Experimental and numerical investigation of liquid circulation induced by a bubble plume in a baffled tank

Bubble induced liquid circulation is important in applications such as bubble columns and air-lift reactors. In this work, we describe an experimental and numerical investigation of liquid circulation induced by a bubble plume in a tank partitioned by a baffle. The baffle divides the tank into two compartments. Liquid can flow from one compartment to the other through openings at the top and the bottom of the baffle. Gas (air) was injected in the riser section in the form of bubbles at one corner of the tank. The temporal and spatial variation of velocity field in the liquid as a function of the gas flow rate was measured using particle image velocimetry (PIV). At a constant gas flow rate, the liquid flow field is unsteady due to the interaction with the bubbles. The time scales associated with the velocity-time series and the bubble plume thickness variation were calculated. The time averaged-velocity field was used to quantify the variation of the liquid circulation rate with gas flow rate. The turbulence in the liquid was measured in terms of turbulent intensities. These were calculated from the experimental data and were observed to be less than 3 cm/s. A 2-d Euler-Euler two-fluid model with buoyancy and drag as the interaction terms was used to simulate the flow. The parameters chosen for the simulations were selected from literature. It is shown that inclusion of turbulence model such as k-ε is necessary to capture the overall flow behavior. Good agreement was observed between experimentally obtained velocity profiles and the recirculation rates with the simulation results.     

Experimental study on the effects of gas permeation through flat membranes on the hydrodynamics in membrane-assisted fluidized beds

Recently, many novel reactor concepts based on membrane fluidized bed reactors have been proposed. In this work, the effects of gas permeation through flat membranes on the hydrodynamics in a pseudo-2D membrane-assisted gas–solid fluidized bed have been investigated experimentally. A combination of the non-invasive techniques (Particle Image Velocimetry (PIV) and Digital Image Analysis (DIA)) was employed to simultaneously investigate solids phase and bubble phase properties in great detail. Counter-intuitively, addition of secondary gas via the membranes, that constituted the confining walls of a gas–solid suspension at conditions close to incipient fluidization, did not result in a larger, but in a smaller equivalent bubble diameter, while gas extraction on the other hand, resulted in a larger equivalent bubble diameter, although in this case the effect was less pronounced. This could be explained by changes in the larger scale particle circulation patterns due to gas extraction and addition via the membranes.     

Experimental and numerical investigation of structure and hydrodynamics in packed beds of spherical particles

In chemical industry, flows often occur in nontransparent equipment, for example in steel pipelines and vessels. Magnetic resonance imaging is a suitable approach to visualize the flow, which cannot be performed with classical optical techniques, and obtain quantitative data in such cases. It is therefore a unique tool to noninvasively study whole‐field porosity and velocity distributions in opaque single‐phase porous media flow. In this article, experimental results obtained with this technique, applied to the study of structure and hydrodynamics in packed beds of spherical particles, are shown and compared with detailed computational fluid dynamics simulations performed with an in‐house numerical code based on an immersed boundary method‐direct numerical simulation approach. Pressure drop and the radial profiles of porosity and axial velocity of the fluid for three packed beds of spheres with different sizes were evaluated, both experimentally and numerically, in order to compare the two approaches. © 2018 The Authors AIChE Journal published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers AIChE J, 64: 1896–1907, 2018     

