Prediction of wear and its effect on the multiphase flow and separation performance of dense medium cyclone

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.     

CFD modeling and analysis of the multiphase flow and performance of dense medium cyclones

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 E_p 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.     

Applicability of a coarse-grained CFD–DEM model on dense medium cyclone

The combined computational fluid dynamics and discrete element method (CFD–DEM) approach has been proved to be an effective tool to study the fundamentals of different particle–fluid flow systems, but suffers high computational cost problem. Recently, various treatments such as parcel–particle concept, coarse-grained model, similar particle assembly and representative particle model have been developed to reduce the computational cost of CFD–DEM approaches. These treatments are basically empirical and thus their applicability is likely system-dependent. Until now, there are still no general agreements on the formulation of those models and their accuracy and general applicability are largely unknown.In this work, a coarse-grained (CG) (CFD–DEM) model is developed to model the swirling multiphase flow in a dense medium cyclone (DMC) and the error caused by the CG concept is quantified by carrying out controlled numerical calculations to directly compare the simulated results between a standard CFD–DEM model and a CG CFD–DEM model. It demonstrates that when the flow is dilute, the results are independent on the size of the grain (also called as parcel or model particle in this work). Nonetheless, when the flow is dense, small discrepancies are observed between the two models. This work suggests that the CG CFD–DEM model is indeed a useful tool to quickly evaluate the flow and performance of large-scale DMCs and the simulation results should be useful at least qualitatively, if not quantitatively.     

How to optimize design and operation of dense medium cyclones in coal preparation

The dense medium cyclone (DMC) is a high-tonnage device widely used to upgrade run-of-mine coal in the modern coal industry. It is known that a small improvement on the performance of DMC may greatly enhance industrial profitability. Therefore, it is very useful to develop an effective method to help optimize the design and operation of DMCs. Recently, based on the numerical experiments performed by Computational Fluid Dynamics and its combination with Discrete Element Method; the authors have established a PC-based mathematical model that looks promising to achieve this design and operational goal. In this paper, the authors will first discuss how to use this model to design high capacity or high efficiency DMCs for coal preparation through representative examples, in comparison with several typical designs in the industry. Some rules for DMC scale-up are then proposed for general application. The results further demonstrate that this DMC model can indeed offer a convenient way for optimum design and/or operation of DMCs under different conditions.     

Using an axial electromagnetic field to improve the separation density of a dense medium cyclone

To produce an on-line control method to improve the separation density of a given suspension density of a dense medium cyclone, a thin solenoid coil was placed in the cylindrical part of a cyclone. The dense medium distribution test and −3 + 0.125-mm coarse slime separation tests for different electric currents were performed. Float-and-sink analysis was performed for the separation products. The magnetic force of the particles under a magnetic field was also simulated. The results indicated that the presence of a magnetic field can improve the separation density by increasing the “separation cone density” caused by the inward radial motion and the upward axial motion of the magnetic particles. This approach provided a new separation density manipulation method for dense medium cyclones via application of a magnetic field.     

重介质旋流器的推广及新技术的应用

介绍了重介质旋流器选煤技术的发展现状,唐山研究院根据市场需求,利用计算流体力学技术结合重介质旋流器分选理论,研制出多供介无压给料三产品重介质旋流器、非金属矿用旋流器和难选煤专用的新型重介质旋流器,介绍了它们各自的特点.     

重介质旋流器分选效果的研究

针对影响石板选煤发电厂重介质旋流器分选效果的有关因素进行分析,阐述了进行技术改造所采取的相应措施。技术改造前后的对比试验表明,改造后的重介质旋流器的分选精度得到提高,分选效果良好。     

重介质旋流器流场湍流数值计算模型的选择

简述了重介质旋流器理论研究的现状,给出了计算流体力学基本方程及各种湍流数值计算模型.在相同边界条件下采用k-ε,RNG,RSM三个模型对重介质旋流器流场进行试探性数值模拟,并与相同边界条件的试验测量结果进行比较,最后选定RSM模型用于重介质旋流器流场数值模拟的湍流模型.     

电磁场对重介质旋流器分选密度的影响

为研究重介质旋流器外部电磁场的磁场位置、强度对其分选密度的影响,以磁铁矿粉为介质,将重介质旋流器置于同轴的螺线圈磁场中进行试验。试验结果表明:当磁场位于重介质旋流器柱体上端时,低磁场强度可提高其分选密度;当磁场位于重介质旋流器锥体下端时,高磁场强度可降低其分选密度。通过对重介质旋流器外部电磁场位置和强度的调节,可以达到调节其分选密度的目的,为实现其二段分选密度的在线调控提供一定的理论支持。     

