水力旋流器内固体颗粒径向运动规律的研究

本文采用二维激光多普勒测速仪,对水力旋流器内悬浮液中颗粒的径向进行了实测研究,得到实验条件下颗粒径向速度准数与操作参数间的一组准数方程。论证了径向速度与旋流器锥段、底部直流的关系和径向速度的方向。根据实验结果,可以评价颗粒分离行为的好坏,可以预测旋流器的分离效率,有助于旋流器结构参数的优化设计。     

数值研究液-液水力旋流器内颗粒浓度分布特性

根据计算流体力学(CFD)的原理和方法,应用Fluent软件对液-淡水力旋流器内部颗粒浓度分布进行数值模拟,得到了不同进口流量和不同进料浓度下旋流器内各个截面上浓度分布情况.结果表明:旋流器内各截面上浓度变化曲线呈现抛物线型;进口流量越大,在管壁处浓度值越小,油滴不断向管芯聚集;进料浓度越大,在旋流器的大部分区域中,浓...     

充气水力旋流器内颗粒的受力与运动

对充气水力旋流器内颗粒的径向受力和轴向受力进行了分析，并分别研究了亲水性颗粒与疏水性颗粒的运动行为，为深入认识充气水力旋流器内颗粒的分选过程行为提供了重要的理论依据。     

水力旋流器固相颗粒径向速度研究

利用粒子动态分析仪(PDA)对水力旋流器内固相颗粒的径向速度进行了实测研究,得出了固粒径向速度的经验公式,并从理论上分析了固相颗粒的径向速度。本研究还探讨了固相颗粒径向速度对分离性能的影响,给出了一个描述水力旋流器内固粒径向速度与分离效率间的数学关系式。     

水力旋流器内流体流动的湍流数值模拟研究进展

对水力旋流器内流体流动的湍流数值模拟研究进行了综合介绍,并就各种水力旋流器的湍流模型进行了分析和评述,提出了水力旋流器的湍流数值模拟研究的新方向。     

水力旋流器湍流数值模拟及湍流结构

采用湍流代数应力模型对水力旋流器内湍流场进行了数值模拟,数值计算结果验证了实测数据。用数值计算与实验研究相结合的方法深入揭示了旋流器内的湍流结构,结果表明,旋流器内湍动能分布呈两边高中间低的不对称鞍形;湍动勇耗散率分布与湍动能的分布有十分相似的规律;溢流管端以下内旋流区域中湍流压力脉动强度以及压力相对脉动强度均很大。     

水力旋流器内固相颗粒时均流场及脉动特性的研究

利用一种新型的激光多普勒测速仪———粒子动态分析仪（ＰＤＡ），对水力旋流器中固相颗粒的时均流速、绝对湍流度、相对湍流度等流动参数进行了实测研究，给出了更全面的关于水力旋流器内固相颗粒流速分布的信息，并探讨了湍流脉动对水力旋流器分离过程的影响。     

水力旋流器内固体颗粒的沉降

水力旋流器内固体颗粒的离心沉降是一种复杂的两相流行为。从分析固液体系的相互作用，分别讨论了颗粒的自由沉降、干涉沉降问题，提出了高浓度下水力旋流器内颗粒之间的几种碰撞模式，阐述了旋流器内颗粒沉降的特殊性及其与使流器工作和旋流器结构间的可能联系，并指出了离心沉降本身有待深入研究的若干问题。     

水力旋流器湍流场数值模拟

<正> 引言 水力旋流器作为一种简便、易行和高效率的分离、分级和离心沉降设备,已被广泛应用于化工、冶金、石油等众多工业领域中.以往的水力旋流器设计主要是根据大量物理模型试验得出的经验准数方程来求出旋流器的几何结构参数和操作参数.然而,随着水力旋流器应用范围的迅速扩大和人们对其分离(级)性能指标的要求日益提高,传统的按经验或半经验公式进行旋流器设计方法的局限性越来越明显,以及物模试验的耗时费钱,已促使人们开始采用数值模拟的方法,通过对旋流器内部流体运动的深入研究,弄清旋流器的分离机理,以便为提高水力旋流器的分离效率和分级准确度予以理论指导.本文采用了适于水力旋流器液相(水)流场的K-ε湍流数学模型,对水力旋流器内的湍流运动规律进行了数值模拟并根据激光实测结果对部分模型常数进行了修正.     

