Experimental and CFD study on a cyclonic classifier with new flow pattern

Physical principle of conventional top-inlet classifier (CTC) with reverse-flow pattern leads to the heavily fine particles entrainment in coarse fraction. Present work concentrates on the flow-field design for less downward airflow at near-wall region of the classifier. A new middle-inlet classifier (NMC) is proposed and analyzed using computational fluid dynamics (CFD) method and powder classification experiments. The results showed that new flow pattern characterized by a pair of vortexes was created in the new classifier. The upper vortex with 80% of the total air volume moves upward and forms the washing effect at near-wall region, which effectively reduces the fine particles entrainment in coarse fraction. The downer vortex with reverse-flow pattern discharges the coarse particles timely. The radial centrifugal sedimentation combined with the axial counter-current washing effect dominates the particle classification in the NMC. Compared to the CTC, classification accuracy index of the NMC with double-vortex averagely increases by 27% with a pressure drop reduction of more than 38%. This work offers a new principle for high-efficiency particle classification and new strategy for improving the classification performance of turbo air classifiers and hydrocyclones.     

Geometrical configuration of hydrocyclone for improving the separation performance

The inherent defect of particle misplacement in traditional hydrocyclones is the main reason for the deterioration of separation accuracy and has obtained wide attentions. This paper presents three novel hydrocyclones based on the traditional cylindrical-conical type and cylindrical type. The effect of the hydrocyclone structure on the flow field characteristics and separation performance is investigated by a validated two fluid model. The numerical results show that a higher turbulence intensity deteriorates the separation performance of CCB type (cylindrical-conical-flat bottom type) and C type (cylindrical type), while a greater tangential velocity and velocity gradient improve the separation accuracy of MS type (multi-stage cylindrical type). Hydrocyclones with a flat-bottom structure reduces the misplaced fine particles due to the effect of the internal swirling flow and the axial circulation flow in the spigot. The MS type has the lowest imperfection value and the cut size of 76.5 μm implying a sharpness separation sharpness and an appropriate separation result. CC type has the minimum cut size of 62.21 μm, while the maximum cut size of 120.68 μm for C type.     

Role of tool pin profiles on wear characteristics of friction stir processed magnesium alloy ZK60/silicon carbide surface composites

Poor friction and wear resistance are the major drawbacks that restrict structural applications of ZK60 magnesium alloys. The surface properties of magnesium alloy can be enhanced by reinforcing particles in the surface using friction stir processing (FSP). Tool pin profile is the significant process parameters which influences the material flow, particle breakups and its distribution in the processed zone. In this study, an attempt was made in order to understand the major effects of tool pin profiles namely, cylindrical thread (CT), plain cylindrical (PC), plain tapered cylindrical (PTC) and square (SA) pin profiles on the microstructure characteristics and particle distribution of friction stir processed/silicon carbide particle surface composites. The surface composites fabricated by plain tapered cylindrical pin profile yield superior properties which is attributed to the higher shear force and balanced state of material flow and heat generation in the processed zone. The formation of smaller grains and hardness enhancement due to dispersion strengthening are the main causes to get better wear resistance of friction stir processed/silicon carbide particle magnesium alloy.     

Performance evaluation of a hydrocyclone with a spiral rib for separation of particles

Several design modifications have been done to improve particle separation efficiency in a hydrocyclone. The effects of a rib which is introduced into the cylindrical part of the hydrocyclone are discussed here. CFD (Computational fluid dynamics) is a useful tool to study the velocity and pressure distribution of complex turbulent flow in a hydrocyclone. Flow simulations are carried out using a three-dimensional double precision, segregated, steady-state solver tool. Reynolds stress model is employed for turbulent model which is suitable for the anisotropic turbulent flow. A comparison study for pressure drop and flow velocity for the conventional and ribbed hydrocyclone have done. The obtained CFD simulated results in correlation with experimental data shows that the pressure drop reduces by 13.9% at a velocity of 2.5 m/s by using rib. An experimental finding shows that the cut size particle diameter for conventional and ribbed hydrocyclone are 36 µm and 28 µm respectively at the velocity of 2.5 m/s.     

