DEM simulations of spherical particle–particle collisions

The randomness, diversity, and complexity of the high-speed particle crushing process bring great difficulties to the theoretical analysis of powder engineering. In this paper, the discrete element method is used to simulate the collision of spherical particles, which provides a reference for studying the process and mechanism of crushing between particles under impact load. The Hertz–Mindlin with bonded contact model is used as the particle–particle contact model. The central collisions of particles with different diameter ratios under different high-speed motions and the eccentric collisions with different eccentricities are discussed. The results show that the bond damage increases with the increase of relative velocity in both centre impact and eccentric impact. In centre collisions, particles of smaller objects are more fragmented than particles of larger objects. For smaller target particles, the larger the diameter ratio is, the more particle elements are detached from the target particles, and the greater the bond breakage rate. For larger target particles, the larger the diameter ratio is, the less the particle element falls off and the smaller the bond breakage rate. This provides guidance for the collision and crushing of particles with different particle size ratios and different eccentricities during high-speed motion in engineering applications in the future.     

Trapping–detrapping behavior in CVD diamond particle detectors

Diamond based particle detectors were built up using high quality diamond films grown by microwave chemical vapor deposition (CVD). The efficiency (η) and charge collection distance (CCD) of such devices were tested by a 5.5 Mev ²⁴¹Am α-particle source. Their response times were then carefully investigated both in the as-grown normal state and after irradiation with β-particles for approximately 60 h, in order to bring the detectors in the so called pumped state. A drastic change in the time evolution, the signal amplitude and the symmetry of the pulse shapes recorded with positive and negative polarization of the detector is observed as soon as the priming procedure takes place. This behavior is explained in the framework of a model in which a trapping–detrapping mechanism for CVD diamond is accounted for. Two different kinds of trapping centers for electrons and holes are proposed as the limiting factor in the diamond detection performance. A very good agreement between the simulation and the experimental pulse shapes is observed, thus allowing a better understanding of the priming procedure and the possible identification of the crystal defects limiting the efficiency of diamond based particle detectors.     

Gas–particle partitioning of organic compounds in the atmosphere

Gas–particle partitioning of condensable organic compounds in the atmosphere is described using two methods. The first method is based on the use of a comprehensive mechanistic model of adsorption/absorption processes. The second method is based on aerosol yields estimates. The model parameters in the adsorption/absorption model are evaluated from experimental data. The concepts of concentration of adsorbed molecules on the surface of aerosol particles and diffusion of adsorbed molecules in the liquid phase are used for determining the importance of the adsorption/absorption mechanisms. The model calculations showed a qualitative agreement with available experimental data for alkanes. Furthermore, a modified version of the Carbon Bond IV chemical mechanism including an aerosol yields method to model the formation of organic aerosols in reactive plumes is used in combination with a plume dispersion model. The formation of secondary organic matter in plumes contributes significantly to the total secondary aerosol mass produced.     

Bubble–particle collision and attachment probability on fine particles flotation

Particle size is an important parameter in flotation and has been the focus of flotation research for decades. The difficulty in floating fine particles is attributed to the low probability of bubble–particle collision. In this research, the influence of hydrodynamic parameters on collision probability of fine particles was investigated. Collision probability was obtained using Stokes, intermediate I and intermediate II and potential equations. Maximum collision probability was 5.65% obtained with impeller speed of 1100 rpm, air flow rate of 30 l/h and particle size of 50 μm. Also, attachment probability under Stokes flow, turbulent and potential flow conditions was calculated 100, 99.49 and 81.87% respectively. Maximum attachment probability was obtained with impeller speed of 700 rpm, contact angle of 90°, particle size of 20 μm and air flow rate of 15 l/h. Collision angles were obtained between 60.71° and 60.18° and attachment angles were obtained between 9.15° and 59.83°.     

Magnetothermal study of nanocrystalline particle formation in amorphous electroless Ni–P and Ni–Me–P alloys

The microstructure of amorphous Ni–P and Ni–Me–P materials and especially its change during the heat treatment is of great importance for their magnetic, mechanical and corrosion behavior. A new magnetic phase analysis method (magnetothermal) is presented that reveals the precipitation of nanoparticles with strong magnetic properties during phase transformation upon heat treatment. It is applied to electroless Ni–P, Ni–Cu–P and Ni–Sn–P amorphous alloys. The results acquired by this method are compared with data obtained by differential scanning calorimetry, as well as by microhardness measurements using identical heat treatment in all three cases. Due to the high sensitivity of the magnetothermal method a more detailed picture of the precipitation processes in Ni–P alloys is obtained and the new information is discussed. Magnetothermal measurements reveal several stages of precipitation of a phase with strong magnetic properties. This phase is Ni in the Ni–P alloy, and Ni(Me) solid solution in the Ni–Me–P alloys. Though Sn has a stronger effect on the Ni magnetization, Cu is more effective in preventing the appearance of high magnetization in a thermally treated Ni–Cu–P alloy. This is due to Cu incorporation in Ni particles in a quantity above four times larger than Sn.     

