Industrial cyclones, such as gas, hydro and dense medium, are widely used in chemical, mineral and process industries to separate particles from fluids or classify particles by size, density or other ...solid properties. It is well known that the loading of particles can significantly affect fluid flow in such cyclones but the specific effect can be confusing due to a limited fundamental understanding of the working mechanisms involved. In this work, the problem is partially tackled by analysing the influence of particles of different size on the medium flow in a dense medium cyclone (DMC) by using a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) (CFD-DEM). In the CFD-DEM model, CFD is used to model the motion of slurry medium by numerically solving the local-averaged Navier-Stokes equations at a computational cell scale, facilitated by the Volume of Fluid (VOF) and Mixture multiphase models. DEM is then used to model the motion of discrete particles by applying Newton's laws of motion, facilitated by a coarse-grained (CG) method to model small coal particles. It is found that both the particle and medium flows are sensitive to the particle size. The most notable finding is that spatial distributions of solid flow patterns form due to particles of different size have different trajectories in the cyclone. These lead to significant different distribution of volumetric particle-fluid interaction forces which cause different effects on the fluid flow. The findings are useful for developing a better understanding the working mechanisms of solids loading effects in DMCs and also in other similar swirling multiphase flow systems.
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•The solids loading effect in a dense medium cyclone heavily depends on particle size.•Different particle size leads to different spatial distribution of particle-fluid interaction force.•80% of quite large and heavy particles can be misplaced to overflow.•The effect of particle top size is predicted for the first time.
Hydrocyclones are studied numerically. The effects of conical shape and length on flows and performance are quantified at different feed solids concentrations. A new hydrocyclone featured with a long ...convex cone is proposed, which has a better performance than the conventional hydrocyclone, because of its larger separation region with relatively smaller tangential velocities. Display omitted
•Hydrocyclones are studied in dilute and dense regimes by the mixture model.•The effects of cone length and shape on flows and performance are quantified.•A new hydrocyclone featured with a long convex cone is hence proposed.•The new hydrocyclone has a better performance than the conventional cyclone.
Hydrocyclones generally follow a conventional design and may have some limitations on separation performance. This paper presents a numerical study of hydrocyclones with different conical configurations by a recently developed computational fluid dynamics method. The feed solids concentration considered is up to 30% (by volume), which is well beyond the range reported before. The numerical results show that the cyclone performance is sensitive to both the length and shape of the conical section, as well as the feed solids concentration. A longer conical section length leads to decreased inlet pressure drop, cut size d50, and Ecart probable Ep, and at the same time, an increased water split (thus larger by-pass effect). When conical shape varies from the concave to convex styles gradually, a compromised optimum performance is observed for the cyclone with a convex cone, resulting in a minimum Ep and relatively small inlet pressure drop and water split. Almost all these effects are pronounced with increasing feed solids concentration. Based on the numerical experiments, a new hydrocyclone featured with a long convex cone is proposed. It can improve the performance of the conventional cyclone at all the feed solids concentrations considered.
Dense medium cyclone (DMC) is widely used to upgrade run-of-mine coal in modern coal preparation plants. The flow within it is very complicated, with multi-phases involved including air, water, coal ...and magnetic/non-magnetic particles of different sizes, densities and other properties. In this work, the effect of coal particle density distribution, one of the most important variables and highly related to coal type, is systematically studied using a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In particular, the so called Johnson's SB function, which can describe a wide range of distributions, is employed to represent different particle density distributions. For a given density range, the function is characterized by two parameters: median particle density ρ0.5 and distribution parameter σj, with the latter describes the spread of the distribution. The effects of the two parameters on the flow and performance of a typical DMC are quantified. Moreover, their implication to practical operation, e.g. when the amount of near gravity material is high, is discussed. The results are also analysed in terms of the medium and particle flow fields, particle-fluid, particle-particle and particle-wall interaction forces to understand the fundamentals of the operation. The results obtained in this work should be useful to the design and control of DMC operations with different coal density distributions or coal types.
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•A CFD-DEM model is used to study the multiphase flow in a DMC.•Johnson's SB function is used to describe the particle density distribution.•The effect of particle density distribution on DMC performance is quantified.•The mechanisms are explored in terms of particle-particle and particle-fluid forces.