Simulation Studies of A 76MM Hydrocyclone

The investigation pertains to establishing a simulation methodology for understanding the separation characteristics of a typical hydrocyclone where the work was carried out using a commercially available CFD software. The studies included water flow profiles, water throughput {\&} product split, particle distribution etc. and the simulated results are further validated with suitably performed experiments. The work essentially highlights the performance of the hydrocyclone using numerical studies where water is used as a primary phase and solid particles as secondary ones. This methodology is expected to be useful in the design of hydrocyclones and optimizing the processes.     

Kinematics of Flow of a Slow Suspended-Particle Flux about a Sphere as Applied to the Sedimentation of a Bidisperse Suspension

A study is made of the kinematics of flow of fine particles about a coarse one in the process of sedimentation of the fine particles. An approximate analytical solution for the increase in the settling rate of fine particles, which is in good agreement with the approximation dependence obtained earlier as a result of numerical experiment, has been found. It has been shown that a relatively weak entrainment of most of the fine particles arriving at the peripheral region of the cell around the coarse particle is of primary importance for the effect of acceleration of sedimentation. A comparison with the available experimental data is made.     

Comparison of separation performance between single and two inlets hydrocyclones

Experimental studies have been carried out about the particle separation performance of single and two inlets hydrocyclones. Two types of hydrocyclones having the same total inlet holes area were examined. Hydrocyclone A (with single inlet hole) and hydrocyclone B (with two inlet holes) were used. An aqueous slurry of silica particles (0.5 wt% with a median diameter of about 2.1 μm) was tested using a 20 mm-diameter hydrocyclone with a 15% underflow ratio. It was found that the optimum flow rate which gives the highest separation performance of the hydrocyclone B is the same flow rate for each inlet. Usage of the hydrocyclone B indicated smaller cut size and higher particle collection efficiency compared to that of the hydrocyclone A, under the same flow rate or same pressure drop conditions. The time of flight model was used to predict the 50% cut size diameter for single inlet hydrocyclone, and the model was modified to the two inlets hydrocyclone. It was found that the calculated results agree well with the experimental results for the hydrocyclone A, and for the hydrocyclone B under the low total flow rates conditions.     

Pinched flow fractionation: continuous size separation of particles utilizing a laminar flow profile in a pinched microchannel

A concept of "pinched flow fractionation" for the continuous size separation and analysis of particles in microfabricated devices has been proposed and demonstrated. In this method, particles suspended in liquid were continuously introduced into a microchannel having a pinched segment and were aligned to one sidewall in the pinched segment by another liquid flow without particles. The particles were then separated perpendicularly to the flow direction according to their sizes by the spreading flow profile inside the microchannel. Polymer microbeads were successfully separated, and the effects of the flow rate and channel shapes on the separation performance were examined. Also, separated particles were collected independently by making branches at the end of the pinched segment. Since this method utilizes only the laminar flow profile inside a microchannel, complicated outer field control could be eliminated, which is usually required for other kinds of particle separation methods such as field flow fractionation. Also, this method can be applied both for particle size analysis and for preparation of monodispersed particles, since separation can be rapidly and continuously performed.     

Design of low cost instrumentation to measure flow andconcentration of a copper metallurgical slurry

An instrumentation system with a special pipe inserted in the circuit section that measures the particle sizes in the hydrocyclone on an ore grinding classification system was designed. The instrumentation developed uses a set of ultrasonic transducers which detect either shifts of frequency to measure the flow or attenuation of the signal to measure the concentration of a solid in a copper metallurgical slurry. Real-time measurements of flow and solid concentration make possible the control of the size of the ore particles which go into a flotation cell. In this cell optimal particle size gives the maximum amount of mineral when the particles react with chemicals, thereby improving the yield of metal. The system offers not only a lower cost but also an accuracy comparable to that of instrumentation which measures only one of the two variables     

Particle Separation in an Annular Converging Channel with an Inner Rotating Permeable Baffle

The relation between the separation efficiency of solid particles and the stability of the helical flow of a viscous fluid in a converging channel with an inner rotating permeable cylindrical baffle has been studied. The profiles of the axial and tangential velocities and the separation efficiency of solid particles have been calculated based on the numerical solution of a system of equations describing the hydrodynamics of two-phase media. Analysis of the obtained solutions shows that vortices having an effect on particle separation can appear in the converging channel. Moreover, the larger the size of the converging annular channel, the earlier a loss of stability occurs. It has been found that the formation of vortices is impossible for some flow regimes and, as a result of fluid flow stabilization, the fraction of particles settled on the permeable cylindrical baffle decreases. It has been shown that those regime parameters at which a helical flow exists should be selected for the development of combined action units involving filtering and the separation of the solid dispersed phase.     

