喷动床反应器内流体动力特性的数值模拟

采用欧拉-欧拉双流体模型,颗粒动理学理论模拟颗粒相流动,对喷动床核反应器内的流体动力行为进行了数值模拟.模拟得到了喷动床核反应器内颗粒浓度标准方差、空隙率和颗粒速度分布.研究结果表明喷动床核反应器内颗粒浓度标准方差先逐渐增大至最大值后下降.分析了摩擦应力模型和倒锥体角度等影响因素对喷动床核反应器内的流体动力特性的影响.     

Adhesive particulate flow: The discrete-element method and its application in energy and environmental engineering

In this paper, recent advances in the discrete-element method (DEM) for describing motion, deposition, agglomeration or aggregation of a large number of adhesive spherical particles immersed in fluid flows, termed as adhesive particulate flow, are reviewed. The constitutive equations together with the length and time scales of DEM are compared with those of other similar Lagrangian particle methods, e.g., molecular dynamics (MD), Brownian dynamics (BD), dissipative particle dynamics (DPD). The adhesive contact force and torque models in the presence of different adhesive effects are examined, including van der Waals force, ligand-receptor binding, liquid bridging force, interface adhesion, and sintering forces, all of which play an important role in DEM formulations for different types of adhesive particulate flow problems of interest in energy, combustion and environmental fluid mechanics problems. A summary of various kinds of particle-field interactions is presented, including fluid forces, electric field forces, acoustic force, and thermophoretic force. The computational method is illustrated by application to a series of examples involving capture of spherical particles by a fiber in a uniform upstream flow, examining the deposition/aggregation patterns of both mono-size and binary-size particles on the cylinder with and without the presence of electric field effects, which may be due either to charging of the cylinder or polarization of the particles. Particle capture problems of this sort are commonly encountered in filtration problems and ash-removal problems experienced in environmental and combustion applications, respectively. The article concludes with a discussion of remaining modeling challenges in development of discrete-element methods for adhesive particulate flow fields.     

Combination of the fictitious domain method and the sharp interface method for direct numerical simulation of particulate flows with heat transfer

In this paper, the fictitious domain (FD) method and the sharp interface (SI) method are combined for the direct numerical simulations of particulate flows with heat transfer in three dimensions. The flow field and the motion of particles are solved with the FD method. The temperature field is solved in both fluid and solid media with the SI method. The accuracy of the proposed FD/SI method is validated via two problems: the natural convection in a two- dimensional cavity with fixed solid particles, and the flow over a cold sphere. The method is then applied to the natural convection in a three dimensional cavity with a fixed sphere, the motion of a spherical particle in a non-isothermal fluid, and the rising of spherical catalyst particles in an enclosure. The effects of the thermal conductivity ratio are examined in the first and third problems, respectively, and the significant effects of the thermal expansion coefficient ratio on the particle motion are demonstrated in the second problem.     

Dissipative particle dynamics simulations of water droplet flows in a submicron parallel-plate channel for different temperature and surface-wetting conditions

ABSTRACT

The effects of temperature-dependent thermophysical properties on droplet flow characteristics in a parallel-plate channel at submicron scale are investigated. The dissipative particle dynamics method with many-body (MDPD) and energy conservation (DPDe) configurations (MDPDe) was used. Droplet flows were simulated to study the effects of the temperature difference between top and bottom walls, body force on MDPDe particles, and wall-wetting conditions. The effects on the droplet flow were discussed. Droplet flows with a subzero wall temperature were simulated. An ice layer was formed on the wall. Its thickness and shape changed depending on surface wetting, temperature gradient, and body force.     

Liquid-solid separation phenomena of two-phase turbulent flow in curved pipes

The present study is to contribute some knowledge of phase separation phenomena of liquid-solid two-phase turbulent flow in curved pipes and provide a basis for the invention and development of a new type of curved pipe separator. Firstly, the solid-liquid two-phase flows in two-dimensional (2D) curved channels were numerically simulated using a two-way coupling Euler-Lagrangian scheme. Phase distribution characteristics of 2D curved channel two-phase flow were examined under conditions of different particle size, liquid flowrate and coil curvature. Based on the numerical results, the dynamic effects and contributions to phase separation of particle-subjected forces, including centrifugal force, drag force, pressure gradient force, gravity force, buoyancy force, virtual mass force and lift force, were exposed by kinematic and dynamic analysis along particle trajectories. Secondly, measurement of particle size and concentration profiles in helically coiled tube two-phase flow was conducted using a nonintrusive Malvern 2600 particle sizer based on laser diffraction. Particle size and concentration distribution characteristics of helically coiled tube two-phase flow and the effect of secondary flow on phase separation were analyzed based on experimental data.     