Coal–oil–water multiphase fuel: Rheological behavior and prediction of optimum particle size

As the coal–oil–water slurry is gaining importance in place of fuel oil, a better understanding of handling characteristics is in demand. Therefore, experimental investigations have been carried out to investigate the rheological properties of coal–oil–water suspension containing coal particles of different sizes. Different coal stocks with average particle sizes of 108 μm, 75.7 μm and 62.9 μm have been used. The concentration of solid for the experiment varies from 10% to 50% by weight. All experiments have been carried out in a cup and bob type coaxial cylindrical viscometer. Newtonian, shear thinning and shear thickening behavior of suspension has been observed depending on component content and operating conditions. Study with different particle sizes shows that it is possible to achieve an optimum particle size for better handling of such suspension. A generalized correlation has been developed to predict the apparent viscosity of coal–oil–water suspension incorporating the coal concentration, oil concentration, torque and particle diameter. The experimental data are in well agreement with proposed correlation.     

Effect of anisotropic microstructures on fluid–particle drag in low-Reynolds-number monodisperse gas–solid suspensions

Local structural anisotropy prevails in gas–solid suspensions. It causes strong fluctuations in the drag on individual particles. In this work, the anisotropy of microstructures is quantified by a second-order structure tensor, which is determined with a directionally dependent mean free path length. Direct numerical simulations of low-Reynolds-number flows past anisotropic and isotropic BCC, FCC, and random arrays of monodisperse spheres in sufficiently large domains are performed. The results show that, at the same solid volume fraction, the differences between the mean drag in principal directions of anisotropic arrays and that in isotropic arrays correlate well with functions of eigenvalues of the structure tensor for the anisotropic arrays. Anisotropic drag models for different arrays are proposed. Assessment of the model for random arrays shows that it well captures fluctuations in the mean drag at microscales of several sphere diameters, where the traditional model fails to give satisfactory predictions.     

Matrix–particle interactions in catalyzed and uncatalyzed copper-filled epoxy matrix composites

Epoxy composites filled with copper particles with sizes of the order of 100 μm are studied with the aim of analyzing the particle–matrix interphase. Two matrixes are used: diglycidyl ether of bisphenol A resin (DGEBA)–anhydride catalyzed using a tertiary amine, and uncatalyzed DGEBA–anhydride. The surface of both types of composites was analyzed using scanning electron microscopy, X-ray photoelectron spectroscopy, and instrumented nanoindentation. The formation of Cu(I) and Cu(II) complexes is revealed using X-ray photoelectron spectroscopy, while instrumented nanoindentation measurements allow us to determine regions with different mechanical properties in the uncatalyzed composite. The influence of anhydride and the type of curing reaction on the formation of copper complexes is analyzed. The main results point out that copper particles can interact strongly with the epoxy, depending on the chemistry and kinetics of the curing reaction, to modify the composite. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47511.     

Experimental characterization of liquid film behavior during droplets–polyethylene particle collision

Nowadays, the droplet–particle collision characteristics in the gas-phase ethylene polymerization process are still unclear. The high-speed photography and a quasi-circle imaging approach are employed to study the collision interaction characteristics between liquid droplets and polyethylene particles. The liquid film evolution is studied through variations of the film thickness on the particle north pole, the dynamic contact angle, center angle and film thickness at the maximum extension. Results have found that for n-hexane the threshold temperature of the recoil happening increases with increasing initial Weber number, but for 1-hexene it is stable. Over 70°C evaporation and splash occurs immediately. Under low Weber numbers, the water droplet stays for damping oscillations, the reference stable height of which is linearly related to temperatures. Moreover, three regimes of film thickness variation with time are identified and mathematically described, while Regime 3 characteristics are found strongly dependent on the liquid species, Weber number, and particle temperature.     

On the self-similarity of the aggregation–fragmentation equilibrium particle size distribution

A rather simple (yet general) theoretical analysis is presented concerning the steady-state particle size distribution in combined aggregation–fragmentation processes. The general conditions are provided for this distribution to exhibit self-similar behaviour. The theory is applied to the case of turbulent aggregation-fragmentation of fractal particles. It is shown that by using this analysis, experimental data can be interpreted without resorting to a numerical solution of the population balance equation.     

