A Numerical Simulation of the Boycott Effect

The lattice Boltzmann method has been used to simulate the velocity field induced and the motion of an ensemble of particles during the sedimentation process in inclined tubes. The simulations show the trajectories and flow behavior of individual particles and particle-particle and particle-wall interactions as well as the formation of particle clusters. The global convection motion that was experimentally observed during such processes and tends to enhance the sedimentation process is also reproduced numerically. In addition we have found that smaller intermittent vortices, formed from the wakes of groups of settling particles, also play an important role in the sedimentation process and the final distribution of particles.     

Interaction of two touching spheres in a viscous fluid

The nature and effects of contacts between suspended particles were studied through a process in which a heavy sphere falls past a light sphere in a viscous fluid at low Reynolds number. Teflon and nylon spheres were used for the heavy and light spheres, respectively, with natural surface roughness and with the nylon sphere artificially roughened. Because of the existence of microscopic roughness on the sphere surfaces, the particles are able to make physical contact, breaking the symmetry of the trajectory predicted by hydrodynamic theory for smooth spheres. The experimental results are compared with numerical results calculated according to the theory of Davis (Phys. Fluids A 4 (1992) 2607), with a particular focus on the rotational velocities of the spheres. The numerical results from the roll/slip model provide the best fit of the experimental data. Instead of locking together like a rigid body and rotating together, two spheres initially roll without slipping and then roll with slipping after the maximum friction force is reached.     

Experiment and lattice Boltzmann simulation of two-phase gas-liquid flows in microchannels

The two-phase flows in microchannels have many advantages in heat and mass transfer compared to single-phase flows. In particular, segmented flows such as bubbly and slug flows are often used in microfluidic devices. In the present study, experiments and Lattice Boltzmann simulations were carried out to study the gas-liquid flow in microchannels under various conditions. Two types of mixer geometries were used, including the cross-shape and the converging shape channels. The bubble shape, bubble size, and formation mechanism were investigated for different flow rates and different mixer geometries. The simulation results and the experimental results were compared based on dimensionless numbers, and good agreement was found in general. Different flow regimes with different bubble shapes were found depending on the Capillary number of the flow. The simulation data confirmed that the breakup was induced by the pressure difference in the two phases for small Capillary numbers. The geometry of the mixing section was also observed to have an impact on the size of the gas and liquid slugs.     

节流间隙宽度对直通式迷宫密封泄漏量的影响

应用格子Boltzmann方法计算直通式迷宫密封中不同间隙宽度对迷宫密封流场和泄漏的影响。计算结果与分析表明,不同间隙宽度对密封不可压缩流体流场和泄漏量有一定的影响,随着间隙宽度的增加,泄漏量增大,在保证安全的情况下应尽量减小间隙宽度。     

Sedimentation from surface currents generated by particle-laden jets

In this paper, we develop a theoretical model for the dynamics and deposition patterns from surface currents generated by axisymmetric particle-laden jets. Our model incorporates the interaction between the particles in the environment and the continuing jet. We show that this interaction plays a central role in the evolution of the behaviour of the jet and on the sedimentation patterns on the surrounding floor. We consider surface currents generated by the impingement of the turbulent jet on both a free surface and on a solid surface. Our theoretical predictions are successfully compared with data from laboratory experiments. For the case of a jet impinging on a free surface, we also examine the evolution of the concentration of particles in the environment and in the surface current until steady state is attained. Finally, we propose a simple analytical expression for the sedimentation rate on the floor in the case of a dilute jet.     

Comments on ‘Lattice Boltzmann simulation of alumina-water nanofluid in a square cavity’ by Yurong He,Cong Qi,Yanwei Hu,Bin Qin,Fengchen Li and Yulong Ding

This work presents some comments concerning the paper entitled ‘Lattice Boltzmann simulation of alumina-water nanofluid in a square cavity’ by Yurong He, Cong Qi, Yanwei Hu, Bin Qin, Fengchen Li and Yulong Ding which was published in Nanoscale Research Letters in 2011. The comments are related to the numerical parameters and the computed results of average Nusselt number.     

