

Three commercial silk varieties, namely mulberry, muga and eri, were used to prepare ultra-fine silk particles. Degummed silk fibres were chopped into short snippets and then pulverised using rotary and planetary ball milling. The effects of degree of degumming, size of milling media, water and lubricant on particle refinement were studied. Before milling, single fibre strength tests were conducted on silk fibres degummed under different conditions. The results indicate that while reducing fibre strength via harsh degumming could cut milling time drastically, too severe a reduction in fibre strength is actually detrimental to achievable minimum particle size due to increased particle aggregation. Water played an important role in affecting the performance of ball milling. With the milling processes used in this study, a volume based median particle size (d(0.5)) of around 200 nm was achieved, which is much smaller than previously reported results. To achieve a similar particle size, mulberry silk required more milling time, even though the overall milling behaviour was quite similar for the three silk varieties examined. SEM observations revealed axial spitting and fragmentation of micro and nanofibrillar architecture of silk fibres due to milling. Unlike ball milling, which produced particles with variable shapes, rotary milled particles remained fibrous through the size reduction process.