Computational fluid dynamics-discrete element method simulation of stirred tank reactor for graphene production

Liquid phase exfoliation(LPE)process for graphene production is usually carried out in stirred tank reactor and the interactions between the solvent and the graphite particles are important as to improve the production efficiency.In this paper,these interactions were revealed by computational fluid dynamics–discrete element method(CFD-DEM)method.Based on simulation results,both liquid phase flow hydrodynamics and particle motion behavior have been analyzed,which gave the general information of the multiphase flow behavior inside the stirred tank reactor as to graphene production.By calculating the threshold at the beginning of graphite exfoliation process,the shear force from the slip velocity was determined as the active force.These results can support the optimization of the graphene production process.

Computational fluid dynamics simulations of respiratory airflow in human nasal cavity and its characteristic dimension study

To study the airflow distribution in human nasal cavity during respiration and the characteristic parameters of nasal structure, three-dimensional, anatomically accurate representations of 30 adult nasal cavity models were recons- tructed based on processed tomography images collected from normal people. The airflow fields in nasal cavities were simulated by fluid dynamics with finite element software ANSYS. The results showed that the difference of human nasal cavity structure led to different airflow distribution in the nasal cavities and variation of the main airstream passing through the common nasal meatus. The nasal resistance in the regions of nasal valve and nasal vestibule accounted for more than half of the overall resistance. The characteristic model of nasal cavity was extracted on the basis of characteristic points and dimensions deduced from the original models. It showed that either the geometric structure or the airflow field of the two kinds of models was similar. The characteristic dimensions were the characteristic parameters of nasal cavity that could properly represent the original model in model studies on nasal cavity.

Stabilization for Equal-Order Polygonal Finite Element Method for High Fluid Velocity and Pressure Gradient

This paper presents an adapted stabilisation method for the equal-order mixed scheme of finite elements on convex polygonal meshes to analyse the high velocity and pressure gradient of incompressible fluid flows that are governed by Stokes equations system.This technique is constructed by a local pressure projection which is extremely simple,yet effective,to eliminate the poor or even non-convergence as well as the instability of equal-order mixed polygonal technique.In this research,some numerical examples of incompressible Stokes fluid flow that is coded and programmed by MATLAB will be presented to examine the effectiveness of the proposed stabilised method.

MIXED COMPATIBLE ELEMENT AND MIXED HYBRID INCOMPATIBLE ELEMENT VARIATIONAL METHODS IN DYNAMICS OF VISCOUS BAROTROPIC FLUIDS

This paper presents and proves the mixed compatible finite element variationalprinciples in dynamics of viscous barotropic fluids. When the principles are proved, itis found that the compatibility conditions of stress can be naturally satisfied. The gene-rallzed variational principles with mixed hybrid incompatible finite elements are alsopresented and proved, and they can reduce the computation of incompatible elements indynamics of viscous barotropic flows.

CVBEM and FVM Computational Model Comparison for Solving Ideal Fluid Flow in a 90-Degree Bend

While finite volume methodologies (FVM) have predominated in fluid flow computations, many flow problems, including groundwater models, would benefit from the use of boundary methods, such as the Complex Variable Boundary Element Method (CVBEM). However, to date, there has been no reporting of a comparison of computational results between the FVM and the CVBEM in the assessment of flow field characteristics. In this work, the CVBEM is used to develop a flow field vector outcome of ideal fluid flow in a 90-degree bend which is then compared to the computational results from a finite volume model of the same situation. The focus of the modelling comparison in the current work is flow field trajectory vectors of the fluid flow, with respect to vector magnitude and direction. Such a comparison is necessary to validate the development of flow field vectors from the CVBEM and is of interest to many engineering flow problems, specifically groundwater modelling. Comparison of the CVBEM and FVM flow field trajectory vectors for the target problem of ideal flow in a 90-degree bend shows good agreement between the considered methodologies.

