Multi-level coarse-grained discrete element method modeling of cylinder particle flow in a rotating drum

The coarse-grained discrete element method(DEM)is probably a feasible option for simulating an actual drum-type biomass boiler,which contains over 10 million cylinder particles.A multi-level study was conducted based on particle and coarse-grained level data to evaluate the adequacy of the coarse-grained approach in terms of geometrical characteristics,kinematic features,and dynamic properties.Two scaling laws for contact parameters were used and compared during the simulations.The results show that the coarse-grained approach can accurately predict the positions of the free surface and active-passive interface,the mixing index,and the orientation properties.Deviations in the velocity fields may occur due to the worse flowability of coarse-grained particles near the free surface.The efficiency is significantly improved by the coarse-grained model compared with the corresponding original case(the same DEM code without a coarse-grained model was used for the original simulations).

DEM simulation of particle flow on a single deck banana screen

A mathematical study of particle flow on a banana screen deck using the discrete element method (DEM) was presented in this paper. The motion characteristics and penetrating mechanisms of particles on the screen deck were studied. Effects of geometric parameters of screen deck on banana screening process were also investigated. The results show that when the values of inclination of discharge and increment of screen deck inclination are 10° and 5° respectively, the banana screening process get a good screening performance in the simulation. The relationship between screen deck length and screening efficiency was further confirmed. The conclusion that the screening efficiency will not significantly increase when the deck length L≥430 mm (L/B ≥ 3.5) was obtained, which can provide theoretical basis for the optimization of banana screen.

Coupling of discrete-element method and smoothed particle hydrodynamics for liquid-solid flows

Particle based methods can be used for both the simulations of solid and fluid phases in multiphase medium, such as the discrete-element method for solid phase and the smoothed particle hydrodynamics for fluid phase. This paper presents a computational method combining these two methods for solid-liquid medium. The two phases are coupled by using an improved model from a reported Lagrangian-Eulerian method. The technique is verified by simulating liquid-solid flows in a two-dimensional lid-driven cavity.

Flow behavior of non-spherical particle flowing in hopper

Ellipsoidal particles flowing in the hopper were simulated by using the discrete element method （DEM）, and described by the multi-element method. The contact detection algorithm and equations for ellipsoidal particle motion in hopper were developed. And the simulation results were confirmed by experiment. Additionally, the mass flow rate, pressure distribution and velocity distribu- tion of two kinds of particles were examined. The results show that the mass flow rate of ellipsoidal particles is smaller than that of spherical particles. There is a maximum value of pressure drop at the top of the junction. Besides, the pressure drop decreases with the discharging time increasing. The velocity of spherical particle is larger than that of ellipsoidal.

Discrete element simulation of crushable rockfill materials

A discrete element method was used to study the evolution of particle crushing in a rockfill sample subjected to triaxial shear. A simple procedure was developed to generate clusters with arbitrary shapes, which resembled real rockfill particles. A theoretical method was developed to define the failure criterion for an individual particle subjected to an arbitrary set of contact forces. Then, a series of numerical tests of large-scale drained triaxial tests were conducted to simulate the behaviors of the rockfill sample. Finally, we examined the development of micro-characteristics such as particle crushing, contact characteristics, porosity, deformation, movement, and energy dissipation. The simulation results were partially compared with the laboratory experiments, and good agreement was achieved, demonstrating that the particle crushing model proposed can be used to simulate the drained triaxial test ofrockfill materials. Based on a comparison of macro behaviors of the roekfill sample and micro structures of the particles, the microscopic mechanism of the rockfill materials subjected to triaxial shear was determined qualitatively. It is shown that the crushing rate, rather than the number of crushed particles, can be used to reflect the relationship between macro- and micro-mechanical characteristics of rockfill materials. These research results further develop our understanding of the deformation mechanism of rockfill materials.

