Particle Dispersion Characteristics Research Articles

Numerical simulation on the larger concentration difference characteristics of dense granular jet in a coaxial gas stream

AbstractThis work is devoted to the numerical study of particle dispersion prediction of gas–solid coaxial jet with a larger concentration difference. In pulverized coal gasifier, the pulverized coal is very dense at the nozzle outlet, and its volume fraction can reach 0.3. Then, it is dispersed rapidly under the action of high‐speed annular gas, and the particle volume fraction in the space is less than 0.002. In order to better predict the motion characteristics of a gas–solid jet with a high concentration gradient, the applicability of the three models is compared. The results show that the dense discrete phase model (DDPM) and Eulerian two‐fluid model (TFM) models considering particles collision can accurately predict the dense jet flow at low annular gas velocity. The DDPM and discrete phase model (DPM) show a good simulation for the dispersion characteristics of particles at high annular velocities where the particles' collision could be ignored. Therefore, DDPM has better adaptability for coaxial jet with large concentration gradient. The DDPM was used to predict the particle velocity and concentration for different annular gas velocities and different particle mass loads. It is found that particle flow is contracted first and then dispersed gradually under the action of airflow. The particle dispersion range increases with the increase of solid loading rate, and the corresponding radial distribution of particle velocity is greater.

Super-quadric CFD-DEM simulation of chip-like particles flow in a fluidized bed

Chip-like particles may be practised in fluidized beds in some emerging industries, but its fluidization is not well understood. In this work, the computational fluid dynamics-discrete element method (CFD-DEM) is extended to integrate with a super-quadric model to study the fluidization behaviours of chip-like particles in a three-dimensional bubbling fluidized bed (BFB). After model validation, the typical fluidization behaviours of chip-like particles in the BFB are described in terms of bubble dynamics, pressure signals, particle orientation, particle mixing, and particle dispersion. Particularly, the ordered arrangement of chip-like particles appears at the near-wall region while random arrangement occurs in other regions. The effects of superficial gas velocity and aspect ratio on the mixing and dispersion characteristics of chip-like particles are discussed. The results show that increasing aspect ratio induces a higher mean pressure drop and promotes the initial mixing state. Particle dispersion coefficients range from 10-3 m2/s to 10-2 m2/s, whose vertical component is one order of magnitude larger than the horizontal components. Larger aspect ratios and superficial gas velocities lead to larger particle dispersion coefficients. The fundamental study sheds light on the design and optimization of fluidized beds of chip-like particles including solar panel waste recycling.

Improving particle dispersion characteristics with a novel cleaning screen: parameter design and numerical simulation

To reduce particle accumulation on a traditional planar-linear screen, a novel screen with sliding fingers and ladders was designed by numerical simulation and theoretical analysis. The dispersion behaviors of particles under the action of sliding finger and ladder were analyzed using the CFD-DEM (Computational Fluid Dynamics and Discrete Element Method). The effects of traditional and novel screens on the airflow distribution, particle motion and particle dispersion were investigated. The results indicated that the novel screen enhanced the airflow velocity and discharge in the traditional cleaning unit. This screen made the grains penetrate the sliding fingers, and the impurities were thrown behind the screen. It improved the particle accumulation on the traditional screen. The comprehensive dispersion degree of particles perpendicular to the entire novel screen surface was 8.14% higher than that with the traditional screen. The comprehensive dispersion degree of particles along the entire novel screen surface was higher by 6.87%.

직렬로 배열된 두 원형 실린더 주위를 지나는 입자가 포함된 유동

Numerical simulations are performed for particle-laden flow around two circular cylinders in tandem arrangements. The present study aims to understand the effects of cylinder spacing and particle size on particle dispersion and deposition characteristics. We consider the Reynolds number of 100 at which vortex shedding takes place, and the density ratio of particle to fluid is fixed as 1,000. For a parametric study, we vary the Stokes number from 0.1 to 10 for the center-to-center spacing of 2D, 3D, and 4D, where D is the cylinder diameter. As the spacing decreases from 4D to 2D, vortex shedding becomes weaker and lift fluctuations are reduced, agreeing well with the previous results. The dispersion pattern of particles around the cylinder changes significantly depending on the spacing. The overall deposition efficiency of the upstream cylinder is very similar to that of a single cylinder. However, the efficiency of the downstream cylinder has much smaller values with a strong dependency on the spacing. The local deposition efficiency and the deposition location are also analyzed in detail.