Application of the Hierarchical Functions Expansion Method for the Solution of the Two Dimensional Navier-Stokes Equations for Compressible Fluids in High Velocity

This work presents a new application for the Hierarchical Function Expansion Method for the solution of the Navier-Stokes equations for compressible fluids in two dimensions and in high velocity. This method is based on the finite elements method using the Petrov-Galerkin formulation, know as SUPG (Streamline Upwind Petrov-Galerkin), applied with the expansion of the variables into hierarchical functions. To test and validate the numerical method proposed as well as the computational program developed simulations are performed for some cases whose theoretical solutions are known. These cases are the following: continuity test, stability and convergence test, temperature step problem, and several oblique shocks. The objective of the last cases is basically to verify the capture of the shock wave by the method developed. The results obtained in the simulations with the proposed method were good both qualitatively and quantitatively when compared with the theoretical solutions. This allows concluding that the objectives of this work are reached.

A Generic Computational Solution of a Natural Convection Flow past an Infinite Vertical Porous Plate

A simulation was carried out on an unsteady flow of a viscous, incompressible and electrically conducting fluid past an infinite vertical porous plate. A generic computer program using the Galerkin finite element method is employed to solve the coupled non-linear differential equations for velocity and temperature fields. The diffusion equation, the energy equation, the momentum equations and other relevant parameters are transformed into interpretable postfix codes. Numerical calculations are carried out on the flow fields both in the presence of cooling and heating of the plate by free convection currents. The effects of the dimensionless parameters, namely, the Prandtl number, the Eckert number, the modified Grashof number, the Schmidt number and the time on the temperature and velocity distributions are discussed.

Analysis of gas-solid flow and shaft-injected gas distribution in an oxygen blast furnace using a discrete element method and computational fluid dynamics coupled model

lronmaking using an oxygen blast furnace is an attractive approach for reducing energy consumption in the iron and steel industry. This paper presents a numerical study of gas-solid flow in an oxygen blast fur- nace by coupling the discrete element method with computational fluid dynamics. The model reliability was verified by previous experimental results. The influences of particle diameter, shaft tuyere size, and specific ratio （X） of shaft-injected gas （51G） flowrate to total gas flowrate on the SIC penetration behavior and pressure field in the furnace were investigated. The results showed that gas penetration capacity in the furnace gradually decreased as the particle diameter decreased from 100 to 40 mm. Decreasing particle diameter and increasing shaft tuyere size both slightly increased the SIG concentration near the furnace wall but decreased it at the furnace center. The value of X has a significant impact on the SIG distribution. According to the pressure fields obtained under different conditions, the key factor affecting SIG penetration depth is the pressure difference between the upper and lower levels of the shaft tuyere. If the pressure difference is small, the SIG can easily penetrate to the furnace center.

恶劣海况下船舶砰击颤振响应特性数值计算与试验研究

[目的]针对恶劣海况下船舶所受砰击颤振响应现象,探究船舶非线性波浪载荷与瞬态高幅值砰击载荷的耦合作用。[方法]采用计算流体动力学与有限元方法(CFD-FEM)相结合的双向流固耦合方法对S175集装箱船进行数值仿真计算,并与试验结果及切片理论计算结果进行对比验证;采用分段变截面弹性龙骨梁模型开展船舶的砰击颤振特性模型试验,基于CFD-FEM双向流固耦合方法开展船艏砰击载荷及高频非线性砰击颤振响应特性分析,并与模型试验结果进行对比验证。[结果]结果显示,波浪砰击载荷对船艏颤振响应的影响不可忽视,6级海况下由砰击颤振诱发的二阶高频成分分量占低频波浪弯矩的59.86%。[结论]采用基于CFD-FEM的双向流固耦合方法可准确计算船首砰击颤振响应;在高海况下船舶所受非线性波浪载荷及结构动态响应易受船首瞬态砰击载荷的影响,在船舶结构设计与安全评估中需考虑高频砰击颤振的情况。