A numerical study of particle wall-deposition in a turbulent square duct flow

The deposition of dense solid particles in a downward, fully developed turbulent square duct flow at Re_τ = 360, based on the mean friction velocity and the duct width, is studied using large eddy simulations of the fluid flow. The fluid and the particulate phases are treated using Eulerian and Lagrangian approaches, respectively. A finite-volume based, second-order accurate fractional step scheme is used to integrate the incompressible form of the unsteady, three-dimensional, filtered Navier-Stokes equations on an 80 × 80 × 128 grid. A dynamic subgrid kinetic energy model is used to account for the unresolved scales. The Lagrangian particle equation of motion includes the drag, lift, and gravity forces and is integrated using the fourth-order accurate Runge-Kutta scheme. Two values of particle to fluid density ratio (ρ_p/ρ_f = 1000 and 8900) and five values of dimensionless particle diameter (d_p/δ × 10⁶ = 100, 250, 500, 1000 and 2000, δ is the duct width) are studied. Two particle number densities, consisting of 10⁵ and 1.5 × 10⁶ particles initially in the domain, are examined.Variations in the probability distribution function (PDF) of the particle deposition location with dimensionless particle response time, i.e. Stokes number, are presented. The deposition is seen to occur with greater probability near the center of the duct walls, than at the corners. The average streamwise and wall-normal deposition velocities of the particles increase with Stokes number, with their maxima occurring near the center of the duct wall. The computed deposition rates are compared to previously reported results for a circular pipe flow. It is observed that the deposition rates in a square duct are greater than those in a pipe flow, especially for the low Stokes number particles. Also, wall-deposition of the low Stokes number particles increases significantly by including the subgrid velocity fluctuations in computing the fluid forces on the particles. Two-way coupling and, to a greater extent, four-way coupling are seen to increase the deposition rates.     

A numerical investigation of aerosol dynamics in a wall-less reactor

This paper describes a numerical investigation of aerosol formation during silane decomposition in a wall-less reactor. The wall-less reactor is amenable to numerical investigation because the homogeneous chemical reactions leading to the formation of solid particles are isolated from heterogeneous effects, such as occur at the walls of a laminar flow aerosol reactor. The flow/heat transfer and gas-phase chemical kinetics are simulated utilizing separate one-way coupled models. The aerosol dynamics model is based on a simplified sectional model originally developed by Okuyama et al. This model is modified to allow for the simulation of particle growth via condensation. Simulations have been performed which indicate that particle growth via condensation may be an important process. Additionally, the effects of total reactor pressure, temperature and inlet silane concentration on the dynamics of the aerosol population have been investigated. Conditions which result in the formation of larger and more numerous particles have been identified.     

不同尺寸异径管的冲蚀数值模拟研究

运用Fluent软件中的DPM模型对不同入口颗粒浓度下不同尺寸异径管的冲蚀磨损进行数值模拟研究。通过数值模拟得出异径管冲蚀情况与入口浓度和尺寸之间的关系。结果表明,不同尺寸异径管的严重冲蚀区域都位于变径区域壁面;在相同的入口速度下,异径管的冲蚀磨损随着入口浓度的增大而上升;异径管的冲蚀磨损随着变径角度的上升成先上升后下降的现象,且冲蚀严重区域形状随角度的上升从斑点状向环状转变,并存在最严重冲蚀角度。     

Direct numerical simulation of particle collisions in two-phase flows with a meshless method

The element-free Galerkin (EFG) method was used to simulate particle motion in two-phase flows with a new node distribution arithmetic suitable for meshless methods. The control equations were discretized with the standard Galerkin method in space and a fractional step finite element scheme in time. Regular background cells were used for the quadrature. The forces of the fluid on the particles were obtained by integrating the stress and shear forces on the particle surfaces. Simulation of the movement of a particle in a channel showed the Karman vortex street forming behind the particle with increasing particle velocity. Simulation of the movement of two particles in a channel showed the well-known drafting, kissing and tumbling, which has been obtained experimentally. Multi-particle flows were simulated with the results showing that meshless methods are capable of dealing with real particle collisions, which makes meshless methods superior to all mesh-based methods. Moreover, the theoretical relation of interparticle collision rate derived using kinetic theory was compared with the present numerical results and it is found that the collision rate is much lower than the theoretical estimation based on kinetic theory.     

