Effect Of Particle Collisions Research Articles

Improve removal efficiency of PM 1 depending on collision enhancement and agglomeration effect

In this paper, aiming at PM 1, namely submicron particles which are the most difficult to remove in traditional purification technologies, it tries to improve their removal efficiency depending on collision enhancement and agglomeration effect. The effect of electric field and plate structure on collision enhancement is studied. Besides, a novel structure of corrugated plate is put forward and its removal performance is compared with parallel plate. Results show that it confirms the key function of the external electric field on the enhancement of particle collision through the trajectory visualization of particle streams with different polarity charging types. The intense turbulence quickens the particle collision and agglomeration in corrugated plate, and the enhancement of electric field around the bend can bring a quick capture firstly, and then the nonuniformity of electric field also leads to the positive effect of particle collision. Corrugated plate can achieve a 12% improvement of removal efficiency for PM 1 compared with traditional parallel plate, under consideration of collision enhancement brought by the airflow arrangement and agglomeration effect under Coulomb force between positive and negative particles.

Modeling particle collisions in moderately dense curtain impacted by an incident shock wave

The interactions between an incident shock and a moderately dense particle curtain are simulated with the Eulerian–Lagrangian method. A customized solver based on OpenFOAM is extended with an improved drag model and collision model and then validated against two benchmark experiments. The results show that the collision model has a limited impact on curtain morphology compared with the improved drag model. In this work, parametric studies are performed considering different particle sizes, volume fractions, and curtain thicknesses. Smaller particle sizes and larger volume fractions lead to stronger reflected shock and weaker transmitted shock. Attention is paid to the particle collision effects on the curtain evolution behaviors. According to our results, for the mono-dispersed particle curtain, the collision effects on curtain front behaviors are small, even when the initial particle volume fraction is as high as 20%. This is due to the positive velocity gradient across the curtain after the shock wave passage, leading to the faster motion of downstream particles than the upstream ones, and hence, no collision occurs. For the bi-dispersed particle curtain, the collision effects become important in the mixing region of different-size particles. Collisions decelerate small particles while accelerating large ones and cause velocity scattering. Moreover, increasing the bi-dispersed curtain thickness leads to multiple collision force peaks, which is the result of the delayed separation of different particle groups. Our results indicate that the collision model may be unnecessary to predict curtain fronts in mono-dispersed particles, but in bi-dispersed particles, the collision effects are important and, therefore, must be modeled.

Model Experiment Exploration of the Kinetic Dissipation Effect on the Slit Dam with Baffles Tilted in the Downstream Direction

Slit dams can eliminate the risk of particle overload accumulation, which can be safer in controlling debris flow compared with a completely closed dam. In attempting to better use the energy dissipation effect of particle collision and reduce the impact of the dam body, referring to the traditional slit dam, this paper proposed one with tilted baffles in the downstream direction. Discrete element simulation and several flume model experiments were carried out herein to verify the advantages and explore the applicable conditions of this tilted baffle slit dam, in which the particle trapping efficiency and the change law of impact force of the tilted baffles under the conditions of different inclined angles, opening sizes, and particle sizes were studied. The results show that: 1. when the inclination angle is 30° ≤ θ ≤ 45°, the tilted baffles can dissipate more particle kinetic energy than the transverse baffles; 2. the maximum impact force and trapping efficiency of the tilted baffles decrease with the increase in the width diameter ratio b/d, with the opening width b of the slit to the particle diameter d; 3. with the given particle size of 6 mm ≤ d ≤ 14 mm, the range that the tilted baffles can effectively intercept the particles flowing down is 0 ≤ b/d ≤ 4, and it reaches the ideal interception state near 1 ≤ b/d ≤ 2, where, relatively, the impact force is weak, and the interception efficiency is high.