This paper presents a numerical study of multiphase flow in hydrocyclones with different configurations of cyclone size and spigot diameter. This is done by a recently developed mixture multiphase ...flow model. In the model, the strong swirling flow of the cyclone is modeled using the Reynolds stress model. The interface between liquid and air core and the particle flow are both modeled using the so-called mixture model. The solid properties are described by the kinetic theory. The applicability of the proposed model has been verified by the good agreement between the measured and predicted results in a previous study. It is here used to study the effects of cyclone size and spigot diameter when feed solids concentration is up to 30% (by volume), which is well beyond the range reported before. The flow features predicted are examined in terms of the flow field, pressure drop, and amount of water split to underflow, separation efficiency and underflow discharge type. The simulation results show that the multiphase flow in a hydrocyclone varies with cyclone size or spigot diameter, leading to a different performance. A smaller cyclone results in an increased cut size, a decreased pressure drop and a sharper separation, and, at the same time, an increased water split (thus worse bypass effect) and a more possibly unstable operation associated with rope discharge, particularly at relatively high feed solids concentrations. Both large and small spigot diameters may lead to poor separation performance. Accordingly, an optimum spigot diameter can be identified depending on feed solids concentration. It is also shown that for all the considered hydrocyclones, a better separation performance and a smoother running state can be achieved by the operation at a lower feed solid concentration.
Hydrocyclones are numerically studied. The effects of vortex finder diameter, length and shape on flows and performance are quantified at different feed solids concentrations. An optimum performance ...can be identified when vortex finder diameter or shape varies. The optimum shape is inverse conical style, which can best control the flow properties and hence the separation performance. Display omitted
•Hydrocyclones are numerically studied at different feed solids concentrations.•The effects of vortex finder size and shape on flows and performance are quantified.•An optimum performance is identified when vortex finder diameter or shape varies.•The mechanisms underlying the flow control by vortex finder are identified.
This paper presents a numerical study of the multiphase flow and performance of hydrocyclone by means of two-fluid model, with special reference to the effects of diameter, length and shape of vortex finder at a wide range of feed solids concentrations. The considered shapes include the conventional cylindrical style and the new conical and inverse conical styles. The simulation results are analysed with respect to cyclone flow and performance in term of cut size d50, water split, Ecart probable Ep and inlet pressure drop. It is shown that when vortex finder diameter or shape varies, a compromised optimum performance can be identified, resulting in relatively small inlet pressure drop, Ep, and water split. Both d50 and Ep are more sensitive to feed solids concentration than inlet pressure drop and water split. Overall, the effect of vortex finder length on the separation efficiency of particles is much less significant than diameter and shape, which shows opposite trends at low and high feed solids concentrations. All these results can be well explained using the predicted tangential and axial velocities and solid volume fraction.
A mathematical model is developed to study the coal-medium flow in a dense medium cyclone (DMC) of 1000
mm body diameter. In the model, the motion of coal particles is obtained using the Discrete ...Element Method (DEM) facilitated with the concept of “parcel–particle” while the flow of medium as a liquid-magnetite mixture Computational Fluid Dynamics (CFD) based on the local averaged Navier–Stokes equations. In addition the Reynolds Stress Model (RSM) is adopted to describe the anisotropic turbulence, the Volume of Fluid (VOF) model is used to describe the air-core position and multiphase mixture model used to estimate the flow of fine magnetite particles. The simulated medium and coal flows allow estimates to be made of pressure drop, efflux stream medium densities and partition curves for coal particles of different sizes and densities. These estimates are compared favourably with industrial scale measurements of a 1000
mm DMC operating under similar conditions. On this base, the effect of particle density distribution that represents the major difference between two major coal type, i.e., coking coal and thermal coal, is studied. The results are analysed in terms of medium flow pattern, particle flow pattern, partition performance and particle–fluid, particle–wall and particle–particle interaction forces.
A CFD model is proposed to describe the multiphase flow in a dense-medium cyclone (DMC). The model is then used to quantify the effects of the ratios of spigot to vortex finder diameters (U:O) and ...medium to coal (M:C) on DMC flow and performance. The numerical results show that the separation efficiency increases with the increase of U:O ratio, and the partition curve presents a fish-hook shape when U:O ratio is equal to 1 and 1.1.