Effect of particle shape on packing fraction and velocity profiles at outlet of a silo

Many studies on how the particle shape affects the discharge flow mainly focus on discharge rates and avalanche statistics. In this study, the effect of the particle shape on the packing fraction and velocities of particles in the silo discharge flow are investigated by using the discrete element method. The time-averaged packing fraction and velocity profiles through the aperture are systematically measured for superelliptical particles with different blockinesses. Increasing the particle blockiness is found to increase resistance to flow and reduce the flow rate. At an identical outlet size, larger particle blockiness leads to lower velocity and packing fraction at the outlet. The packing fraction profiles display evidently the self-similar feature that can be appropriately adjusted by fractional power law. The velocity profiles for particles with different shapes obey a uniform self-similar law that is in accord with previous experimental results, which is compatible with the hypothesis of free fall arch. To further investigate the origin of flow behaviors, the packing fraction and velocity field in the region above the orifice are computed. Based on these observations, the flow rate of superelliptical particles is calculated and in agreement with the simulated data.

纤维增强树脂矿物复合材料的细观力学特性及损伤机理

纤维增强树脂矿物复合材料因优异的力学性能得到广泛应用,但有关材料损伤机理的研究仅停留在宏观层面,较少关注损伤过程中材料的细观力学特性和微裂纹演化规律。针对这一问题,基于颗粒流离散元技术建立了纤维树脂矿物复合材料的细观模型,分析了材料应力-应变曲线的变化规律,揭示了材料的微裂纹萌生扩展机制和其破坏失效形式。结果表明,相比于树脂矿物复合材料,纤维增强树脂矿物复合材料的抗压强度更高,起裂应力和损伤应力更大;骨料颗粒形状对纤维的增强增韧作用影响较小;纤维对树脂矿物复合材料具有增韧阻裂的作用。研究从细观角度揭示了纤维增强树脂矿物复合材料的损伤性能,能够为该材料的推广应用提供指导。

重力驱动下漏斗出口形状及振动因素对颗粒流的影响分析

为了深入了解漏斗颗粒在外部激励下的运动状态,本文基于离散元方法,探究了重力驱动下漏斗出口倾角、振动幅值、振动方向、颗粒竖向初始速度以及是否加装出口挡板对于颗粒流流量的影响。结果表明:出口倾角和振动幅值的增大都有利于颗粒流量的增加;在不考虑初始速度和挡板情况下,当振动方向由水平逐渐转向竖直这一过程中,颗粒流量随之减小,并且在水平方向振动时,颗粒流量达到最大值,而在加装出口挡板和考虑初始速度两种情况下,结果与之相反;在出口无挡板且振幅为零的条件下,赋予颗粒物质竖直方向速度有益于流量的增加,随着振幅的增大,流量稳步提高,但增长幅度均低于相同环境下无竖直方向速度的流量增长幅度;出口的挡板在低振幅下不利于颗粒物质的排出,但在提高振幅且振动方向逐步转向竖直方向后,颗粒流量大幅提高。

螺旋输送机卸料口流量脉动产生原因及降低方法研究

为探明螺旋输送机卸料口流量脉动产生的原因,基于离散单元法(DEM)模拟球形颗粒在螺旋输送机中的流动过程,对螺旋周期结构和颗粒周期流动进行分析,得出了流量脉动产生的原因并提出了降低流量脉动的方法。结果表明:料槽中颗粒的周期性不均匀分布和螺旋面对颗粒的周期性阻挡造成了流量脉动;不同结构形式的螺旋输送机卸料口流量脉动对比为:无轴螺旋输送小于有轴螺旋输送,双轴螺旋输送小于单轴螺旋输送,斜卸料口结构小于正常卸料口结构;所提斜卸料口双轴螺旋输送的脉动值(1.14)与普通螺旋输送(2.25)相比降低了49.33%,基本实现了螺旋输送机卸料口颗粒的稳定流出。