Intricate relations among particle collision, relative motion and clustering in turbulent clouds: computational observation and theory

Abstract. Considering turbulent clouds containing small inertial particles, we investigate the effect of particle collision, in particular collision–coagulation, on particle clustering and particle relative motion. We perform direct numerical simulation (DNS) of coagulating particles in isotropic turbulent flow in the regime of small Stokes number (St=0.001–0.54) and find that, due to collision–coagulation, the radial distribution functions (RDFs) fall off dramatically at scales r∼d (where d is the particle diameter) to small but finite values, while the mean radial component of the particle relative velocity (MRV) increases sharply in magnitude. Based on a previously proposed Fokker–Planck (drift-diffusion) framework, we derive a theoretical account of the relationship among particle collision–coagulation rate, RDF and MRV. The theory includes contributions from turbulent fluctuations absent in earlier mean-field theories. We show numerically that the theory accurately accounts for the DNS results (i.e., given an accurate RDF, the theory could produce an accurate MRV). Separately, we also propose a phenomenological model that could directly predict MRV and find that it is accurate when calibrated using fourth moments of the fluid velocities. We use the model to derive a general solution of RDF. We uncover a paradox: the past empirical success of the differential version of the theory is theoretically unjustified. We see a further shape-preserving reduction of the RDF (and MRV) when the gravitational settling parameter (Sg) is of order O(1). Our results demonstrate strong coupling between RDF and MRV and imply that earlier isolated studies on either RDF or MRV have limited relevance for predicting particle collision rate.

A direct relaxation process for particle methods in gas-kinetic theory

The multi-scale flow mechanism is crucial for the force and heat loaded on near-space vehicles, the control of spacecraft, and the propelling and cooling of microelectromechanical systems. Since the continuum and rarefied flows often coexist, the prediction of multi-scale flow is complicated. One efficient way is constructing numerical methods by adopting the multi-scale temporal integral solutions (or characteristic line solutions) for model equations in the gas-kinetic theory. The model equations can be classified into the Fokker–Planck type and Bhatnagar–Gross–Krook type (BGK-type). Since these numerical methods are strictly based on model equations, they are also restricted by the model equations. The difficulty in constructing a model equation that has complete asymptotic preserving property for gas mixture with non-equilibrium internal energy will prevent the further extension of these methods. Therefore, this paper addresses the question whether a multi-scale numerical method can be established by directly adopting the relaxation rates of macroscopic variables, such as stress and heat flux, because these relaxation rates are the aggregate effect of particle collisions and are the essential constrains when constructing model equations. Since the particle-BGK method is concise, its collision term is replaced by the direct relaxation process, where the macroscopic variables first evolve according to their relaxation rates, and then, the after-collision molecules get their velocities from the after-evolution macroscopic variables. Therefore, the modified particle-BGK method does not depend on model equations. Finally, the validity and accuracy of the present method are examined with homogenous relaxation case, shock tube, shock structure, cavity flow, and hypersonic cylinder flow in transitional regime.

Simulation study of the effect of the restitution coefficient on interphase heat transfer processes and flow characteristics in a fluidized bed

This article discusses a simulation study performed to investigate the effect of particle collision on inter-particle and gas–solid heat transfer processes, and other related bed flow characteristics. The effect of particle elasticity is presented using different values of the particle–particle coefficient of restitution. The simulation study was carried out using a two-dimensional model of a fluidized bed reactor incorporated to ANSYS Fluent 16.2 software. Two different materials, steel beads and sand particles, were used as the bed material fluidized by air. The simulation results are compared to those from previous studies on fluidized bed reactors containing a single bed material. The coefficient of restitution affected the bed hydrodynamics. Specifically, an increasing coefficient of restitution resulted in an increasing bed pressure drop and decreasing void fraction, granular temperature, particle velocity, and collision frequency. Conversely, increasing the particle coefficient of restitution resulted in decreasing the particle–particle heat exchange coefficient and the gas–particle heat transfer coefficient. The gas–particle heat transfer coefficient for sand particles was higher than that for steel beads. The effect of the coefficient of restitution on the flow characteristics from a binary mixture bed was quite similar to those of single material beds found in previous studies. This study demonstrated that the restitution coefficient clearly affected both the particle–particle and gas–particle heat transfer processes.