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•A CFD model is developed to study dense medium cyclones.•The applicability of the model is verified.•The effect of medium-to-coal volume ratio on performance is quantified.•The effect of spigot-to-vortex finder diameter ratio on performance is quantified.•A fish-hook phenomenon related to particle density is numerically observed.
A computational fluid dynamics (CFD) model is proposed to describe the multiphase flow in a dense-medium cyclone (DMC). In this model, the volume of fluid (VOF) multiphase model is first used to determine the initial shape and position of the air core, and then the so called mixture model is employed to describe the flows of the medium, coal particles and air, where the turbulence is described by the Reynolds stress model. The validity of the proposed approach is verified by the reasonably good agreement between the measured and calculated results in terms of separation efficiency. On this base, this model is used to quantify the effects of the ratios of spigot to vortex finder diameters (U:O) and medium to coal (M:C) on performance. The results are shown to be generally comparable to those reported in the literature. It reveals that when vortex finder or spigot diameter is varied at the same U:O ratio, the offset and medium split nearly remain the same, however, the coal feed rate and Ep are different under the conditions considered. It is also shown that the fish-hook phenomenon is observed when spigot diameter is equal to or slightly larger than vortex finder diameter, and a normal operation becomes less stable with decreasing U:O ratio. The key phenomena predicted are explained by the calculated inner flows.
Diabetes is on the rise as the worldwide population ages. While physical activity can help protect against diabetes, ageing is commonly associated with reduced physical activity. This study aimed to ...examine if physical activity differs by diabetes status in mid-aged adults, how this association changes over time, and whether physical activity-related sociodemographic factors and health indicators differ in those with and without diabetes. Data came from four waves of the How Areas in Brisbane Influence HealTh and AcTivity (HABITAT), a longitudinal study of mid-age adults living in Brisbane, Australia. Random effects/Expectation-maximisation (RE-EM) regression trees were used to identify factors affecting physical activity among those with and without diabetes, both separately and combined. At study entry, those with diabetes had a higher median age of 58 years (95% CI: 57–60) and a lower median physical activity of 699 MET.min/week (95% CI: 599–799) than people without diabetes (53 years (95% CI: 53–53) and 849 MET.min/week (95% CI: 799–899)). However, the strongest factors influencing physical activity were BMI and gender, not diabetes status. It is vital to promote physical activity among adults, in particular among those with high BMI and women, as well as those with and at high risk of diseases like diabetes.
A CFD-DEM approach, coupled with the Finnie wear model, has been developed to predict the wear rate of a DMC under various operational, material and geometrical conditions. The figure above shows the ...predicted wear rate at the DMC walls for different coal types: (a), coking coal (particle size is 25mm); (b), thermal coal (particle size is 25mm); and (c), thermal coal (particle size is 0.25mm). Display omitted
•Wear rate of DMC walls is numerically predicted using a CFD-DEM approach.•The approach is coupled with the Finnie wear model.•The locations of severe wall wearing are predicted.•Wear rate of DMC walls varies with both operational and material conditions.•The effect of a worn spigot wall on the DMC performance is predicted.
Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in modern coal preparation plants. It is known that wear is one of the problems in the operation of DMCs, but it is not well understood. In this work, the wear rate of DMC walls due to the impact of coal particles is predicted by a combined computational fluid dynamics and discrete element method (CFD-DEM) approach, using the Finnie wear model from the literature. In the CFD-DEM model, DEM is used to model the motion of discrete coal particles by applying Newton’s laws of motion and CFD is used to model the motion of the slurry medium by numerically solving the local-averaged Navier–Stokes equations together with the volume of fluid (VOF) and mixture multiphase flow models. According to the Finnie wear model, the wear rate is calculated according to the impact angle of particles on the wall, particle velocity during an impact and the yield stress of wall material; the relevant particle-scale information can be readily obtained from the CFD-DEM simulation. The numerical results show that the severe wear locations are generally the inside wall of the spigot and the outside wall of the vortex finder. The wear rate depends on both the operational conditions and solids properties. It increases generally with the decrease of medium-to-coal (M:C) ratio. For a given constant M:C ratio, the wear rate for thermal coal is higher than that for coking coal, especially at the spigot. Large particles may cause a non-symmetric wear rate due to the gravity effect. The effect of a worn spigot wall on the multiphase flow and separation performance is also studied. This work suggests that the proposed approach could be a useful tool to study the effect of wear in DMCs under different conditions.
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