Large eddy simulation of particle response to turbulence along its trajectory in a backward-facing step turbulent flow

In order to understand particle response to turbulence along its path, properties of the particle phase and gas phase are compared and analyzed for a turbulent flow over a backward-facing step. The turbulent gas phase is simulated numerically using large eddy simulation and the particle phase is modeled by means of Lagrangian methods. The particle Stokes number ranges from 0.01 to 111.18 and the Reynolds number is 18,400, based on the step height and the inlet mean velocity. Particle velocities, fluid velocities and vorticity along particle trajectory as well as particle dispersion are obtained so as to provide information on particle response to turbulence. Results show that particle behavior depends heavily on the local fluid turbulence along its path, especially for small particles. Particles follow a path along which the gas-phase vorticity is small. However, large particles maintain the inertia along their trajectories without responding to the fluid fluctuations.     

Numerical calculation of the dynamic behavior of solid particles through the inlet box with complex geometry of a fluid machine

A finite element method is used to calculate the three-dimensional gas flow field in the inlet box of a large-size fluid machinery,The dynamic behaviors of solid particles through the inlet box are obtained numerically by the “one way coupling” method .The trajectories of different-sized particles and the distribution of particle positions on the outlet plane of inlet box are investigated.     

Numerical prediction of a two-phase fluid driving system using cavitating flow of magnetic fluid

A new concept of a two-phase fluid driving system using cavitating flow of a magnetic fluid is proposed, and the driving and acceleration performance of the system is numerically predicted. A typical computational model for cavitating flow of a magnetic fluid is proposed and several flow characteristics, taking into account the strong nonuniform magnetic field, are numerically investigated to realize the further development and high performance of the proposed new type of two-phase fluid driving system using magnetic fluids. Based on numerical results, the two-dimensional structure of the cavitating flow as well as the cloud cavity formation of the magnetic fluid through a vertical converging–diverging channel are shown in detail. The numerical results demonstrate that an effective two-phase magnetic driving force and fluid acceleration can be obtained by the practical use of magnetization of the working fluid. Also clarified is the cavitation number in the case of a strong magnetic field with a larger value than that in the case of a nonmagnetic field. Magnetic control for suppression of cavitation bubbles is remarkably enhanced in the condition of high Reynolds number. Further clarified is the precise control of the cavitating flow of magnetic fluid that is possible by effective use of the magnetic body force that acts on cavitation bubbles.     

Thermophoresis of particles in aqueous solution in micro-channel

A theoretical analysis of micro motion of particles under the action of thermophoresis in aqueous electrolyte solution is presented in this paper. A two-dimensional, incompressible and laminar fluid flow model is proposed to simulate the fluid flow and particle motion trajectory in micro-channel. The effects of thermophoretic force and viscous force on the particle are calculated. The particle trajectories considering the effects of particle size, inlet flow velocity and temperature difference between bottom side and top side in micro-channel with a length of 2000 μm and a height of 500 μm are simulated. The effect of thermophoresis on separation motion of particles in micro-channel is analyzed. The results show that thermophoretic force increases with the increase of temperature gradient in micro-channel and particles with smaller diameter, smaller density and smaller gravity acceleration can pass the micro-channel more easily.     

Adaptive-Network-Based Fuzzy Inference System Analysis to Predict the Temperature and Flow Fields in a Lid-Driven Cavity

Heat transfer behavior in a 2-D square lid-driven cavity has been studied for various pertinent Reynolds and Rayleigh numbers. The lattice Boltzmann method, a numerical tool based on the particle distribution function is applied to simulate a thermal fluid flow problem. Bhatnagar-Gross-Krook (BGK) is combined with the double population thermal Lattice Boltzmann model to solve mixed convection in a square cavity. An adaptive-network-based fuzzy inference system (ANFIS) method is trained and validated using BGK Lattice Boltzmann model results. The results show that the trained ANFIS model successfully predicts the temperature and flow fields in a few seconds with acceptable accuracy.     