Experimental and numerical investigation on three-point bending behaviors of unidirectional warp-knitted composites

This study develops a novel specimen model, which considers the progressive damage of the layers, the knitting yarns, and the interlaminar cohesive zone, to investigate the flexural properties and the interlaminar shear properties of the unidirectional warp-knitted composites. Three-point bending tests are conducted as verification of the numerical model. Improved strain-based Hashin criteria are proposed to analyze the shear nonlinearity. Results show that specimens with small span-to-thickness ratios exhibit obvious nonlinear behaviors and the corresponding simulation results are sensitive to the value of the shear nonlinear factor. Failure mechanism and stress distributions are analyzed based on numerical simulations. The effect of the specimen size on bending behaviors is discussed. The influence of the width is found to be negligible but that of the span-to-thickness ratio is significant. The ranges of the span-to-thickness ratio corresponding to different failure modes are given. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48132.     

Experimental investigation of particle contact time at the wall of gas fluidized beds

The contact time of particles at the walls of gas fluidized beds has been studied using a radioactive particle tracking technique to monitor the position of a radioactive tracer. The solids used were sand or FCC particles fluidized by air at room temperature and atmospheric pressure at various superficial velocities, covering both bubbling and turbulent regimes of fluidization. Based on the analysis of tracer positions, the motion of individual particles near the walls of the fluidized bed was studied. The contact time, contact distance and contact frequency of the particles at the wall were evaluated from these experimental data. It was found that in a bed of sand particles, the mean wall contact time of the fluidized bed of sand particles decreases by increasing the gas velocity in the bubbling and increases in the turbulent fluidization. In other words, the particle-wall contact time is minimum at the onset of turbulent fluidization in the bed of sand particles. However, the mean wall contact time is almost constant in both regimes of fluidization in the bed of FCC particles. All the existing models in the literature predict a decreasing contact time when the gas velocity in the bed is increased. It was also shown that the contact distance increases monotonously by increasing the gas velocity in the bed of sand particles, while it is almost constant for the bed of FCC particles. Contact frequency has a trend similar to that of the contact time for both sand and FCC particles.     

Mechanical behaviour of porous lanthanide oxide microspheres: Experimental investigation and numerical simulations

Actinide oxide microspheres are considered as promising substituents to powder precursors for the production of ceramic pellets of nuclear fuel or targets. Porous microspheres of sub-millimetric size are synthesised using the Weak Acid Resin process. Controlling their microstructure and their mechanical properties is essential to predict the microstructure of green compacts and sintered pellets. Here, cerium and gadolinium are used to mimic actinides as metal cation. Single microspheres are crushed experimentally using a micropress in a Scanning Electron Microscope (SEM) to investigate their mechanical properties and visualise their fracture behaviour. The results are compared to numerical simulations based on the Discrete Element Method (DEM). In DEM, a microsphere is modelled as an assembly of bonded spheres representing aggregates. Bonds may fracture in tension or shear. A limited number of material parameters (aggregate elastic modulus, bond strength) are sufficient for the accurate simulation of the fracture behaviour of a microsphere.     

Experimental and numerical analysis of a lab-scale fluid energy mill

This study investigates the grinding performance of Fluid Energy Mill (FEM) through experimental studying and numerical simulation. The experimental parametric study shows that the mean product particle size decreases with grinding pressure (GP) and increases with the solid feed rate (SFR). In comparison, the influence of the feed pressure (FP) on the product size is much less significant. Visualization study indicates the existence of a particle-concentrated layer near the peripheral wall region, named the grinding region in this article since most of the collision-induced size reduction occurred in this region. The grinding air streams re-orient the particles, facilitating particle-particle and particle-wall collisions downstream in the grinding region. To understand the influence of the particle-wall collision, the peripheral wall of FEM was coated with a foam film in some experiments. The particle-wall collision was found to play a significant role in size reduction, especially under low air pressure.The gas flow inside the grinding chamber was simulated as the initial step to the ongoing 2-phase flow simulation of the milling process in the FEM. The simulation results show that eddies are formed at the feed air entrance, which explains the tendency of fine particle deposition in this region. The simulation results also suggest a strong relationship between the GP and the mean gas velocity in the grinding region.     