Exploring contraction–expansion inertial microfluidic-based particle separation devices integrated with curved channels

Separation of particles or cells has various applications in biotechnology, pharmaceutical and chemical industry. Inertial cell separation, in particular, has been gaining a great attention in the recent years since it has exhibited a label-free, high-throughput and efficient performance. In this work, first, an inertial contraction–expansion array microchannel device, capable of passively separating two particles with diameters of 4 and 10 μm, was numerically studied. Then, the validated model was combined with curved geometries in order to investigate the effect of curve features on the separation process. The overall purpose was to investigate the interaction between the two different separation methods (separation with curved channels and with contraction–expansion arrays) to find an ideal model that can enjoy the merits of both contraction–expansion and curved channel based methods. Moreover, the relation between separation strength and the aspect ratio in the contraction and expansion zones of the simulated model as well as its height were examined. Then, a new model that combines the curved and the contraction–expansion geometries was tested for its efficiency. This new geometry showed that separation could be achieved with shorter lengths compared with straight contraction–expansion geometries.     

Discrete particle simulation of solids motion in a gas–solid fluidized bed

The solids motion in a gas–solid fluidized bed was investigated via discrete particle simulation. The motion of individual particles in a uniform particle system and a binary particle system was monitored by the solution of the Newton's second law of motion. The force acting on each particle consists of the contact force between particles and the force exerted by the surrounding fluid. The contact force is modeled by using the analogy of spring, dash-pot and friction slider. The flow field of gas was predicted by the Navier–Stokes equation. The solids distribution is non-uniform in the bed, which is very diluted near the center but high near the wall. It was also found that there is a single solids circulation cell in the fluidized bed with ascending at the center and descending near the wall. This finding agrees with the experimental results obtained by Moslemian. The effects of the operating conditions, such as superficial gas velocity, particle size, and column size on the solids movement, were investigated. In the fluidized bed containing uniform particles better solids mixing was found in the larger bed containing smaller size particles and operated at higher superficial gas velocity. In the system containing binary particles, it was shown that under suitable conditions the particles in a fluidized bed could be made mixable or non-mixable depending on the ratios of particle sizes and densities. Better mixing of binary particles was found in the system containing particles with less different densities and closer sizes. These results were found to follow the mixing and segregation criteria obtained experimentally by Tanaka et al.     

Numerical simulation of turbulent gas–particle flow in a 90° bend: Eulerian–Eulerian approach

A numerical investigation into the physical characteristics of dilute gas–particle flows over a square-sectioned 90° bend is reported. The modified Eulerian two-fluid model is employed to predict the gas–particle flows. The computational results using both the methods are compared with the LDV results of Kliafas and Holt, wherein particles with corresponding diameter of 50 μm are simulated with a flow Reynolds number of 3.47 × 10⁵. RNG-based κ–? model is used as the turbulent closure, wherein additional transport equations are solved to account for the combined gas–particle interactions and turbulence kinetic energy of the particle phase turbulence. Moreover, using the current turbulence modelling formulation, a better understanding of the particle and the combined gas–particle turbulent interaction has been shown. The Eulerian–Eulerian model used in the current study was found to yield good agreement with the measured values.     

Analysis of particle dispersion in a turbulent flow considering particle rotation

Non-spherical particles exist widely in natural and industrial fluid systems and the motions of nonspherical particles are significantly different from that of spherical particles. In this paper, a simplified model of non-spherical particles considering particle drag correction, lift, and rotation was established.Based on the Eulerian–Lagrangian simulation, the dispersion characteristics of spherical and nonspherical particles with different Stokes numbers in a high-speed turbulent jet were anal...     

Phenomenological study of saltating motion of individual particles in horizontal particle–gas systems

A comprehensive use of particle–fluid conveying systems for a wide range of industries requires a deep understanding in all interactions of the particular conveying process. One of the most common particle motions occurring in conveying systems is the saltating motion of particles. Although the literature abounds with theoretical, empirical and numerical studies that investigate the saltation phenomenon, there remain many questions and misunderstandings. Some of the recently solved issues are: which non-dimensional groups are introducing the particle saltating motion, how the saltation length might be predicted, how the pipe diameter and the coefficient of restitution influence the saltation velocity and length.The present work investigates the motion of individual saltating particles and presents a wide range of experimental measurements of the conveying length for a variety of particulate solids, sizes and shapes. The total conveying length was divided into three lengths: the first flight, the rebound and the rolling/sliding and each one of them is theoretically and empirically analyzed and compared. This phenomenological study presents the theoretical evidence to previously empirical findings. The theoretical analysis is further used to define the border conditions between various mechanisms. The results show that for coarse particles the rebound and rolling/sliding motions might be presented by a simple relationship between the Reynolds and Archimedes numbers. Additionally we find that the preferred saltation mode of fine powders depends on the conveying system length and diameter. For example for large pipe diameters and short length the first flight mode is the dominant; however, for small pipe diameters and long systems length the rebound mode is the dominant.     