Numerical investigation of isothermal and non-isothermal ice slurry flow in horizontal elliptical pipes

This study reports an analysis of isothermal and non-isothermal (with melting) ice slurry flow in horizontal elliptical pipes. A three dimensional Euler-Euler granular multiphase turbulent CFD model has been constructed accounting the effects of particle collisions, drag, lift and turbulence dispersion. The model is applied to study isothermal and non-isothermal ice slurry flow in horizontal elliptical pipes and the characteristics, such as stratified flow development, heterogeneity in ice fraction distribution, pressure drop, melting and heat transfer are described. Isothermal ice slurry flow in elliptical pipes results in slightly lower heterogeneity as compared to circular pipes for a given constant flow rate. The heterogeneity of ice fraction distribution in the elliptical pipe increases as the size of the ice particles increase. For non-isothermal ice slurry flow, as compared to circular pipe elliptical pipe results in less melting tendency of ice particles.

The Removal Efficiency of PM2.5 with Different Particle Size of Dust Removers

In the present test, the particle concentration in each particle size range of PM2.5 was monitored in the front and rear of dust collector using electrical low pressure impactor, and it was found that electrostatic precipitator has a higher removal efficiency of PM2.5, and the removal efficiency is the lowest when the range of particle size is 0.1~1.0μm, which is the transition zone between the two kinds of dust removal mechanism, diffusion one and charge one. The bag filter has the lowest removal efficiency for the particle with the size of 0.3μm, and the dust of the bag surface increases the effect of particle collision, interception and diffusion, which improves the efficiency of dust removal. The water film dust collector’s removal efficiency for the PM2.5 is lower, and the minimum removal efficiency occurs at 0.12μm particle diameter which is located at the transition area of two kinds of dust removal mechanism, the diffusion and the collision one, and the interception one.

Effect of particle collisions in dense suspension flows.

We study nonlocal effects associated with particle collisions in dense suspension flows, in the context of the Affine Solvent Model, known to capture various aspects of the jamming transition. We show that an individual collision changes significantly the velocity field on a characteristic volume Ω_{c}∼1/δz that diverges as jamming is approached, where δz is the deficit in coordination number required to jam the system. Such an event also affects the contact forces between particles on that same volume Ω_{c}, but this change is modest in relative terms, of order f_{coll}∼f[over ¯]^{0.8}, where f[over ¯] is the typical contact force scale. We then show that the requirement that coordination is stationary (such that a collision has a finite probability to open one contact elsewhere in the system) yields the scaling of the viscosity (or equivalently the viscous number) with coordination deficit δz. The same scaling result was derived [E. DeGiuli, G. Düring, E. Lerner, and M. Wyart, Phys. Rev. E 91, 062206 (2015)PLEEE81539-375510.1103/PhysRevE.91.062206] via different arguments making an additional assumption. The present approach gives a mechanistic justification as to why the correct finite size scaling volume behaves as 1/δz and can be used to recover a marginality condition known to characterize the distributions of contact forces and gaps in jammed packings.

On collisional diffusion in a stochastic magnetic field

The effect of particle collisions on the transport in a stochastic magnetic field in tokamaks is investigated. The model of resonant magnetic perturbations generated by external coils at the plasma edge is used for the stochastic magnetic field. The particle collisions are simulated by a random walk process along the magnetic field lines and the jumps across the field lines at the collision instants. The dependencies of the local diffusion coefficients on the mean free path λmfp, the diffusion coefficients of field lines DFL, and the collisional diffusion coefficients, χ⊥ are studied. Based on these numerical data and the heuristic arguments, the empirical formula, Dr=χ⊥+v||DFL/(1+Lc/λmfp), for the local diffusion coefficient is proposed, where Lc is the characteristic length of order of the connection length lc=πqR0, q is the safety factor, R0 is the major radius. The formula quite well describes the results of numerical simulations. In the limiting cases, the formula describes the Rechester-Rosenbluth and Laval scalings.

Constitutive flow models for characterizing the rheology of fresh mortar and concrete

Constitutive equations for fresh mortar and fresh concrete provide the characterization of the mixture’s flow and the quantification of the rheological properties. This paper presents a constitutive material model for mortar and concrete that builds on previous research. It postulates that (a) the shear stress is the sum of three components: static interaction between particles, dynamic interaction between particles, and collision of particles; and (b) the cell is a representative volume of the mixture. For fresh concrete, the effects of particle collisions are negligible due to high concentration, and the equation reduces to the Bingham model. Experimental data reported in the literature was employed to evaluate the predictive capabilities of the constitutive equations. The model results are found to compare very well with the measured experimental data and the difference is within the measurement errors.