料斗倾角对气基竖炉布料过程中物料流动及分布的影响

球团矿离开料斗时的颗粒偏析会影响其在气基竖炉内的分布,分布不合理将影响还原气的利用率建立20°~70°倾角的料斗、导料管、溜槽、气基竖炉三维模型,基于离散元法研究物料从料斗到气基竖炉布料过程中的流动和分布结果表明:排料时,20°~25°倾角的料斗内炉料流型均为整体流,30°~55°倾角的料斗内炉料流型均为混合流,60°~70°倾角的料斗内炉料流型均为漏斗流漏斗流促使料层形成快速流动的中心通道,大颗粒会更早地离开料斗进入炉内,在落点区域堆积,导致料斗内大颗粒负偏析、竖炉内落点区域大颗粒正偏析20°~35°倾角的料斗布料后,炉内孔隙率偏差分布在000383~000468,倾角增大,孔隙率偏差增大改变炉料-料斗静摩擦系数、炉料批重、料斗半径等参数,发现料斗倾角是影响布料过程和结果的主要因素,孔隙率偏差仍随料斗倾角的增大而增大。

基于计算流体力学-离散单元法的U形管冲蚀磨损数值模拟研究

采用计算流体力学-离散单元法(CFD-DEM)和切向撞击能模型(SIEM)来研究气固两相流中高速煤粉颗粒冲击导致的U形弯管冲蚀磨损问题。分析了颗粒粒径、气体流速、固体颗粒形状对其冲蚀磨损特性的影响,并验证粗粒化模型应用于此磨损过程的准确性。结果表明,U形弯头的冲蚀磨损位置主要有3处;磨损率随粒径的增大呈现整体减小的趋势;U形管拱背磨损率随气速的增加而增大,且前2处磨损率峰值位置随气速的增加向入口方向偏移;长椭球对U形管造成的磨损最严重,扁椭球次之,球形最轻;粗粒化CFD-DEM能够基本准确预测U形管冲蚀磨损,且显著降低计算时间。

基于离散元的废旧二灰碎石与土混合料承载比细观力学特性研究

建筑垃圾应用于路基过程中,其材料承载力对道路服役性能具有重要影响。为评价建筑垃圾用于路基填料时的细观力学特性,该文利用离散元软件对不同含石率的废旧二灰碎石与土混合料进行承载比试验,并利用离散元法揭示强度形成机理。结果表明:混合料的承载比随含石率增大不断增加,含石率为80%时达到最大值。在离散元模拟承载比试验中,空隙率随着二灰碎石土的含石量的增大先降低后增大,当含石率为80%时混合料的空隙率最小,此时能达到最佳的承载效果。随着含石率的逐渐增大,废旧二灰碎石之间的颗粒相互嵌挤,形成骨架结构,因此能够有效承担来自贯入杆的荷载。

岩土球形颗粒体粒间接触力学特性及接触模型验证

针对大理岩和花岗岩两种材料,分别进行了球颗粒法向、切向接触力学试验;以大理岩为例,总结了非线性接触模型及其参数,并进行了球颗粒集合体的离心模型试验,通过数值模拟和试验结果的对比验证了接触模型及其参数的合理性。研究结果表明:颗粒法向接触以脆性断裂为破坏形态,切向接触以摩擦侵蚀为破坏特征;法向、切向刚度曲线都可以利用幂函数进行拟合;利用非线性接触模型及其参数可以有效模拟颗粒集合体的力学性状,同时还能够反映散体模型细部的离散变形特征。