基于CFD-DEM的流态化速冻影响因素研究

利用Fluent多孔介质理论建立了速冻流化床等效流域模型;以表观气速、入口温度、脉动频率为试验因素,根据CFD-DEM数值模拟方法对典型颗粒形状物料的流态化和冻结进行单因素试验分析,探究了各试验因素对物料冻结的影响规律;基于单因素试验结果,以平均降温速率为评价指标,进行三因素、四水平的正交试验,试验结果表明,在进风量一定的情况下,影响程度由强到弱依次为入口温度、脉动频率、表观气速。以期分析结果对脉冲气流式速冻工程实践具有一定的参考作用。

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。

大变幅加卸载下特厚煤层底板断层突水机理模拟研究

特厚煤层采场空间大、扰动范围广,强烈大变幅荷载易导致底板断层破裂加剧并诱发水害。数值模拟是揭示特厚煤层底板断层活化与突水机理的重要方法,准确反映大变幅加卸载下岩体破裂与裂隙水耦合特征是其合理性的关键。构建损伤变量与塑性应变、应力之间的相关关系,得到完整岩块的拉、加压卸载损伤演化方程;以平方拉剪应力与Benzeggagh-Kenane为初始、完全断裂准则,建立塑性位移与强度劣化关系,建立加卸载韧性断裂本构关系;基于实验数据建立贯通裂隙加卸载剪切本构关系。以基本方程与状态方程为基础,结合浸没边界方法,形成裂隙岩体水力学模拟理论。由此编制流体动力学-有限离散元CFD-FDEM耦合程序,模拟研究特厚煤层底板断层活化突水过程。结果表明:CFD-FDEM耦合程序可数值实现特厚煤层底板断层从(准)连续体到离散体转化,以及断层带裂隙水运移过程。底板断层采动破坏包络线呈W形,最深位于断层及其上盘(48.6m),最浅位于断层下盘(23m)。特厚煤层采场底板断层及其上盘受到较大超前集中应力,而后在采空区内大幅卸载,导致该位置出现显著二次破坏,并形成主要导水通道。研究成果为特厚煤层工作面底板断层水害防治提供理论支撑。

考虑跨洋特征及碎冰对快速性影响的极地探险邮轮型线优化

[目的]全球减碳背景下,为应对极地船舶设计建造的环保要求,需开展跨洋航行时碎冰水域对极地探险邮轮船体型线优化设计的影响规律研究,以获得最佳节能型线。[方法]针对极地探险邮轮的跨洋航行特征,采用航区权重方法进行量化评估,分析碎冰对阻力和推进效率的影响。应用计算流体力学耦合离散元法(CFD-DEM)来分析螺旋桨的碎冰水域性能,建立以联合自航功率为目标的优化模型,进而对全船参数化模型开展设计航速下的优化计算。[结果]计算结果表明,优化后的船型可以满足排水量要求,有效降低了2种水域下的航行功率,其联合自航功率降低了9.71%。[结论]研究成果验证了基于权重优化方法的可行性和合理性,可为极地探险邮轮的型线和推进器优化设计提供参考。

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。

脉动流化床内青刀豆颗粒的混合特性数值模拟

青刀豆一般采用流化床装置进行速冻。为了解决其床层堵塞和聚集不良的问题,进一步提高青刀豆的混合效率,课题组建立了三维混合脉动流化床模型,将计算流体力学与离散单元法(computational fluid dynamics-discrete element method,CFD-DEM)相结合,探究1,2,4和8 Hz 4种不同频率的脉动进口风速下青刀豆颗粒的流化混合情况。仿真结果表明:脉动流化床对于青刀豆颗粒混合具有一定改善作用,青刀豆颗粒的轴向混合强于径向混合,4 Hz时颗粒混合标准偏差值最小;当进口风速的频率接近颗粒的自主振动频率时,床层膨胀最明显,出现类似“共振”的现象;2 Hz时颗粒径向、轴向混合效果最佳,频率过大或过小对于流态化增强效果不明显。