基于CFD DEM微细带肋内冷通道中颗粒沉积特性比较研究

航空发动机全天候全域长航时运行时,颗粒随着二次流空气系统进入到涡轮叶片内部,沉积堵塞在涡轮叶片内冷通道中,严重影响了涡轮叶片的冷却性能。本文采用计算流体力学和离散单元法(CFD-DEM)相结合的方法研究了涡轮叶片带肋细小矩形内冷通道中微尘颗粒的流动和沉积特性。所研究的内冷通道肋片周期性布置在通道的一侧,肋片阻塞比和肋间距比分别为0.024和10,考虑了平行直肋、45°斜肋和45°V肋3种肋结构,详细分析了雷诺数、颗粒斯托克斯数、入口颗粒体积分数和肋片的类型对颗粒流动和沉积特性的影响规律。结果表明：颗粒沉积主要发生在第1根肋片的前缘处;颗粒的沉积质量均随着雷诺数、斯托克斯数和颗粒体积分数增加而减小;在所有的肋片类型中,直肋布置时颗粒沉积现象最明显,其次是V肋,斜肋拥有最小的颗粒沉积质量。     

Thermophoresis of a radiating aerosol in thermally developing Poiseuille flow

Radiation and thermophoresis interaction in thermally developing laminar flow in a constantwall-temperature parallel-plate channel is investigated. The fluid is a radiatively nonparticipating gas containing emitting, absorbing, and isotropically scattering gray aerosol particles. The channel walls are opaque, gray and diffuse. Formal relations developed for the radiation part are used together with the discretized forms of the energy and particle conservation equations to solve the problem numerically through an iterative scheme. Various results are presented to illustrate the effects of the parameters of the problem on the temperature and particle concentration distributions, as well as on particle deposition on the channel walls when they are cold and on the development of the particle-free zone along the walls when they are hot.     

Numerical study on bubble behavior in magnetic nanofluid used for waste heat recovery power generation concept

Electrical energy can be generated by the bubble motion inside the magnetic nanofluid under the influence of an external magnetic field. The relative movement of the magnetic particles dispersed in the magnetic fluid is induced through the movement of the bubbles rising by buoyancy force. This disturbs the external magnetic field associated with the generator coil, and electrical energy can be generated. The bubble movement in this complex physical environment was studied through 2D numerical analysis. Commercial magnetic fluids EFH1 and EFH3, manufactured by Ferrotec, were selected as the working fluid for the investigation. A level set method was used to analyze the 2‐phase flow of bubbles motion in the magnetic fluid. The effect of magnetic particle concentration on the behavior of bubbles and the change of bubble flow patterns through interaction between bubbles were observed by analysis. In addition, the influence of the magnetic force caused by the external magnetic field on the behavior of the bubble was also investigated. The following results can be obtained through the analysis of this study. The high concentration of magnetic particles increases the viscosity and attenuates the rising velocity and the lateral oscillation of the bubbles. The interaction of the 2 bubbles depends on the initial relative distance. Merging occurs only between 2 bubbles within a certain initial distance, which maximizes disturbance of the surrounding magnetic fluid. The magnetic force exerted by the permanent magnets externally applied is relatively small in comparison with gravity. Therefore, the effect on the rise behavior of the bubble is not significant. In consideration of the overall external force and flow conditions, the pattern of the bubble flow that maximizes the efficiency in the present electric energy generation concept was found.     

Numerical simulation of fluid flow and convection heat transfer in sintered porous plate channels

Fluid flow and convective heat transfer of water in sintered bronze porous plate channels was investigated numerically. The numerical simulations assumed a simple cubic structure formed by uniformly sized particles with small contact areas and a finite-thickness wall subject to a constant heat flux at the surface which mirrors the experimental setup. The permeability and inertia coefficient were calculated numerically according to the modified Darcy’s model. The numerical calculation results are in agreement with well-known correlation results. The calculated local heat transfer coefficients on the plate channel surface, which agreed well with the experimental data, increased with mass flow rate and decreased slightly along the axial direction. The convection heat transfer coefficients between the solid particles and the fluid and the volumetric heat transfer coefficients in the porous media predicted by the numerical results increase with mass flow rate and decrease with increasing particle diameter. The numerical results also illustrate the temperature difference between the solid particles and the fluid which indicates the local thermal non-equilibrium in porous media.     