Multiscale lattice Boltzmann modeling of two-phase flow and retention times in micro-patterned fluidic devices

Recent advances for fabricating micro-featured architectures such as posts or pillars in fluidic devices provide exciting opportunities for multiphase flow management. Here we describe a novel, multiscale modeling approach for two-phase flows in microfeatured architectures developed within the Shan and Chen Lattice Boltzmann method. In our approach a fine scale is used to resolve the true microfeatured architecture, with a coarser scale used to model the gross geometry of the device. We develop the basic features of the approach and demonstrate its applicability to modeling retention times of droplets of a dispersed phase in an array of microposts – an architecture used in microfluidic reactors, bioreactors, and biomedical devises. Additionally we show that it is feasible to model the microfeatured geometry in a piecewise manner which includes extrapolating dispersed phase flow characteristics in the entire system based on simulations in smaller subdomains.     

COMPUTATIONAL CLARIFICATIONS OF EXPERIMENTAL BLEND MORPHOLOGY TRANSITIONS IN IMMISCIBLE POLYMER MELTS ORGANIZED BY CHAOTIC ADVECTION

Chaotic advection has been used recently to organize immiscible polymer melts into thousands of alternating thin layers and controllably obtain from layer breakup a wide variety of useful derivative blend morphologies. Experiments suggested that morphology changes in the multiple layers occur progressively, volumetrically, and interactively via gradual layer refinement and the formation and growth of holes (i.e., ruptures). To theoretically evaluate the experimental findings and also clarify morphology development in multilayer melts, interactive hole growth in multiple layers and associated morphology transitions have been investigated for experimentally observed random and ordered-hole arrangements using the lattice Boltzmann method (LBM). Results also provide a three-dimensional generalization to multiple layers of prior studies focused on hole growth in thin film coatings.     

Fluid-particle interaction from lattice Boltzmann simulations for flow through polydisperse random arrays of spheres

Fluid-solid drag force correlations, such as the Ergun relation, are widely used in many areas of chemical engineering. In many practical applications, the solid phase consist of an assembly of spheres which are, more often than not, polydisperse. In this paper we report on a study of the fluid-particle interaction by fully resolved DNS-type simulations (lattice Boltzmann) of flow through polydisperse random arrays of spheres, both for log-normal and Gaussian size distributions. In a recent paper [Van der Hoef, M.A., Beetstra, R., Kuipers, J.A.M., 2005. Lattice Boltzmann simulations of low Reynolds number flow past mono- and bidisperse arrays of spheres: results for the permeability and drag force. J. Fluid Mech. 528, 233] we have shown that a correction factor should be applied to the monodisperse drag force relations, when used for bidisperse systems. On the basis of the data reported in this paper, we conclude that the correction factor also applies to general polydisperse systems.     

COMPUTATIONAL CLARIFICATIONS OF EXPERIMENTAL BLEND MORPHOLOGY TRANSITIONS IN IMMISCIBLE POLYMER MELTS ORGANIZED BY CHAOTIC ADVECTION

Chaotic advection has been used recently to organize immiscible polymer melts into thousands of alternating thin layers and controllably obtain from layer breakup a wide variety of useful derivative blend morphologies. Experiments suggested that morphology changes in the multiple layers occur progressively, volumetrically, and interactively via gradual layer refinement and the formation and growth of holes (i.e., ruptures). To theoretically evaluate the experimental findings and also clarify morphology development in multilayer melts, interactive hole growth in multiple layers and associated morphology transitions have been investigated for experimentally observed random and ordered-hole arrangements using the lattice Boltzmann method (LBM). Results also provide a three-dimensional generalization to multiple layers of prior studies focused on hole growth in thin film coatings.     