Study of heat transfer by using DEM–CFD method in a randomly packed pebble-bed reactor

The pebble-bed reactor is one of the most promising designs for the nuclear energy industry. In this paper,a discrete element method-computational fluid dynamics(DEM-CFD) approach that includes thermal conduction, radiation, and natural convection mechanisms was proposed to simulate the thermal-fluid phenomena after the failure of forced circulation cooling system in a pebble-bed core. The whole large-scale packed bed was created using the DEM technique, and the calculated radial porosity of the bed was validated with empirical correlations reported by researchers. To reduce computational costs, a segment of the bed was extracted, which served as a good representative of the large-scale packed bed for CFD calculation. The temperature distributions simulated with two different fluids in this DEM-CFD approach were in good agreement with SANA experimental data. The influence of the natural convection mechanism on heat transfer must be taken into account for coolants with strong convective capacity. The proposed DEM-CFD methodology offers a computationally efficient and widely applied method for understanding the heat transfer process in a pebble-bed core. The method can also be easily extended to assess the passive safety features of newly designed fluoride-salt-cooled pebble-bed reactors.

CFD-DEM simulation of fluid-solid flow of a tapered column separation bed

Research on recycling waste Printed Circuit Boards(PCB) is at the forefront of preventing environmental pollution and finding ways to recycle resources.The Tapered Column Separation Bed(TCSB) is invented aiming at disposing the problem that fine particles of waste printed circuit boards cannot be separated efficiently so as to obtain further insight about the underlying mechanisms and demonstrate the separation feasibility in the tapered column separation bed.In this work,a Computational Fluid Dynamics(CFD) coupled with Discrete Element Method(DEM) model for two-phase flow has been extended to simulate the fluid-solid flow in the tapered column separation bed.Its validity is demonstrated by its successful capturing the key features of particles' flow pattern,velocity,the pressure distribution,the axial position with time and axial force for particles with different densities.Simulation results show that the plastic particles and resin particles become overflow,while copper particles,iron particles and aluminum particles successively become underflow,with a discharge water flow rate of 1 m^3/h,an obliquity of 30°.The simulated results agree reasonably well with the experimental observation.Using this equipment to separate waste PCBs is feasible,theoretically.

共轴对转螺旋桨气动与噪声特性研究进展

共轴对转螺旋桨由于拉力大、气动效率高而备受人们的关注。本文从共轴对转螺旋桨的试验研究、气动性能评估、气动模拟、气动噪声预测、气动噪声优化设计等方面进行了综述。首先,总结了目前国内外在共轴对转螺旋桨气动及噪声方面的试验成果;随后,介绍了共轴对转螺旋桨气动力计算的工程方法,并详细阐述了北京航空航天大学陆士嘉实验室提出的共轴对转螺旋桨气动力评估的片条理论发展与参数选择;然后,介绍了共轴对转螺旋桨数值模拟方法,并针对共轴对转螺旋桨气动噪声预测的频域方法和时域方法进行了论述;最后,介绍了目前共轴对转螺旋桨气动噪声优化设计的研究成果。

基于浸入边界法的高解析度CFD-DEM流固耦合方法

根据流固耦合问题的普遍性,提出了基于浸入边界法的高解析度计算流体力学-离散元法(CFD-DEM)流固耦合方法.新方法采用基于欧拉框架的计算流体力学方法描述流体的运动,采用基于拉格朗日框架的离散元法描述固体的运动及碰撞,在离散单元的表面布置浸入边界点,解决固体运动过程中与流体间的移动且未知的边界问题.为验证方法的准确性,模拟了圆柱绕流涡激振动、方块驰振两个经典算例,计算结果与数值解吻合度高,说明新方法能够准确描述流固耦合作用.最终,将该方法应用于多块体沉降的模拟,结果表明新方法能够反映流场的复杂变化,有效处理包含大量任意形状离散块体碰撞的流固耦合问题.