基于CFD-DEM的超细碳酸钙螺旋输送仿真分析

【目的】为提高螺旋输送机的输送效率,降低输送机的功耗与磨损,探究在不同进料速率、螺旋轴转速与几何体摩擦系数下,超细碳酸钙在水平变径变距螺旋输送机内的颗粒流动状态、出口质量流量、输送机功耗与磨损分布。【方法】使用计算流体动力学(computational fluid dynamics,CFD)与离散单元法(discrete element method,DEM)双向耦合数值模拟的方法,对螺旋输送机在不同转速下的质量流率进行分析对比,验证数值模型的正确性。【结果】摩擦系数对颗粒的运动有较大影响,颗粒流的轴向速度峰值和质量流率峰值随着摩擦系数的增加先增大再减小;随着下料速度和摩擦系数的增大,输送机功率明显增大,且摩擦系数在高进料速度与低转速的情况下对功耗的影响相对于低进料速度和高转速更加明显;磨损较严重的区域集中在下料口处的螺旋轴与螺旋叶片的边缘处。【结论】简单增大或减小摩擦系数并不能提高颗粒的轴向速度和质量流量,而是存在一个局部最优参数组合;适当地提高转速能够减小颗粒密实度与颗粒停留时间,从而减小输送机的功耗与几何体磨损。

基于CFD-DEM的湿颗粒气力输送数值模拟

【目的】为了探索水平管中湿颗粒气力输送的内在机制,开发液桥轮廓由凸到凹的液桥力模型,实现对颗粒含水率的精准调控,分析不同含水率颗粒在输送过程中的动力学特性变化规律。【方法】采用计算流体力学(computational fluid dynamics, CFD)和离散元法(discrete element method, DEM)双向耦合的数值模拟方法,通过对弯头外侧中心线的颗粒速度分析对比,验证数值模型的正确性以及网格的无关性。【结果】干颗粒沉降在管道底部,表现为管底流的运动状态,湿颗粒因液桥力的作用而形成紧密的颗粒团块,以单粒子和颗粒团2种形式进行运动,并且颗粒含水率越大,颗粒团聚现象越严重;湿颗粒的输送速度明显比干颗粒低,且随着颗粒含水率的增加,颗粒的平均输送速度呈下降趋势。【结论】相对于干颗粒输送,湿颗粒输送流动性更弱、输送效率更低以及能耗更高,在实际工业应用中,应当对湿颗粒进行前处理,以便于气力输送的无故障进行。

基于离散元法的三轴试验数值仿真分析

基于离散元法的思想,利用Fish语言编写程序在PFC软件上实现了三轴试验的数值模拟。实现了土体细观参数和宏观参数的标定,对于内摩擦角2°-30°的土体,可以从文章的数值模拟结果中找到大致对应的细观参数。得出细观摩擦系数、法向粘结强度和切向粘结强度等细观参数会影响应变的峰值强度与宏观内摩擦角和粘结力,细观摩擦系数与内摩擦角成正相关,与粘结力成负相关。根据数值模拟的结果,提出粘结力与细观摩擦系数关系公式和内摩擦角与细观摩擦系数关系公式,利用室内试验结果验证了拟合公式的准确性和可行性。

密相栓流湿颗粒气力输送数值模拟

鉴于目前对于湿颗粒密相气力输送的研究缺乏成熟的体系与结论,本文探究了湿颗粒密相气力输送的流动机理与流动形态,为气力输送的优化设计提供理论支持。研究采用计算流体力学(computational fluid dynamics,CFD)和离散元法(discrete element method,DEM)双向耦合的数值模拟方法,对水平管道中干湿颗粒料栓的运动形式和颗粒运动轨迹进行了分析,深入研究了颗粒含水率对输送流型、输送效率以及输送压降的影响。结果表明,同等气速下,湿颗粒可以形成段塞流,而干颗粒先是段塞流,随后料栓被气体击穿演变为沙丘流,最后成为管底流;随着气体速度的增大,单位长度的水平管道压降数值呈现先降低后增长的趋势,在质量流率0.5 kg/s、含水率5%的工况下,输送速度为9 m/s时管道压降数值最低,为最佳经济速度值;湿颗粒的压力曲线一直处于波动较大的不稳定状态,当两个监测点位于段塞外部时,轴向压差几乎为零,而当两个监测点位于段塞内部时,轴向压差可以达到-600 Pa。通过对湿颗粒输送时的压降变化进行分析和监测,可及时发现设备故障的迹象,预警可能的问题并采取预防措施,避免设备损坏和生产中断。