Sub-micron particle dewatering using hydrocyclones

Hydrocyclones are used for dewatering of solid–liquid suspensions in many industries. Generally, however, large diameter cyclones are used and their application is restricted to large (>25 μm) particles. Small diameter (10 mm) hydrocyclones have the potential to be applied to fine particle (<10 μm) suspensions and, in particular, to collect the sub-micron fraction. This is due to the very small cutsizes that are achieved in these cyclones. In order to apply these small hydrocyclones industrially, knowledge of the range of their classification performance is required. It is found that these cyclones exhibit a fish-hook partition curve, and a high bypass fraction. The very small cutsize (<5 μm) and the relatively large bypass makes the effective collection of sub-micron particles possible. While in most hydrocyclone applications it is found that the bypass fraction equals the water recovery to the underflow, in 10 mm hydrocyclones the bypass fraction is considerably larger than the water recovery. This results in a high particle recovery to the underflow, as well as low water recovery, resulting in a high concentration ratio. Results will be presented to show the separation performance of different hydrocyclone outlet configurations and pressure drops. A general model will be presented that describes the fish-hook and that gives an explanation for its origin. It will be shown that 10 mm hydrocyclones yield a new operating regime for their application to sub-micron solid–liquid separation, as a result of high solids recoveries and low water recoveries.     

Modeling of particle deposition in a vertical turbulent pipe flow at a reduced probability of particle sticking to the wall

Particle deposition on the wall in a dilute turbulent vertical pipe flow is modeled. The different mechanisms of particle transport to the wall are considered, i.e., Brownian motion, turbulent diffusion and turbophoresis. The Saffman lift force, the electrostatic force, the virtual mass effect and wall surface roughness are taken into account in the model developed. A boundary condition that accounts for the probability of particle sticking to the wall is suggested. An analytical solution for deposition of small Brownian particles is obtained. A particle relaxation time range, where the model developed is reliably applicable, is evaluated. Computational results obtained at different particle-wall sticking probabilities in the wide particle relaxation time range are presented and discussed.     

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.     

Part I: Dynamic evolution of the particle size distribution in particulate processes undergoing combined particle growth and aggregation

The present study provides a comprehensive investigation on the numerical problems arising in the solution of dynamic population balance equations (PBEs) for particulate processes undergoing simultaneous particle growth and aggregation. The general PBE was numerically solved in both the continuous and its equivalent discrete form using the orthogonal collocation on finite elements (OCFE) and the discretized PBE method (DPBE), respectively. A detailed investigation on the effect of different particle growth rate functions on the calculated PSD was carried out over a wide range of variation of dimensionless aggregation and growth times. The performance (i.e., accuracy and stability) of the employed numerical methods was assessed by a direct comparison of predicted PSDs or/and their respective moments to available analytical solutions. It was found that the OCFE method was in general more accurate than the discretized PBE method but was susceptible to numerical instabilities. On the other hand, for growth dominated systems, the discretized PBE method was very robust but suffered from poor accuracy. For both methods, discretization of the volume domain was found to affect significantly the performance of the numerical solution. The optimal discretization of the volume domain was closely related with the satisfactory resolution of the time-varying PSD. Finally, it was shown that, in specific cases, further improvement of the numerical results could be obtained with the addition of an artificial diffusion term or the use of a moment-weighting method to correct the calculated PSD.     

Spherical particle movement in dilute pneumatic conveying

A numerical simulation of a particle in a horizontal pipe has been carried out, and the variation of aerodynamic forces is described. The major forces that control particle motion are drag in the axial direction, and lift due to air velocity gradient and due to spin in the transverse direction. An elastic contact model based on rigid body sliding has been incorporated, which avoids particle settlement without having to use any form of irregular bounce. The results from the simulation agree closely with experimental time-of-flight measurements.     

Flow patterns in conical and cylindrical hydrocyclones

Using a two beam, 300 mW laser Doppler velocimeter, the tangential and axial velocity fields were determined for the water flow in a 102-mm modular hydrocyclone. The body of the equipment could be changed to transform it into a conical or a flat bottom hydrocyclone. During the tests the pressure drop and the diameter of apex and vortex were varied and the axial and the tangential velocities and their turbulence intensity were measured. The results shows that the inlet pressure affects only the magnitude of the velocities, but does not change the flow pattern. The tangential velocity is similar in both types of hydrocylones while the axial velocity is different. In both hydrocyclones the axial velocity is a function of the radial position but, while it is a linear function of the vertical coordinate in the cylindrical hydrocyclone, this is not the case for the conical vessel.     