Dynamic modelling of a bubble column for particle formation via a gas–liquid reaction

This paper presents a dynamic model of a bubble column reactor with particle formation, accomplished by adopting a hybrid CFD-reaction engineering approach. CFD is employed for estimating the hydrodynamics and is based on the two-phase Eulerian–Eulerian viewpoint. The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model is then applied to the precipitation of CaCO₃ via CO₂ absorption into Ca(OH)₂aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution.     

Fluid–particle correlated motion and turbulent energy transfer in a two-dimensional particle-laden shear flow

Discrete vortex simulations of a dilute two-dimensional particle-laden shear layer with one-way coupling were performed to study fluid–particle correlated motion and the transfer of turbulent kinetic energy between the phases. The resulting modification of carrier phase turbulence, estimated according to current computational models, was evaluated. Particle Stokes numbers were between 1.0 and 4.5, so that the particles showed considerable temporal concentration fluctuations due to centrifuging by the fluid flow structures, and the mass loading was 12% corresponding to a volume fraction of 6.0×10^?5.Fluid velocities and particle concentration and velocities and their covariances, which appear in a commonly used model equation for carrier phase turbulence modification, were evaluated. Additionally, the probability density functions of fluid velocity fluctuations viewed by the particles are presented and compared with their Eulerian counterparts. It was found that particles view reduced velocity fluctuations due to preferential clustering. The model for carrier-phase turbulence modification predicted turbulence reduction, depending on the particle Stokes number. The mechanism responsible for turbulence reduction was the correlated velocity fluctuations of fluid and particles and this reduction could reach values up to one third of the fluid flow dissipation. Preferential particle concentration together with a relative velocity between the phases could generate turbulent kinetic energy of the gas phase, however this production was nearly an order of magnitude smaller compared to reduction of turbulence due to the correlated motion. The findings were compared with experiments available in the literature and help to clarify the view when turbulence reduction or augmentation occurs.     

Predicting turbulence modulations at different Reynolds numbers in dilute-phase turbulent liquid–particle flow simulations

The present work examines the predictive capability of two-fluid CFD model based on the kinetic theory of granular flow in capturing the Reynolds number (Re) dependence of fluid-phase turbulence modulations in dilute-phase turbulent liquid–particle flows. The model predictions are examined using turbulent liquid–particle flow data in a vertical pipe at Re=17,000, 48,000, 65,000, and 76,000 in the particle concentration range of between 0.5% and 4.0% (v/v). The experimental data indicate that the fluid-phase turbulence intensities are enhanced with respect to the single-phase flow at Re≤48,000 but are attenuated at Re≥65,000. The simulation results indicate that the CFD model can successfully predict the turbulence modulations at Re=17,000, 65,000, and 76,000 both qualitatively and quantitatively, but not at the intermediate Re of 48,000. In this regard, (1) different drag correlations to describe the fluctuating drag force are needed to accurately predict the trends in the turbulence modulations as a function of Re, and (2) appropriate combinations of the drag correlations and turbulence closure models to describe the long-range fluid–particle interactions must be identified in each phase at different Re in order to accurately predict the turbulence modulation, granular temperature, and particle radial concentration profile.     

Numerical simulation of fine particle liquid–solid flow in porous media based on LBM-IBM-DEM

Fine particle liquid–solid flow in porous media is involved in many industrial processes such as oil exploitation, geothermal reinjection, and filtration systems. It is of great significance to master the behaviours of the fine particle liquid–solid flow in porous media. At present, there are few studies on the influences of the migration of fine particles on the flow field in porous media, and the effects of the porosity of porous media and inlet fluid velocity on the migration behaviours of fine particles in porous media. In this paper, a liquid–solid flow model was established based on the lattice Boltzmann method (LBM)-immersed boundary method (IBM)-distinct element method (DEM) and verified by the classical Drag Kiss Tumble (DKT) phenomena and flow around a cylinder successfully. In this model, the interaction between solid particles is analyzed using the distinct element method, and the interaction between fine particles and flow field is handled by IBM. Then, the migration and blockage of fine particles in porous media was studied using this model. It is found that, in addition to the blockage, a large amount of blocked-surface sliding-separation occur in fine particles. At the same time, the decrease in porosity increases the damage degree of fine particles on the permeability. The porosity exerts great influence on the penetration rate and dispersion behaviour of fine particles. The inlet fluid velocity mainly affects the residence time of fine particles and the average velocity of motion in the direction perpendicular to the main flow direction.