基于离散元法模拟的冻融砂岩细观破裂演化特征研究

为探究冻融岩石细观损伤累积及受载破裂过程,基于离散元提出一种水冰颗粒相变耦合膨胀方法,借助颗粒流程序实现岩石冻融过程模拟,结合室内试验验证模拟结果的可靠性,定量表征孔隙水颗粒冻胀评价指标λv,建立λv与冻融循环次数N之间的函数关系,并对冻融岩石受载过程中微裂纹−位移场−力链场演化及破裂特征进行评估。结果表明:(1)冻融循环作用下岩石内部孔隙水体积膨胀及持续补水是造成其损伤的本质原因;冻融过程中试样细观微裂纹由拉伸裂纹主导,呈“先慢后快”趋势演化,且外围岩石颗粒位移较内部更显著。(2)试样受载时微裂纹呈“慢→缓→陡”发展,冻融循环次数N与受载产生的微裂纹数量呈正相关,但与微裂纹起裂应力σi却呈负相关。(3)冻融前后试样破裂过程与形态明显不一,受载趋于峰值强度σf时,试样微裂纹分布−位移场−力链场有“异常信号”出现,可作为破坏前兆识别信息;冻融循环影响下,试样内部微裂纹空间排布方式更为复杂,由拉伸微裂纹主导破裂向拉−剪混合微裂纹主导转变。该研究可为探索冻融岩石破坏行为提供一种新的思路和方法。

颗粒阻尼器耗能特性及振动抑制研究

机电控制系统受振动影响易发生故障,严重影响飞行安全,本文通过颗粒阻尼器对机电控制系统进行振动抑制研究,采用离散元仿真方法研究阻尼器的耗能变化规律与振动幅值、振动频率和颗粒数量的影响关系,并通过BP神经网络对颗粒阻尼器耗能数据进行训练和预测;通过机电控制器的随机振动试验,验证离散元仿真结论与BP神经网络预测模型的准确性。结论表明,离散元仿真在振动频率20~40Hz、激励幅值2~16mm范围内,其他条件一定时,阻尼器耗能随频率和幅值的增大而增大,随颗粒填充率先增大后减小,在57%~70%填充率范围内具有最佳耗能效果;在机载系统随机振动试验中,颗粒阻尼器填充率处于30%~90%范围内均表现出较好的振动抑制效果。仿真和试验结果对颗粒阻尼器在机电控制系统中进一步应用具有指导意义。

潮湿颗粒电解质电化学机械抛光铜工件的接触特性研究

潮湿颗粒电解质电化学机械抛光(Moist particle electrolyte electrochemical mechanical polishing,MPEECMP)作为新兴技术,仍存在难以获得高表面质量的问题。为解决该问题,深入研究电解质颗粒与工件的接触特性,采用离散元仿真软件Altair EDEM探究了工件倾斜角、转速对接触数量、接触力的影响规律,并进行MPE-ECMP工艺试验。研究结果表明,倾斜角为30°时,单位时间内电解质颗粒与工件的接触数量最多,且切向力最大,为3.38 mN;在90°时,切向力最小,为1.21 mN。随着工件转速增大,单位时间内电解质颗粒与工件的接触数量变少,电解质颗粒与工件接触的法向力、切向力呈增大趋势。当抛光电位(vs.Hg/Hg_(2)SO_(4))为0.8 V,工件倾斜角为30°,抛光1 h,表面粗糙度从S_(a)433.51 nm降低到S_(a)22.43 nm,降低了94.8%。结果证明了工件倾斜角、转速的调整可有效提高MPE-ECMP的抛光精度,表面粗糙度的降低是由接触数量及接触力共同决定的,EDEM可有效模拟电解质颗粒运动的流态特性,为MPE-ECMP的进一步研究奠定了基础。