Experimental investigation of bubble and particle motion behaviors in a gas-solid fluidized bed with side wall liquid spray

Bubble and particle motion behaviors are investigated experimentally in a gas solid fluidized bed with liquid spray on the side wall. The particles used in the experiment are classified as Geldart B particles. The results reveal that when the fluid drag force is less than the liquid bridge force between particles, liquid distribute all over the bed. Bubble size increases as the increase of inter-particle force, then decreases owing to the increase of particle weight with increasing liquid flow rate. When the fluid drag force is greater than the liquid bridge force, liquid mainly distribute in the upper part of the bed. And it is difficult for the wet particles to form agglomerates. Bubble size decreases with increasing liquid flow rate due to the increasing of minimum fluidization velocity. Besides, the acoustic emission (AE) measurements illustrate that the liquid adhesion and evaporation on particles could enhance the particles motion intensity. Consequently, the bubble and particle behaviors change due to the variation in fluidized gas velocity and liquid flow rate should be seriously considered when attempting to successfully design and operate the side wall liquid spray gas solid fluidized bed.     

Effect of collision angle on particle-particle adhesion of colliding particles through liquid droplet

In wet granulation processes, a particle adhesion mediated by a liquid bridge is one of the quite important phenomena. In an actual process, the liquid bridge shows dynamic motion due to continuous motion of the particles. Therefore, understanding of the particle adhesion phenomenon by a dynamic liquid bridge is essential to adequately and precisely control wet granulation processes. This study presents a direct numerical simulation of the particle–particle adhesion by a dynamic liquid bridge. Collision of a dry particle and a wet particle was simulated at various collision angles. In particular, translational and rotational motions of the particle at different collision angles were discussed through comparison with a conventional static liquid bridge force model. As a result, it was found that both translational and rotational motions were largely different between simulation results of the direct numerical simulation and static liquid bridge force model, especially at the tangential collision. To understand these results, we focused on the rotational behavior of the particle and deformation of the liquid bridge. It was concluded that the non-slip behavior of the liquid bridge on the particle surface is a key phenomenon for the particle-particle adhesion by the dynamic liquid bridge at the tangential collision.     

Numerical Investigation on Turbulence and Bubbles Distribution in Bubbly Flow Under Normal Gravity and Microgravity Conditions

Two-phase flows of gas and liquid are increasingly paid much attention to space application due to excellent properties of heat and mass transfer, so it is very meaningful to develop studies on them in microgravity. In this paper, gas-phase distribution and turbulence characteristics of bubbly flow in normal gravity and microgravity were investigated in detail by using Euler–Lagrange two-way model. The liquid-phase velocity field was solved by using direct numerical simulations (DNS) in Euler frame of reference, and the bubble motion was tracked by using Newtonian motion equations that took into account interphase interaction forces including drag force, shear lift force, wall lift force, virtual mass force and inertia force, etc. in Lagrange frame of reference. The coupling between gas–liquid phases was made with regarding interphase forces as a momentum source term in the momentum equation of the liquid phase. Under the normal gravity condition, a great number of bubbles accumulate near the walls under the influence of the shear lift force, and addition of bubbles reduces turbulence of the liquid phase. Different from the normal gravity condition, in microgravity, an overwhelming majority of bubbles migrate towards the centre of the channel driven by the pressure gradient force, and bubbles have little effect on the turbulence of the liquid phase.     

Particle dynamics in helium II

No Heading The recent implementation of the Particle Image Velocimetry (PIV) method in liquid helium has opened the possibility of directly visualising turbulence in helium II. To interpret the data, however, it is necessary to understand the motion of small inertial particles in the presence of normal fluid and superfluid. We present the governing equations of motion and preliminary numerical results of particle trajectory in the initial growth of a turbulent tangle.PACS numbers: 67.40.Vs; 47.27.–i.     

Gas-particle turbulent flow of foursquare tangential jets in a simplified pulverized-coal firing boiler

An experimental cold-model of a simplified tangential firing boiler was established to investigate the mesoscale turbulent flow behaviors, including gas vortex structures, particle motions and interactions between two phases. A modified PIV technology, employing two pairs of lasers and cameras, was applied to measure the velocity and velocity gradient of turbulent flow in foursquare tangential jets alternatively. At a given initial gas velocity and particle mass loading, the interaction between gas and particles was studied at three different particle sizes. It was found that two main coherent vortex structures, circular eddy and hairpin eddy, distributed mainly in low speed area and heavy impingement area, respectively. The characteristics of particle motion in foursquare tangential jets correlated with gas turbulence dissipation, particle size, particle concentration and particle density. Small particles were easily entrained by gas vortex, so that they consumed more turbulence energy and attenuated the gas turbulence intensity. On the contrary, large particles had more inertia and led to heavier impingement in the chamber center, resulting in particle random distribution and complex momentum transfer between gas and particles. Moreover, large particles stretched the coherent vortex to be narrow and long, while small particles pulled down the vortices rotation intensity.     