Development of Multiple Injection Channels During a Sawtooth Substorm Event

AbstractSubstorm injection boundary was proposed to explain the dispersion characteristics of particles during the substorm expansion phase. Sawtooth event, a peculiar quasi‐periodic type of substorms, is believed to have a very wide injection boundary around the geosynchronous orbit. In this study, we combine the numerical simulation of the Rice Convection Model with the IMAGE, LANL, and GOES data to investigate how the injection develops and couples with the ionosphere during a sawtooth substorm. We find the wide injection breaks up into multiple injection boundaries/channels with the local time separation of about 1–2 h. Using backward tracing of energetic particles in the self‐consistently computed electromagnetic field, we can infer the MLT locations of the channels, which are found to be nearly one‐to‐one co‐located with multiple finger‐like auroral structures. Our study provides strong evidence that the interchange instability plays a central role in the development of multiple injection channels.

Investigation on ultralight particle dispersions in dense two-phase flow using a kinetic friction stress model

The graphite particle dispersion characteristics with irregular shapes of dense gas-solid turbulent flow in downer reactor is numerically investigated using an improved kinetic friction stress model and a adopted drag coefficient describing momentum transfer between gas and non-spherical particles. Hydrodynamic predictions are in better line with experimental results. Redistributions of Reynolds normal and shear stresses with strongly anisotropic dispersions are captured. Comparisons to glass bead, ultralight particles elongate the second acceleration length, gather the concentration peaks at near wall region, and weak the maximum dense ring. Maximum kinetic energy at fully development region is approximately 1.2 times larger than that of second acceleration region. Lower particle sphericities are contribution to the enhancement of particle velocity, particle concentration, particle shear stress, particle collision frequency, and granular temperature, except for the reduction of turbulent kinetic energy. The negative amplitudes of shear stresses at fully development regions are approximately 4.0 times larger than those of second acceleration regions.

Local dispersion characteristics of dust in large open-air piles under the action of one-way wind.

A large amount of dust particles produced by the wind in an open-air pile is one of the important reasons for air pollution. Studying the law of dust diffusion in local areas is of great significance for the atmospheric particulate control. In this study, a pile of sodium carbonate in a large open-air pile in Weifang, China, is regarded as the research object. The dispersion characteristics of dust particles around the pile under the action of unidirectional wind are studied through wind tunnel test and numerical simulation. The complex atmospheric environment is simplified as unidirectional wind, and the influence of different wind speeds on the dispersion of particles with diverse sizes in the pile is studied. Although a large gap exists between the assumption and the real atmospheric environment, this study provides a reference for the evaluation of the pollution scope of blowing dust and prevention and control of pollution. Results show that a high-concentration range of the dust exists near the pile behind the wind direction and may continue to spread to the height due to the influence of a whirlpool, and the dispersion distance and width can increase with the increase in wind speed. The increase in particle diameter increases the kinetic energy loss of particles for the fluid. Under the same starting speed, the dispersion distance of dust decreases with the increase in particle diameter. With the increase in particle diameter, the dust concentration distribution presents the trend of interior hollowing and high-concentration area fragmenting.

Numerical investigation on mixture particle dispersion characteristics in swirling particle-laden combustion chamber

An improved second-order moment mixture gas-particle turbulent model based on the Euler-Euler two continuum approach was developed to indicate the ultralight particle dispersion characteristics in swirling particle-laden combustion chamber. In this model, the fully anisotropic behaviors of heavier and ultralight particle interactions were considered by a improved closure Reynolds stresses transport equations. Numerical simulation was performed and validation for single heavier particle flow by Sommerfeld et al. [4] was obtained with satisfied agreement. Ultralight particle exhibited the distinctively anisotropic dispersion characteristics, particle velocities at central core region, and their fluctuations both axial and tangential directions are smaller than those of heavier particles due to small inertia. At near inlet region, ultralight particle fluctuation velocities are larger than those of heavier particle and then gradually decreased toward downstream direction. Meanwhile, they are prone to be thrown out the out-edge recirculation near wall region. With the increasing ultralight particle mass loading ratio, the interactions between heavier and ultralight particles, and their anisotropic behavior was enhanced. Furthermore, the distributions of normal and shear particles Reynolds stresses, and interactions between ultralight and heavier particles are found to be redistributed.

Deposition and dispersion characteristics of ultrafine particles under different vehicle speeds in road tunnels employing dynamic mesh simulation.