Simulation of particle movement in a pan coating device using discrete element modeling and its comparison with video-imaging experiments

A MATLAB™-based DEM (discrete element method) code was developed to study the particle motion in a pan coating device. The code was developed as a stand-alone program and provides simulation, visualization (GUI interface), and post-processing statistical analysis of particle movement. Results of DEM simulations are compared with those obtained from video-imaging experiments for 9 mm spherical polystyrene balls in a 58 cm diameter pan. The parameters compared are dynamic angle of repose and particle velocities in the x- (axial) and y- (parallel to cascading layer) directions within the spray zone in the cascading layer. The effects of pan loading and pan speed (6, 9, 12 rpm) on particle motion are compared and discussed. Good agreement was obtained between the DEM simulation and experimental results. The dynamic angle was found to increase with increasing pan speed and pan loading. The average cascading velocity was found to increase linearly with pan speed for both DEM and experiments. The velocity distributions both in the x- and y-direction were compared from simulation and experiments and found to be in good agreement. Velocity profiles along the entire top cascading layer of particles were also obtained using DEM. The particles in the cascading layer were found to reach their maximum velocity at positions close to the mid-point of the cascading surface. Comparison of simulated velocity profiles showed good agreement with published scaling laws for rotating drums, and a new correlation for scaling with respect to the pan loading is proposed.     

A particle analyser for measuring particle size distribution and shape,on-line or in the laboratory

Norsk Hydro has developed a Particle Analyser for on-line or laboratory use, which measures particle size, size distribution and the deviation from sphericity (called nonspherical).The principle for this system is that the particles fall in a monolayer curtain in front of a high resolution CCD camera. The computer unit in the analyser measures and calculates the particle size and a sphericity factor for each particle. The data are presented as four real time trend curves, shown simultaneously for the on-line version. These curves show % oversize and fines, d₅₀ and the percentage nonspherical particles. For the laboratory version the data are presented in table and cumulative form.The on-line particle analyser has been installed in two of Norsk Hydro's prilling plants in Norway. The analyser has in both plants improved the product quality during the two years of installation.     

Experimental study of particle trajectory in electrostatics powder coating process

Experimental studies of particle trajectory were conducted in an isolated Plexiglass coating booth. Polyester particles were injected with a Norsdon® electrostatics spray gun with a fixed distance from the gun to a grounded plate. Using a Dantec® Particle Dynamic Analyzer, the particle velocity and size distribution were measured simultaneously. The experimental results showed that the effect of electrostatics charging on the particle trajectory is strong in the close vicinity of the target, but can be neglected at locations away from the target. The influence on particle size and velocity profile due to the electric field between the charged particles and grounded target was weakened as more particles deposited on the target. When no charge is introduced on the coating powder, particle segregation is observed for particles larger than 100 μm. Particle gravitational settlement is noticed even near the gun tip. However, particle charging largely eliminates the segregation at a gun-to-target distance of 25 cm and helps break agglomerates formed in the spray system. The gun-to-target velocities of larger particles exhibited noticeable deviation from those of the flow field as the grounded target was approached. The study revealed that the onset of electrostatic coating is an important period that can affect the transfer efficiency and film thickness.     

Challenges in particle size distribution measurement past, present and for the 21st century

The field of particle size distribution (PSD) characterization and measurement has experienced a renaissance over the past ten years. This revitalization has been driven by advances in electronics, computer technology and sensor technology in conjunction with the market pull for PSD methods embodied in cost effective user friendly instrumentation. The renaissance can be characterized by at least four activities. (1) End user innovation exemplified by techniques such as hydrodynamic chromatography (HDC), capillary hydrodynamic fractionation (CHDF) and field flow fractionation methods (SdFFF, FlFFF, and ThFFF). (2) Revitalization of older instrumental methods such as gravitational and centrifugal sedimentation; (3) Evolution of research grade instrumentation into low cost, routine, user friendly instrumentation exemplified by dynamic light scattering (DLS). (4) The attempt to meet extremely difficult technical challenges such as: (a) providing a single hybrid instrument with high resolution over a very broad dynamic range (4+ decades in size; e.g., Fraunhofer/Mie; photozone sensing/DLS); (b) PSD measurement of concentrated dispersions (acoustophoretic, dielectric measurements, fiber optic DLS (FOQELS)); (c) in-situ process particle size sensors (in-line or at line, e.g., FOQELS); (d) routine measurement of particle shape and structure (e.g., image analysis). Instrumental methods resulting from these activities are discussed in terms of measurement principles and the strengths and weaknesses of these methods for characterizing PSDs. Business and societal driving forces will impact customer perceived instrumentation and knowledge needs for the 21st century and the ability to meet the specific difficult technical challenges in particle size distribution characterization mentioned above. Anticipated progress toward meeting these technical challenges is discussed in conjunction with the associated anticipated advances in required technologies.