EULERIAN AND LAGRANGIAN SIMULATIONS OF PARTICLE DEPOSITION FROM A POINT SOURCE IN A TURBULENT CHANNEL FLOW

Results comparing Eulerian and Lagrangian simulations of particle deposition from a point source in a channel are presented. The mean turbulent flow field is simulated using a two-equation k-ε turbulence model. In the first, approach, diffusion of aerosol particles is studied by solving the corresponding advection-diffusion equation. Deposition of particles in the intermediate size range are analyzed by considering both the turbulent eddy diffusion and the eddy impaction processes, as well as the Brownian diffusion effects. In the second approach, the turbulence fluctuating velocity field are numerically simulated as a Gaussian random process. The Lagrangian trajectories of aerosol particles in the channel are then evaluated by solving the corresponding particle equation of motion. Effects of Brownian diffusion on particle motions are also included. A series of digital simulations for particles of various sizes which are released at different locations across the channel are carried out. Depositions of different size particles on the wall under a variety of conditions are analyzed. The relative significance of turbulence and Brownian effects are also discussed.     

Optimizing micromixer design for enhancing dielectrophoretic microconcentrator performance

We present an investigation into optimizing micromixer design for enhancing dielectrophoretic (DEP) microconcentrator performance. DEP-based microconcentrators use the dielectrophoretic force to collect particles on electrodes. Because the DEP force generated by electrodes decays rapidly away from the electrodes, DEP-based microconcentrators are only effective at capturing particles from a limited cross section of the input liquid stream. Adding a mixer can circulate the input liquid, increasing the probability that particles will drift near the electrodes for capture. Because mixers for DEP-based microconcentrators aim to circulate particles, rather than mix two species, design specifications for such mixers may be significantly different from that for conventional mixers. Here we investigated the performance of patterned-groove micromixers on particle trapping efficiency in DEP-based microconcentrators numerically and experimentally. We used modeling software to simulate the particle motion due to various forces on the particle (DEP, hydrodynamic, etc.), allowing us to predict trapping efficiency. We also conducted trapping experiments and measured the capture efficiency of different micromixer configurations, including the slanted groove, staggered herringbone, and herringbone mixers. Finally, we used these analyses to illustrate the design principles of mixers for DEP-based concentrators.     

Dynamic simulation of rod-like and plate-like particle dispersed systems

A particle simulation method (PSM) is presented to simulate the dynamics of rod-like and plate-like particle dispersed systems. In this method, the particle is modeled with arrays of spheres connected by three types of springs. The motion of particles in flow is followed by solving the translational and rotational equations of motion for each constituent sphere. The mobility matrix for each particle is calculated to obtain the hydrodynamic force and torque exerted on each sphere. For the hydrodynamic interaction among particles, the near-field lubrication force is considered. The method was applied to the simulation of the transient behavior of particles in a shear flow by dispersing them into a cell with periodic boundaries. In semi-dilute to concentrated systems, the overshoot of viscosity was observed for rigid rod-like particle dispersed systems, but not for flexible ones. This was due to the transient change of the microstructure from the flow-directional orientation to the planar one of particles. The normal stress appeared in the flexible particle dispersed systems because of the deformation of particles. In the rectangular plate-like particle dispersed system, the planar orientation of particles was observed and furthermore the orientation of the major axis of particles in the shear direction appeared.     

EULERIAN AND LAGRANGIAN SIMULATIONS OF PARTICLE DEPOSITION FROM A POINT SOURCE IN A TURBULENT CHANNEL FLOW

ABSTRACT

Results comparing Eulerian and Lagrangian simulations of particle deposition from a point source in a channel are presented. The mean turbulent flow field is simulated using a two-equation k-? turbulence model. In the first, approach, diffusion of aerosol particles is studied by solving the corresponding advection-diffusion equation. Deposition of particles in the intermediate size range are analyzed by considering both the turbulent eddy diffusion and the eddy impaction processes, as well as the Brownian diffusion effects. In the second approach, the turbulence fluctuating velocity field are numerically simulated as a Gaussian random process. The Lagrangian trajectories of aerosol particles in the channel are then evaluated by solving the corresponding particle equation of motion. Effects of Brownian diffusion on particle motions are also included. A series of digital simulations for particles of various sizes which are released at different locations across the channel are carried out. Depositions of different size particles on the wall under a variety of conditions are analyzed. The relative significance of turbulence and Brownian effects are also discussed.     