无压三产品重介旋流器的应用与研究进展

主要介绍了新型无压三产品重介旋流器的结构、原理、分类和优缺点及适用场合,并总结了该设备的应用情况和研究进展。指出该设备具有广阔的发展前景,将朝着大型化、自动化、新材料方向发展。     

重介质旋流器壁面处理对湍流数值模拟的影响

采用3种不同的近壁模型和壁面处理方式,对重介质旋流器中的湍流流场进行模拟,将数值模拟数据与试验测量数据进行比较.结果表明,对近壁网格加密后采用增强壁面处理方法,能够得到更为接近实际流场的模拟结果.以2种不同入料口形式的重介质旋流器流场数值试验为例,分析了采用壁面处理后,旋流器内流场的变化情况.数值试验表明,旋流器的入料口采用弧形入料形式可以降低能耗,提高分选精度,增大处理量.     

基于粒度效应的重介质旋流器分选效果研究

为了研究不同粒度在重介质旋流器中的分选效果,分析了旋流器入料和产品的粒度和密度组成,计算了各粒级的可能偏差和错配物指标,对比了各粒级物料的分选效果。结果表明:各粒级物料的分选效果有所区别,粒度为13~6mm分选效果最好,粒度较大或较小均会使分选效果变差。粒度为13~6mm时,可能偏差为0.043,错配物总量为1.6%;粒度大于13mm时,分选效果变差,可能偏差增大,错配物总量增大至1.7%;粒度小于6mm时,分选效果下降明显,可能偏差超过0.050,错配物总量上升至2.0%以上。粒度为13~6mm时,分选效果最好,粒度较大或较小均会使得分选效果变差。     

CFD–DEM study of the effect of particle density distribution on the multiphase flow and performance of dense medium cyclone

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.     

重介质旋流器二段安装角度与物料排出关系的研究

分析了无压给料三产品重介质旋流器二段安装角度对铁器等重产物排出的影响,并用计算流体动力学软件FLUENT对其二段水平和倾斜两种安装方式进行了流场模拟和对比分析,结果表明:合理调整设备二段安装角度可加速重产物的排出,降低重产物对旋流器内壁的磨损,提高使用寿命,并确定无压给料三产品重介质旋流器二段最佳安装角度为-13°。     

重介质旋流器陶瓷衬里的磨损特性及模拟测试方法分析

简要介绍了重介质旋流器的分选原理,研究了该设备自身的磨损特性;阐述了重介质旋流器内衬材料的发展概况及陶瓷等脆性材料的冲蚀磨损特性,并对重介质旋流器陶瓷内衬材料性能的模拟测试进行了简要分析。     

影响重介质旋流器分选效果的因素分析

在总结重介质旋流器分选原理及理论研究的基础上,分析了影响分选效果的因素,并指出在实际生产中应根据煤质情况选用合适的重介质旋流器,同时应通过优化设备各参数提高分选效果。     

Computational modelling of shear-layer instabilities and vortex formation in DC plasma arcs

The direct-current plasma arc is the principal heating and stirring element in plasma arc furnaces. The arc is a highly dynamic system operating at very short time scales (milliseconds or less). This dynamic behaviour can be understood as the combination of several modes of instability acting together. These modes can include fluid dynamic/shear layer instabilities, near-electrode instabilities resulting from steep local temperature gradients and other electrode surface effects, or helical/twisting instabilities resulting from the magnetic field generated in one part of the arc column exerting a force on another.Computational magneto-hydrodynamic models of the arc which are able to reproduce the highly transient behaviour of the system are developed to study certain dynamic modes – in particular, the formation and evolution of axisymmetric vortices and shear layer instabilities in the arc jet. Results from this work are then compared to qualitative data from high-speed photographic imaging of large plasma arcs (up to 3 kA current).     

DWP重介质旋流器流场的数值模拟

选用雷诺应力湍流计算模型（RSM）,对两种直径为1 300 mm的DWP型重介质旋流器的流场进行了数值模拟,并对结构参数和能耗进行了研究.对流场三维速度、压强、密度和空气柱的数值模拟研究表明,在旋流器内存在短路流现象;双入介DWP旋流器分选效果优于单入介DWP旋流器,处理能力高于单入介DWP旋流器,能耗比单入介DWP旋流器减少33%.     

先排矸三产品重介质旋流器流场的数值模拟

使用3种湍流模型对先排矸三产品重介质旋流器的流场进行了数值模拟和实验研究.采用RSM模型进行的流场数值模拟结果表明,在旋流器密度场中,一段旋流器流场中存在着完整的空气柱,二段空气柱是气水混合中心低密度区;在旋流器压力场中,以轴线为中心存在着负压区,沿径向负压逐渐升高至0,然后达到正压力;在旋流器速度场中,轴向速度沿轴线向下形成内旋流,沿半径加大到0,形成了零速包络面,一段的切向速度和DWP旋流器相近,二段的切向速度比DWP旋流器更高.