Effect of a cylindrical shroud on particle conditions in high velocity oxy-fuel spray process

Simulations of solid particles in a highly compressible gas flow in the high velocity oxy-fuel (HVOF) process are presented. The Eulerian formulation is used for the gas flow, and the particl phase is modeled by the Lagrangian method. Effects of attaching a cylindrical shroud to the end of the supersonic HVOF nozzle on gas and particle flows are analyzed. We found that the shroud significantly reduces the oxygen content in the field by protecting the supersonic jet from ambient air entrainment. The validation experiments were performed for the majority of process parameters such as shock formation, particle conditions, and coating oxygen content.     

An experimental study on a horizontal-vertical pneumatic conveying system using oscillatory flow

To reduce the power consumption of a horizontal-vertical pneumatic conveying system, an oscillator is mounted with a 45° oblique plane through the pipe axis in this study. This experimental study focuses on the effect of oscillatory flow using the oscillator on the horizontal-vertical pneumatic conveying system in terms of the overall pressure drop of the system, power consumption, local pressure drop, and particle velocity. Compared with conventional pneumatic conveying (axial-flow), the pressure drop and power consumption can be reduced using the oscillatory flow in a lower air velocity range. Meanwhile, the particle axial velocity of the oscillatory flow is higher than that of the axial-flow near the bottom of pipe. This outcome indicates that the accelerating effect of oscillatory flow is obvious near the bottom of the pipe, and the particle vertical velocity of the oscillatory flow is positive, whereas the particle vertical velocity of the axial-flow is almost negative. This result shows that the particles of the oscillatory flow are suspended sufficiently, but the particles of the axial-flow have a tendency of deposition. Furthermore, the fluctuation intensity of the particle velocity of the oscillatory flow is higher than that of the axial-flow, especially near the bottom of the pipe.     

IMPROVEMENTS IN THE CLASSIFICATION EFFICIENCY OF A HYDROCYCLONE WITH AN IMPELLER INSTALLATION AROUND THE VORTEX-FINDER

Study of flow mechanisms in hydrocyclones shows that about10 to 15 percent of the particles entering the overflow through a leakage stream are not subject to the full centrifugal force. Hence an attempt was made to improve the separation efficinecy through design modifications. An impeller was installed through a central hole in the vortex-finder, and a series of experiments were carried out to compare the performance of an impeller-assisted hydrocyclone with that of a normal hydrocyclone. It was found that, with the introduction of the impeller, the classification efficiency was improved. A computer simulation of a grinding circuit using the impeller-assisted hydrocyclone showed that the capacity of the circuit can be increased by 35 percent.     

Wet centrifugal classification of calcite fines — effect of feed size

The wet classification of various fine calcite materials (<8, <12 and <45 μm) by a diskstack nozzle centrifuge is presented and the results are discussed with respect to feed size. It has been found that the influences of disk-geometry, G-force and feed rate on the classification performance are not related to feed size. However, the selection of a split suitable for an efficient separation depends on the particle size distribution of the feed material. Feed size has an impact on the residence time of particle separation. The results showed that an optimum efficiency can be achieved when a calcite material with a particle size below 12 μm is treated. An excessive amount of fine or coarse calcite particles in the feed affects the efficiency of the classification in the centrifuge. It is also indicated that an effective classification of calcite fines requires a moderate G-force and high flow rates through a disk section bound by stud spacers in the centrifuge.     