Simulation of 3D velocity and concentration profiles in a packed bed adsorber by lattice Boltzmann methods

Full 3D simulations of velocity and concentration profiles were carried out for the several ordered packing arrangements of spherical particles with small tube-to-particle diameter ratio (<10) using lattice Boltzmann methods. The effects of voids and diffusion coefficients on the adsorption concentration profiles in a packed bed of circular cross-section were investigated. In particular, the radial (r) and circumferential (θ) dependencies of the concentrations due to non-uniform velocity and particle voids across tube's cross-section, especially near the walls, were ascertained. The lattice Boltzmann technique allows simultaneous solution to velocity and concentration fields at all locations inside the packed tube without using any empirical correlations for certain transport parameters, for example, dispersion coefficient. Depending upon the packing arrangements and the magnitudes of diffusion coefficient, the concentration gradients in r- and θ-directions were found to be significant. The lattice model simulation results were also compared to the tomographic data obtained in a tubular adsorber packed with the zeolites coated glass beads and were found to be in reasonably good agreement.     

管道泄漏污染物一维迁移的LBM模拟

石油管道泄漏现象时有发生,对环境造成了危害,研究埋地管道石油污染物泄漏尤为关键。在此采用Boltzmann研究方法,通过多尺度技术和局部平衡态分布函数的Chapman-Enskog展开得到运算的平衡态方程,并给出了石油管道污染物泄漏迁移的一维有源扩散方程的格子Boltzmann模型,通过C++软件数值模拟进行运算。最终得出结果与理论解一致,验证了用Boltzmann方法研究污染物泄漏迁移的可行性。     

Numerical model for the computation of permeability of a cemented granular material

We present a 3D model designed to compute permeability in a cemented polydisperse granular material composed of spherical grains. A non-cohesive granular deposit is constructed by means of the Discrete Element Method (DEM) then cement is deposited on grains using three simple models. Finally the solid sample is subjected to an upward hydraulic gradient in order to measure permeability. The fluid flow through the connected sample pores is modeled using the Lattice Boltzmann Method (LBM). The computed permeability coefficients are in good agreement with the existing classical values. The evolution of permeability with the cement deposit growth is studied for the three proposed cementation models.     

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method

Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (T_H), and the top wall is kept at a low constant temperature (T_C)) filled with Al₂O₃/H₂O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.     

界面Rayleigh对流现象模拟的格子Boltzmann方法

采用格子Boltzmann方法中带有双分布函数的LBGK模型,在介观尺度上实现了对气液传质过程中界面对流现象的模拟,比较了不同Sc数、Ra数以及有局部浓度扰动的界面溶质扩散,对发生界面Rayleigh对流的临界条件进行了研究,发现临界Ra数在1.0×104—2.0×104之间。模拟结果和已有实验现象一致,证明双分布的LBGK模型适用于界面对流及扩散现象的模拟。     

Simulation of packed bed reactors using lattice Boltzmann methods

Lattice Boltzmann (LB) methods are used to simulate hydrodynamics, reaction and subsequent mass transfer in a disordered packed bed of catalyst particles at sub-pore length-scales. In contrast to previous studies, a variety of modifications are introduced in the LB method enabling particle Pe numbers up to 10⁸, and hence realistic values of diffusivity, to be accessed. These include decoupling the hydrodynamics from mass transfer and the use of a rest fraction in the LB formulation of mass transfer. In addition the mass transfer simulations are modified to permit spatially varying values of diffusivity, essential to differentiate between intra- and inter-particle diffusivity (D_intra and D_inter, respectively). The simulation method is applied to both a disordered and ordered 2D packing for a range of Pe (15.6-1557.8) and Re (0.16-1.56) numbers, as well as various ratios of D_intra/D_inter (0-1), whilst simulating an esterfication reaction catalyzed by an ion-exchange resin. The value of D_intra is found to have limited effect, whilst reducing Pe number results in a considerable increase in overall conversion. The simulation method is then applied to a 3D lattice for which experimental conversion data is available. This experimental data is straddled by the simulation case of D_intra=0 and D_intra=D_inter, as expected.