Effect of turbulent flow field properties on improving the removal of coal-fired fine particles in chemical-turbulent agglomeration

Chemical-turbulent agglomeration is a promising coupling agglomeration method to improve the removal of fine particles, in which the turbulent flow field plays an important role on the collision of chemical droplets and fine particles. However, there is no specific study about the effect of turbulent flow field properties on the agglomeration and removal of fine particles. In this work, three kinds of turbulent vortex sheets with different structures were designed to generate vortexes with different scales and generation dimensions, the particle agglomeration effect and characteristics, as well as the particle removal effect by ESP with different turbulence generators were investigated. The results demonstrated that the turbulence generator with large-scale and two-dimensional vortexes in flow field had the best effect on improving the agglomeration and removal of fine particle. Besides, the motion trajectories and the turbulent agglomeration kernel of chemical droplets and fine particles were calculated to further explore the interaction mechanism of particle agglomeration and flow field properties. The results proved that the turbulent flow field containing large-scale and two-dimensional vortexes can effectively enlarge the capture area of fine particles by chemical droplets and promote the collision probability of them, thus improving the agglomeration and removal of fine particles.     

Capillary attraction of floating rods

The capillary attraction of two parallel cylinders with circular cross-section representing slender particles floating at the interface between two immiscible fluids is considered. Given the particle separation, the elevation of the particle centers in hydrostatics is computed to satisfy the vertical force balance involving the buoyancy force, the capillary force, and the particle weight. A numerical procedure is developed for calculating the horizontal force exerted on a pair of cylinders in solitary or periodic arrangement. The results confirm that the particles attract each other under the conditions considered. The particle motion and transient flow due to the particle attraction are computed using a boundary-integral method for Stokes flow. In the algorithm, the particle center velocity of translation and angular velocity of rotation are calculated to satisfy force and torque balances. Numerical simulations using a boundary-element method subject to an initial state provided by hydrostatics illustrate the nature of the motion and furnish estimates for the particle velocity induced by capillarity.     

Stress wave in monosized bead string with various water contents

Wet granular materials exhibit unique physical and mechanical properties, especially in relation to wave propagation, which is quite different from dry granular materials. In this paper, by introducing the capillary bridge force into the discrete element method, the stress wave in mono-sized bead string with various water content has been studied. First the vibration of two particles with liquid bridge has been analyzed. The presence of the liquid bridge force causes the kinetic response of the particles to exhibit completely different properties than that of the dry particles. The equilibrium position is affected by both the physical properties of the particles and the liquid bridge properties. Then the wave propagation behaviors in a mono-sized bead string have been analyzed. According to whether the liquid bridge volume has an effect on granular motion, the whole process can be divided into two stages. Stage I, particles are physically contacted with each other directly. The influence of liquid bridge force is independent of the bridge volume. Stage II, particles start to oscillate back and forth at their equilibrium positions, the influence of the liquid bridge force becomes related to the bridge volume. The kinetic energy dissipation first decreases and then increases. A U-shaped trend appears throughout the dissipation process. In our work, the mechanical properties of wet granular materials are studied from two levels: particle vibration and wave propagation, which will provide theoretical guidance for the application of granular materials in aqueous environment.     

大悬浮轻颗粒固液两相流中的有序结构

应用三维颗粒图像跟踪技术,对竖直管内向上大悬浮轻颗粒固液两相流中分散相即颗粒相瞬时分布进行非接触测量,由此对顺流方向颗粒串组成的有序相分布结构进行观察研究。实验发现,当液体流动速度大于某一确定值时会有明显的颗粒串出现,此时颗粒由于受液体速度梯度诱导的强升力作用而紧贴管壁运动;随着液体流动速度的降低,颗粒串逐渐消失而颗粒沿管径向的分布会向着管中心方向发展;当液流速度进一步降低,颗粒开始在水平方向团聚。分析表明液体流动的剪切作用是颗粒串生成和稳定的机制。实验还显示,随着颗粒相平均份额的增加,流动中串间颗粒的相互作用加强,颗粒分布结构也随之受到影响。     

Influence of particles on the transition to turbulence in pipe flow

We investigate by experiment the influence of suspended solids upon the transition to turbulence in pipe flow. The particles are monodisperse and neutrally buoyant with the liquid. The role of the particles on the transition depends upon both the volume fraction, phi, and particle size. Below a critical particle diameter, particles alter the transition to larger critical Reynolds numbers for all phi. In contrast to this, larger particles move the transition to smaller Reynolds numbers for small phi, but they delay the transition at larger concentration.     