气固流化床的离散颗粒运动-碰撞解耦模型与模拟

基于分子动力学和气固两相流体动力学，建立流化床稠密气-固两相离散颗粒运动-碰撞解耦模型，采用硬球模拟方法处理颗粒与颗粒之间的碰撞，及大涡模拟方法处理气相湍流流动．单颗粒运动满足牛顿第二定律，颗粒相和气相相间相互作用的双向耦合由牛顿第三定律确定，数值模拟二维鼓泡流化床内稠密气-固两相流动，得到了气泡的形成、发展及颗粒的流化过程，计算结果表明颗粒弹性恢复系数影响气-固两相流动特性。     

散粒土管涌临界水力梯度的研究

针对传统管涌判别方法的局限性问题,通过对渗流场中土体孔隙通道内的可动颗粒进行力的平衡分析,得到管涌发生的临界水力梯度表达式。在进行颗粒受力分析时,根据流体力学中相对运动原理,通过静水中运动物体之间的相互作用力,进而推求出受周围颗粒影响的水流拖曳力大小。该判别方法除了考虑周围颗粒对起动颗粒水流拖曳力的影响,同时还考虑了颗粒半径、孔隙率、土体组成、土颗粒所处的位置及孔隙水流方向等各方面因素的影响,最后比较该方法计算的理论值与既有管涌试验结果,两者吻合度较好。研究成果可为散粒土管涌临界水力梯度的确定提供理论依据。     

A Numerical Analysis of the Fiber Web Effects on the Pressure Drop,Particles Collection,and Heat Transfer in a Microchannel

In this paper, pressure drop, heat transfer characteristics, and particle deposition in a microchannel with a fiber web at the inlet are investigated numerically. The fiber web was made up of fibers several hundred micrometers in length caught at the entrance of the channels. Governing equations for the flow field are solved by an Eulerian approach, while the equations of particle motion in the flow are solved by a Lagrangian approach. Assuming the symmetry in the domain, one channel and the corresponding inlet and outlet plenums are selected as the computational domain. Several fiber webs with various fiber numbers, orientations, and dimensions are modeled. The increase in the pressure drop and the decrease of heat transfer due to the fiber web are computed and discussed. A correlation is developed for pressure drop as a function of the fiber web blockage ratio, microchannel geometry, and flow characteristics. The deposition of the microparticles with various diameters on the fiber webs is investigated, as well. Deposition of the particles on the fiber web is because of two different mechanisms, inertial impaction and interception. The numerical results indicate that the fiber webs have no considerable effect on the heat transfer characteristic of the channel under constant pumping power.     

煤粉颗粒对管道壁面磨损的数值模拟研究

应用计算流体动力学（CFD）研究煤灰颗粒穿过槽道中10×11错列排列AISI 304不锈钢管束时槽道壁面受到的磨损状况。流场采用直接数值模拟,流场与管束之间的耦合采用内嵌边界技术求解,颗粒轨道采用拉格朗日轨道模型求解并考虑颗粒与流体之间的双向耦合。通过选用200μm粒径的颗粒来分析颗粒对壁面的磨损状况。颗粒对壁面的碰撞频率和壁面局部质量磨损率的分布表明,壁面局部质量磨损率对壁面的磨损量起关键作用,壁面局部质量磨损量的分布也与预测值相符合。     

Lattice-Boltzmann simulations of flow past stationary particles in a channel

Direct numerical simulations of turbulent flow past stationary particles in a channel are carried out to determine the influence of the particle on the flow structure. The position of the particles in the channel, the particle size, the Reynolds number, and the number of particles are varied. The studies are carried out using the lattice-Boltzmann method. The details of the flow field are analyzed to provide insight into the factors that control the distance of influence. In the case of a single particle, when the particle is close to the wall, the mean wake moves away from the wall. Moreover, there is only one dominant vortex in the wake instead of two symmetric vortices. In the case of multiple particles, the vortices shed from the particles interact with each other. In the single particle cases considered, the effect of the particle is felt for about 20 diameters downstream. When multiple particles are present, interaction between the vortices shed by the particles lengthens the distance of influence.