The purpose of this study is to analyze the unsteady turbulence, deposition, and dispersion characteristics of ultrafine particles of different sizes during the passage of vehicles through a road tunnel. We present a realizable k-ε turbulence model based on a combination of Eulerian passive scalar and dynamic mesh models. A 100-m three-dimensional model of a road tunnel containing four vehicles in a row and 10-m long outlets was established using STAR-CCM+ computational fluid dynamics software. Ultrafine particles derived from vehicle exhaust gas were modeled at velocities of 60, 40, and 20 km h-1. The diameter and number of particles ranged from 26 to 287 nm and 1.7 × 104-1.7 × 105 cm-3, respectively. The results show that, in the vertical plane (located 0.1 m behind the exhaust pipe), the deposition effect cannot be ignored at low velocity. Furthermore, the particle concentration distribution both in the breathing zone and the entire tunnel is not significantly affected by deposition.

Dispersion Characteristics of PM10 Particles Identified by Numerical Simulation in the Vicinity of Roads Passing through Various Types of Urban Areas

The dispersion of particulate matter emitted by road transport to the vicinity of roads is predominantly influenced by the character of the air velocity field. The air flow depends on factors such as the speed and direction of the blowing wind, the movement of cars, and the geometries of the buildings around a road. Numerical modeling based on the control volume method was used in this study to describe the relevant processes closely. Detailed air velocity fields were identified in the vicinity of a straight road surrounded by various patterns of built-up urban land. The evaluation of the results was generalized to exponential expressions, affecting the decrease of the mass concentration of fine particles with the increasing distance from the road. The obtained characteristics of the mass concentration fields express the impact of the building geometries and configurations on the dispersion of particulate matter into the environment. These characteristics are presented for two wind speeds, namely, 2 m·s−1 and 4 m·s−1. Furthermore, the characteristics are introduced in relation to three wind directions: perpendicularly, obliquely, and in parallel to the road. The results of the numerical simulations are compared with those obtained via the in-situ measurements, for verification of the validity of the linear emission source calculation.

Experimental investigation on fly ash particulate reinforcement for property enhancement of PU foam core FRP sandwich composites

Low-cost industrial byproduct fly ash (FA) particulates were used as reinforcement to improve the mechanical performance of Polyurethane (PU) foam core and PU based laminated sandwich composites. PU foam core specimens were prepared with 5–20 wt % variation of FA fillers with surface treatment using silane solution. Surface treated core specimens were found to exhibit higher shear strength than untreated specimens due to superior interface adhesion properties and uniform distribution of particles. Two different face sheet-core combination sandwiched composite specimens were designed using woven fabric glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) face sheet with surface treated PU foam core. Since shear and bending being the predominant modes of failures in sandwiched composite structures, accordingly shear and three-point bending tests were conducted as per ASTM standard. Enhanced stiffness and up to 39% increase in shear modulus was observed for FA reinforced PU foam (FA-PUF) core than the neat PU core due to a synergistic effect of interfacial bonding between FA and polymer. SEM images explained the enhanced particle dispersion and strut characteristics. With an optimized FA reinforcement, the mechanical performance of the sandwiched composites was found to improve significantly as evidenced from the flexural and shear test diagrams.

Dispersion characteristics of typical non-spherical particles in a high-speed round jet

Dispersion characteristics of typical non-spherical particles in a high-speed round jet

Study on quantification method for dispersion and distribution of sphere-like particles and relationship with AC/DC breakdown strength in polymer nanocomposites

Polymer nanocomposites are more and more researched and employed as dielectrics in electrical power equipment. However, the bad dispersion and distribution of particles are often reported to deteriorate the dielectric properties of polymer nanocomposites. In most cases, researchers tend to use SEM/TEM images with imprecise descriptions to describe these two factors, and there was still no clear relationship between their quantified particle dispersion characteristics and dielectric properties of polymer nanocomposites. This work is to, first, propose a combined quantification method to estimate the dispersion and distribution of spherical/ellipsoidal particles/aggregates in polymer nanocomposites based on SEM images of epoxy SiO 2 nanocomposites. Based on the proposed quantification method, epoxy nanocomposite with surface treated SiO 2 shows overall better dispersion and distribution of particles/aggregates than those with untreated ones. The presence of agglomerations would lead to the enhancement of mobility of charge carriers and thus reduction of breakdown strength, which become more obvious with the growth of filler loadings. It is found that, in AC breakdown tests, dispersion and distribution of particles/aggregates show little influence on the reduction of AC breakdown strength. However, those should be the main factor which influences the DC breakdown strength in epoxy nanocomposites with a variation of filler loading concentrations.