Three-dimensional numerical simulation of a horizontal rotating fluidized bed

Three-dimensional numerical simulations of a horizontal rotating fluidized bed (RFB) containing glass bead particles (d_s = 82 μm, ρ_s = 2450 kg/m³) and washed alumina (d_s = 89 μm, ρ_s = 1550 kg/m³) were performed. FLUENT 6.1 software was used to carry out our simulation. The numerical results were compared with the experimental data of Qian and Pfeffer et al. [G.H. Qian, I. Bagyi, I.W. Burdick, R. Pfeffer, H. Shaw, Gas-Solid Fluidization in a Centrifugal Field.” AIChE J. 47 (5) (2001) 1022-1034]. The rotating speed of the RFB was set at 325 rpm (34 rad/s), which is equivalent to a centrifugal acceleration of 7 g.The flow behavior of the solid particles was analyzed; the bed thickness and the calculated pressure drop were compared with the experimental results. Our calculated pressure drop agreed very well with the experimental results.     

Experimental and numerical investigation on performance of a porous medium burner with reciprocating flow

Experimental and two-dimensional numerical investigations on the performance of an inert porous media burner with reciprocating flow are presented. Attention was focused on the combustion temperature and pressure loss in the burner, which was, respectively, packed with 4PPC (Pores Per Centimeter) ceramic foams or alumina pellets with various sizes. Results show that material and structures of porous media have significant influence on the burner performance, and that ceramic foam with high porosity is suitable for using in the combustion region whereas alumina pellets should be placed in the heat exchange zone. In addition, the highly two-dimensional characteristics of the porous media burner are validated by the numerical model, which include temperature distributions, species profile and flame structure. Numerical results were validated against experiment data.     

Experimental study on the time change in fluidity and particle dispersion state of alumina slurries with and without sintering aid

The time change of the fluidity and particle dispersion state of alumina slurries with and without a sintering aid were investigated. The apparent viscosity of the slurry was measured at certain intervals. The hydrostatic pressure of the slurry, which represents the particle dispersion state, was also measured. We showed that the pH value, adsorbed amount of dispersant, and Mg ion concentration of the slurry hardly changed with time even though the apparent viscosity of the slurry increased with time. We suggest that the time change in the apparent viscosity, that is, the behaviour in which the apparent viscosity of the slurry increased with time, occurs when the electrostatic repulsion force is insufficient to maintain the particle dispersion state for a long time, such as shortage in the dispersant to its saturated amount.     

Experimental and numerical investigation on the strength of polymer-metal hybrid with laser assisted metal surface treatment

The aim of this study was to evaluate the effect of laser assisted treating metal surface on the strength of polymer-metal hybrid. The oxide film on the metal surface was removed by caustic soda and nitric acid solution. After that, the metal surface was treated by fiber laser, and the hot pressing connection between polymer and metal was completed by plate vulcanizing machine. And then, the tensile strength was obtained by using universal testing machine. The effect of different laser power, different scanning line width and different scanning speed on the bonding strength of polymer-metal hybrid was investigated. The correlation between the characteristics of metal surface and the bonding strength of polymer-metal hybrid was analyzed based on the micro structure morphology and scanning electron microscope (SEM). The results show that the bonding strength of polymer-metal hybrid increases first and then decreases with the increase of laser power. With the increase of scanning line width, the strength of polymer-metal hybrid increases. When the scanning speed is 500?mm/s, the strength of polymer-metal hybrid is the lowest. Based on the experiment, a simplified model is established and analyzed. Through using ABAQUS to conduct the numerical simulations, the results are consistent with the theoretical analysis and experimental data.     