Mechanisms of Spontaneous Reconnection: From Magnetospheric to Fusion Plasma

Very often space plasma is treated as collisionless. We check the validity of this paradigm considering various regimes of tearing mode (spontaneous reconnection) including effects of particle collisions and shear of magnetic field. We briefly describe Pitaevskii’s effect of effective modification of collision frequency due to the finite particle Larmor radius in the presence of magnetic field. This effect results in a significant increase of the role of collisionality, especially in a weakly magnetized systems. Another popular paradigm is related with application of MHD description to collisionless or weakly collisional systems. We show, that for current sheets observed in the Earth magnetotail and magnetopause as well as for current sheets formed in Solar corona and in laboratory devices most appropriate is the kinetic semi-collisional tearing regime. Role of “collisions” could play usual Coulomb pair collisions of electrons and ions (e.g. in Solar corona) or effective collisions (scattering) of electrons with the microturbulence wave modes. Transition to real MHD modes requires either very large collisions frequencies and/or very large amplitudes of the magnetic field shear. The largest domain in the parameter space is occupied by the kinetic regimes of tearing mode growth where dissipation is provided either by Landau damping or by real (or effective) collisions.

Prediction of Release Characteristics of Film‐Coated Urea from Structure Characterization Data of the Film

AbstractAn efficient method for the prediction of the release characteristics of film‐coated urea from its film structure is proposed. A model spray‐coating apparatus was built to prepare a plane film to directly measure the film permeability coefficients by simulating the characteristics of randomness, intermittence and multiple layers of the coating, and eliminating particle collision effects. From analysis, the spray‐coated film structure can be modeled as comprising a dense structure and a defect structure. Both structures contributed to the penetration flux and the apparent permeability coefficients of the film for water and urea diffusion. The permeability coefficients of the plane films were calculated from the film structure model, and were shown to be close to the measured values.

Effect of Particle Collision and Rebound Behavior on Adhesion Characteristics on the Wall of Honeycomb Shaped Catalyst

Effect of Particle Collision and Rebound Behavior on Adhesion Characteristics on the Wall of Honeycomb Shaped Catalyst

Particle Collisions in a Lumped Particle Model

AbstractThis paper presents an extension of the lumped particle model in [1] to include the effects of particle collisions. The lumped particle model is a flexible framework for the modeling of particle laden flows, that takes into account fundamental features, including advection, diffusion and dispersion of the particles. In this paper, we transform a binary collision model and concepts from kinetic theory into a collision procedure for the lumped particle framework. We apply this new collision procedure to investigate numerically the role of particle collisions in the hindered settling effect. The hindered settling effect is characterized by an increase in the effective drag coefficient CD that influences each particle in the flow. This coefficient is given by , where ϕ is the volume fraction of particles, is the drag coefficient for a single particle, and n ≃ 4.67 for creeping flow. We obtain an approximation for CD/CD by calculating the effective work done by collisions, and comparing that to the work done by the drag force. In our numerical experiments, we observe a minimal value of n = 3.0. Moreover, by allowing high energy dissipation, an approximation for the classical value for creeping flow, n = 4.7, is reproduced. We also obtain high values for n, up to n = 6.5, which is consistent with recent physical experiments on the sedimentation of sand grains.