Transfer of discrete inclusions by fluxes with concentrated vorticity

Problems related to modeling of the motion of discrete inclusions (solid particles, drops or bubbles) in flows with concentrated vorticity are considered. A comparative evaluation of the force factors in the equation of motion of a test particle is made. The results of numerical modeling of the motion of discrete inclusions in the gap between concentric rotating cylinders and a vortex flow formed by the liquid rotating with a constant angular velocity over a fixed base are discussed. The coordinates of the points of equilibrium of the test particle in the vortex flow are found. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 2, pp. 36–45, March–April, 2007.     

The Effect of a Short Pulse of Current on Small Particles in a Conducting Fluid

This letter describes the effect of a heavy current pulse, of approximately 10 kA, produced by a charge capacitor bank, on a small volume of liquid metal, confined in a cylindrical container (tube diameter is 1 cm and tube length is about 10 cm). The liquid metal contamination implies the presence of microscopic nonconducting particles. Theoretical consideration shows that a typical heavy current pulse of 15 msec duration leads to extremely large Lorentz force, due to the appearance of a self-induced magnetic field. Even though the particle contaminants are of micron size they can be moved by this force to macroscopic distances. Such a motion phenomenon can be used in the development of a cleanliness control system.     

Numerical simulation of particle motion in dense phase pneumatic conveying

A gas-solids two-dimensional mathematical model was developed for plug flow of cohesionless particles in a horizontal pipeline in dense phase pneumatic conveying. The model was developed based on the discrete element method (DEM). For the gas phase, the Navier-Stokes equations were integrated by the semi-implicit method for pressure-linked equations (SIMPLE) scheme of Patankar employing the staggered grid system. For the particle motion the Newtonian equations of motion of individual particles were integrated, where repulsive and damping forces for particle collision, the gravity force, and the drag force were taken into account. For particle contact, a nonlinear spring and dash pot model for both normal and tangential components was used. In order to get more realistic results, the model uses realistic pneumatic system and material values.     

基于超声驻波场的水下颗粒操控研究

超声波产生的声辐射力可以实现对微小物体的操控。针对微米尺度颗粒在液体环境的操控问题,基于黏性介质中的声辐射力理论,建立由双凹球面聚焦超声换能器驱动下的水下颗粒操控模型。利用COMSOL软件仿真了模型的声场、声流场及颗粒操控动态过程,最后通过水下颗粒操控实验对仿真结果进行验证。研究发现,颗粒在水下操控过程受到声辐射力与声流曳力的共同作用,由声波干涉作用形成的局部驻波场主要依靠声辐射力将颗粒团聚在波节位置,但随着颗粒尺寸的减小,颗粒无法继续束缚,颗粒操控将由依靠声辐射力转变为声流曳力。此外声场强度的增加会增强颗粒操控的抗扰动能力。     

Dynamic forces on an immersed cylindrical tube and analysis of particle interaction in 2D-gas fluidized beds

The interactions of bubbles and particles with fixed cylindrical tubes in two-dimensional fluidized beds were investigated by experiments and by simulations, based on results for single bubbles impinging on a tube. The experimental results based on PIV analysis support our previous force origin model and indicate that the model is able to successfully model bubble behavior and particle motion around fixed objects. The simulation results give useful predictions, dynamic force induced on a tube consists of the force from pressure gradient, fluid viscous force and particle contact force. The predominant force component is from the pressure gradient. As bubbles directly interact with a tube, the particle contact force contribution briefly becomes predominant.Bubble behavior and particle motion are greatly affected by the state of the emulsion phase as the medium of the fluidized bed into which gas is injected. Hence the dynamic forces on immersed objects are directly affected by the state of the emulsion phase.     

Effect of liquid layer on the motion of particle during oblique wet collision

Collisions between particles and the wall covered by a liquid layer play an important role in many different industrial processes (e.g., chemical, pharmaceutical, and transportation). Understanding the rebound motion law of the collision between particles and the wall covered by a liquid layer is vital to ensure the high efficiency of processes such as wet granulation and fluid catalytic cracking. In the present study, we investigated the influence of different collision angles on the liquid bridge geometry, particle motions, particle energy, and other collision details based on the oblique collisions between particles and the target plate covered by a liquid layer. Results showed that the collision angle of particles has a great effect on the liquid bridge geometry. Moreover, the liquid bridge caused by different collision angles initially increases the particle deflection angle difference and then decreases, and this influence gradually increases with the increase of the collision angle. In addition, the collision angle greatly affects the particle’s energy.