Particle migration of concentrated suspension flow in bifurcating channels

Flow of suspension in bifurcating channels has extensive applications in industrial and natural settings. A phenomenon of particular interest during the flow of concentrated suspension is shear-induced particle migration. Previous works on suspension transport in branched channels have been limited to dilute flow conditions. We have carried out experiments using the Particle Image Velocimetry (PIV) technique to study concentrated suspension transport in asymmetric T- and symmetric Y-shape channels. Numerical simulations of fluid flow and particle transport equations were also carried out for the same geometry which was used in the experiments. The migration and transport of particles in the simulations were studied using the Diffusive Flux Model. We have observed in both experiments and numerical simulations that due to the shear-induced migration phenomena the particles move towards the center of the channel, and this gives rise to the blunting of velocity profile before the junction. After the bifurcation, the peak of velocity profile moves in the direction of the outer wall, whereas, the maxima in particle concentration was observed near the inner walls. This causes asymmetry in the velocity and concentration profiles in the daughter branches. As we move towards the downstream positions the maxima in velocity and concentration profiles again shifts toward the center of the channel. The results from the experiments and simulations are observed to be in good agreement.     

Numerical study of flow field in new design cyclones with different wall temperature profiles: Comparison with conventional ones

In this paper, numerical study of flow field in the new design cyclones with five different wall temperature profiles are investigated. The new design cyclone is based on the idea of improving cyclone collection efficiency and pressure drop by increasing the vortex length. In this paper, the five wall temperature profiles are as follows: (A) cooling with uniform distribution, (B) without temperature change, (C) heating with uniform distribution, (D) incremental linear heating, (E) reduction linear heating. Results are compared in new design and conventional cyclones. The Reynolds averaged Navier–Stokes equations with Reynolds stress turbulence model (RSM) are solved. The Eulerian-Lagrangian computational procedure is used to predict particles tracking in the cyclones. The velocity fluctuations are simulated using the Discrete Random Walk (DRW).Results show that generally, heating the bottom zone of the cyclones can improve the collection efficiency and reduce the pressure drop while heating the top zone of the cyclones marginally affects the flow field. Moreover, cooling the cyclones reduces the efficiency and causes a higher pressure drop. Among five different wall temperature profiles, C and E profiles can increase the efficiency about 8% and profile C reduces the pressure drop by about 9%. The mentioned values in different conditions including particle diameter, flow rate, etc. can be different.     

Effect of Fine Particles' Entrainment on Conventional and Column Flotation

The entrainment of particles in a flotation has been considered as one of the significant factors affecting both concentrate grade and recovery for a decades. It is based on the changes depending on the establishment of linear relationship between water recovery and solid recovery. In this study, entrainment of fine particle using a mixture of artificial ore (celestite:calcite; 1:1) was investigated in conventional and column flotation. The effects of frother concentration and particle size were tested. The results showed that the particle size and frother concentration had significant effect on the grade and recovery, flotation time, and fine gangue entrainment. Entrainment factors for the conventional and column flotation were compared. Kirjaveinen (1989) model was used for describing a specific entrained factor (P _i ) of hydrophilic particles. It has been found that Kirjaveinen model supports the results of this study.     

Effect of the particle injection position on the performance of a cyclonic gas solids classifier

This study investigates the effect of the particle injection position on particle classification performance of a cyclonic classifier. The Rankine-type vortex flow in the cyclonic classifier is divided into three characteristic regions from the wall to the center according to the tangential and axial velocity distribution. Three representative particle injection positions are selected within those regions. The particle flow and the particle residence time are investigated via the Reynolds stress and discrete phase models. The simulation result shows that particles injected in the near-wall region provide the highest separation efficiency but the lowest classification accuracy. The residence time of particles injected within the region of maximum tangential gas velocity is shorter, facilitating a rapid and efficient particle classification. The powder classification experiment result confirms the simulation result and the feasibility of this method to enhance the separation degree of particles in the vortex flow.     

Numerical Investigation of Phase Distribution and Liquid Turbulence Modulation in Dilute Particle-Laden Flow

Studies on the motion of particles in turbulence and interactions between particles and turbulence are extremely significant, which can help us to improve the efficiency of industrial processes. In this article, we investigated the particle distribution and particle-turbulence interaction in a solid-liquid channel flow with the Euler-Lagrange two-way model. The liquid phase was solved using direct numerical simulation (DNS), and the particle motion was tracked by Newtonian equations of motion considering effects of drag force, pressure gradient force, and gravity. Two-way coupling was used to explain the effect of particles on the turbulence structure. The results show that the local void fraction of particles indicates the wall-peaked profile, particles scatter uniformly in the spanwise direction, and the injection of particles suppresses the turbulence activities in the near wall region. Suppression of the liquid turbulence is mainly caused by vortexes decay of different sizes.