An improved collision damping time for MP-PIC calculations of dense particle flows with applications to polydisperse sedimenting beds and colliding particle jets

This paper describes several improvements to a numerical model introduced by O’Rourke et al. (2009) for collisional exchange and damping in dense particle flows. O’Rourke et al. (2009) use a Bhatnagar, Gross, and Krook (BGK) approximation to the collision terms in a particle distribution function transport equation to model the effects of particle collisions on damping fluctuating particle velocities and, in gas/liquid/solid beds, fluctuating temperatures and compositions of liquid films on particle surfaces. In this paper we focus on particle flows in which the particles have no liquid films and report on an improved expression we have developed for the collision damping time of particle velocity fluctuations used in the BGK approximation. The improved expression includes the effects on the collision damping time of the particle material coefficient of restitution and of non-equilibrium particle velocity distributions. The collision model improvements are incorporated into the general-purpose computational-particle fluid dynamics (CPFD) numerical methodology for dense particle flows. Three computational examples show the benefits of using the new collision time in calculations of particle separation in polydisperse dense particle flows and calculations of colliding particle jets.

Particle transport by standing waves on an electric curtain

Discrete-element simulations are performed to study particle transport by standing waves on an electric curtain. An electric curtain consists of a series of parallel electrodes with oscillating potential field embedded in a dielectric surface. The study shows that particles can be transported in two different modes under excitation by standing waves. In the first mode, particles roll along the surface in a constant direction with average velocity equal to the wave speed. In the second mode, particles hop along the surface in a manner akin to a Brownian motion. Effect of particle collisions on these transport modes is evaluated.

Effect of particle adhesion and interactions on motion by traveling waves on an electric curtain

A discrete-element/boundary-element method is developed for simulation of adhesive particle transport by traveling waves on an electric curtain. The study shows that both wall adhesion and particle–particle collisions have an important influence on particle transport on electric curtains at different wave frequencies. The most significant effect of particle collisions occurs for cases with medium frequencies in which particles with large negative-charge collect in high-concentration bands and move in a synchronous surfing mode, pushing forward particles with lower charge. Cases with higher and lower frequencies exhibited hopping motion, for which adhesion determines the range of non-transported particles.

Effect of particle collisions and aggregation on red blood cell passage through a bifurcation

Blood flow through bifurcating vessels in the microvasculature leads to uneven distribution of red blood cells (RBC) in the downstream channels when the channels have different sizes or the flow rates through the channels are different. This phenomenon, known as plasma skimming, is responsible for the large variation in hematocrit throughout the circulatory system. Furthermore, the strong streamline curvature present within bifurcations leads to frequent collisions between the blood elements (red and white blood cells and platelets) and the vessel walls, as well as a rearrangement of the distribution of the blood elements in the channels downstream of the bifurcation. Computational models of bifurcation flows typically neglect collision and adhesion of RBCs with each other. In this paper, we use a new type of discrete-element model to investigate the effect of particle–particle collisions and RBC aggregation on modeling of plasma skimming in bifurcations. Cases are examined with and without RBC adhesion and for different hematocrit values, including validation against previous computational results. Results show significant plasma skimming in the low-flow daughter branch and an increase in fractional particle flux with increased hematocrit, as in experimental studies. Accounting for particle–particle collisions leads to a considerable increase in collision rate of particles with the vessel wall. Particles are found to approximately follow fluid velocity streamlines, and consequently particle–particle collisions and aggregation do not significantly affect plasma skimming.

Progress in physics and control of the resistive wall mode in advanced tokamaks

Self-consistent computations are carried out to study the stability of the resistive wall mode (RWM) in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] plasmas with slow plasma rotation, using the hybrid kinetic-magnetohydrodynamic code MARS-K [Y. Q. Liu et al., Phys. Plasmas 15, 112503 (2008)]. Based on kinetic resonances between the mode and the thermal particle toroidal precession drifts, the self-consistent modeling predicts less stabilization of the mode compared to perturbative approaches, and with the DIII-D experiments. A simple analytic model is proposed to explain the MARS-K results, which also gives a qualitative interpretation of the recent experimental results observed in JT-60U [S. Takeji et al., Nucl. Fusion 42, 5 (2002)]. Our present analysis does not include the kinetic contribution from hot ions, which may give additional damping on the mode. The effect of particle collision is not included either. Using the CARMA code [R. Albanese et al., IEEE Trans. Magn. 44, 1654 (2008)], a stability and control analysis is performed for the RWM in ITER [R. Aymar et al., Plasma Phys. Controlled Fusion 44, 519 (2002)] steady state advanced plasmas, taking into account the influence of three-dimensional conducting structures.