Experimental and numerical study of mixing behavior inside droplets in microchannels

For the practical applications of droplet‐based microfluidics, we have paid special attention to the complex hydrodynamics and mixing performance inside microdroplets and the profound process intensification when forcing the droplets to move in winding channels. In this work, experimental studies using micro laser induced fluorescence (μ‐LIF) technique and three‐dimensional simulation based on a multiphase, multicomponents lattice Boltzmann model approach were adopted. The simulation results clearly revealed that the mixing inside the droplet is due to the convection in symmetric vortices in the two hemispheres of the droplet and the diffusion between them. They also showed the fluids inside the droplet could be reoriented due to the winding effect. Three designs of winding channels were studied, where interesting results showed the similar effect of process intensification by breaking up the flow symmetry. The revealed flow mechanism and the mixing performance inside the droplet in droplet‐based microfluidics should be helpful for microdevice design and optimization. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1801–1813, 2013     

A numerical investigation of air flow during tablet compression

This paper presents a systematic numerical investigation of airflow during compression in a simplified tablet press geometry. Air entrainment during tablet compression has been cited as the cause of manufacturing defects and quality problems. The effects of tablet punch profile, speed and punch to die wall clearance on airflow in the system are investigated. Conditions under which air entrainment in the tablet formulation are likely to occur have been identified, and are found to correspond with punch concavity. For a flat profile punch, the effects of punch speed and punch to die wall clearance are shown to be of minor importance.     

Hydrodynamic modeling of particle rotation for segregation in bubbling gas-fluidized beds

A multi-fluid Eulerian model has been improved by incorporating particle rotation using kinetic theory for rapid granular flow of slightly frictional spheres. A simplified model was implemented without changing the current kinetic theory framework by introducing an effective coefficient of restitution to account for additional energy dissipation due to frictional collisions. Simulations without and with particle rotation were performed to study the bubble dynamics and bed expansion in a monodispersed bubbling gas-fluidized bed and the segregation phenomena in a bidispersed bubbling gas-fluidized bed. Results were compared between simulations without and with particle rotation and with corresponding experimental results. It was found that the multi-fluid model with particle rotation better captures the bubble dynamics and time-averaged bed behavior. The model predictions of segregation percentages agreed with experimental data in the fluidization regime where kinetic theory is valid to describe segregation and mixing.     

平板陶瓷毛细芯环路热管的实验与仿真

设计并制作了一种平板陶瓷毛细芯环路热管,以环保型制冷剂R245fa作为工质,实验研究了其传热性能;在传统热阻网络模型基础上,优化了其计算过程,通过两条路径并行计算储液器温度,并以二者残差作为收敛条件,提高了计算速度。实验与仿真结果表明,进入固定热导区后,冷凝器热阻占系统总热阻的90%左右,当系统刚刚进入固定热导区时,蒸发器热阻最小,此时蒸发器传热性能最佳;仿真结果与实验结果吻合度较高,温度计算误差最大不超过5℃,热阻的相对误差最大为17%,通过模型计算得到工质在流经毛细芯内部时产生的压降占系统总压降的90%,蒸发温度随着毛细芯厚度的增大而减小,随着毛细芯有效热导率的增大呈现先减小后增大的趋势;增加毛细芯厚度有利于减小毛细芯向储液器的漏热,毛细芯有效热导率的增大会显著增加漏热,不利于系统运行。     

A numerical study of stir mixing of liquids with particle method

The process of stir mixing of two viscous liquids is simulated using the moving particle semi-implicit (MPS) method. A mixing rate is defined within the particle method to characterize the level of mixing, as the number, position, period, and rotating speed of the stirring stick(s) and liquid viscosity are changed. The motions of liquid particles are tracked to reveal the flow field and mixing mechanisms. The variation of the mixing rate shows that the mixing rate is higher when the sticks are rotating monotonically at high speed, and an optimum position of the stick can be identified. The mixing rate does not enhance significantly when three or more sticks are employed, and the liquid viscosity has minor influences on the mixing rate. These results give useful qualitative suggestions on controlling the mixing rate during chemical reactions.