Intensification on drawdown process of floating particles by circle package fractal impellers

Abstract The floating particle drawdown and dispersion characteristics in a stirred tank with four pitched-blade impellers, fractal impellers, and circle package fractal impellers were studied using computational fluid dynamics (CFD) simulation. The effects of impeller type, impeller speed, impeller spacing, impeller submergence, initial floating particle concentration, and floating particle diameter on the floating particle dispersion quality were investigated. Results showed that the floating particle dispersion degree was improved with an increment in the impeller speed, in term of axial solid concentration profile, solid concentration distribution, and cloud height. The impeller spacing of T2/3 and impeller submergence of T/6 were appropriate for the floating particle drawdown process in this work. Higher initial floating particle concentration was easier to achieve a higher floating particle dispersion degree. Smaller particle diameter resulted in smaller buoyancy and more uniform distribution. Meanwhile, circle package fractal impeller can reduce the power consumption compared with four pitched-blade impeller and fractal impeller at the same impeller speed, and enhance the solid integrated velocity and the level of homogeneity for floating particle dispersion process under the constant power consumption. In addition, the just drawdown speed (Njd) and correlation equation for Njd in the three different impeller systems were obtained.

Analysis and optimisation of particle mixing performance in fluid phase resonance mixers based on Euler/Lagrange calculations

A novel mixing principle utilising oscillating liquid columns was analysed numerically with regard to particle dispersion characteristics. For producing fluid oscillations a pipe (diameter 100 mm) was immersed centrally into a vessel (diameter 450 mm) filled with liquid (filling height 700 mm) and periodically pressurised (frequency 1.2 Hz). The outlet geometry of the central pipe, just ending near the vessel bottom, has a strong effect on mixing and was optimised in this study. The principle of a FPR-mixer does not require rotating stirrers and in the turbulent regime it has power numbers comparable to propellers. The numerical calculations were conducted by a Euler/Lagrange approach neglecting two-way coupling as well as inter-particle collisions for clarity in order to only focus on the effect of interfacial forces on particle dispersion. The continuous phase was calculated in an unsteady way based on the Reynolds-averaged equations combined with the k-ω-SST (shear stress transport) turbulence model. Lagrangian tracking was conducted considering all relevant forces; drag, gravity/buoyancy, fluid inertia, added mass, Basset force and transverse lift forces due to shear and particle rotation. The importance of these forces was analysed with respect to the turbulent particle Stokes number (considered range 0.004 < St < 10.0) and particle/liquid density ratio (i.e. 1.05, 1.5 and 2.5). Finally, the significance of Basset force and shear-rotation lift force (i.e. Magnus effect) on the dispersion process was quantified by mixing parameters.

Effects of swirling flow on ultralight particle dispersion characteristics in coaxial jet combustor

AbstractThe dispersion characteristics of ultralight particle (expanded graphite) in swirling mixture particle‐laden coaxial jet combustor was modeled and numerically predicted using developed second‐order moment gas‐particle multifluid turbulent model. An improved gas‐particle and particle‐particle Reynolds stress transport to describe its anisotropic behaviors was proposed. Experimental validation for single‐phase particle flow has been conducted with satisfied agreement. The multifluid hydrodynamic parameters, that is, number density, particle velocity, fluctuation velocity, Reynolds stress correlation of gas‐particle and particle‐particles, and effects of swirling number are investigated. The results found that the ultralight particles exhibited the distinctive anisotropic behaviors, and their unique flow structures were quite different from those of heavier particles. Under the strong swirling flow condition, the anisotropic dispersions were enhanced and concentration at recirculation regions increased. The axial velocities and fluctuations, and the axial‐axial and tangential‐tangential stress correlations of gas‐particles were weakened. The effect of swirling strength on ultralight particle dispersions is more sensitive in comparison with heavier particle.

Experimental and numerical analysis of turbulent pulverized coal flame in a coaxial burner

A hydrogen supported turbulent pulverized coal combustion flame was investigated experimentally and numerically to analyze combustion characteristics and flame structure. The flame was experimentally investigated by direct measurement methods and non-intrusive optical diagnostic methods. Temperature of gaseous phase was observed by a sheathed thermo-couple, while gas composition was examined by a gas analyzer with a sampling probe. Particle dispersion characteristics and velocity were analyzed by Mie scattering and PIV techniques using an optical measurement system. A numerical analysis was simultaneously performed to investigate the flame in detail by means of LES. The simulation was validated by comparing gaseous temperature, velocity and composition results to the experiment. The issued pulverized coal particles were distributed in limited area even in downstream by forming linear shape of flame. The particles moved by forming a cloud in the flame in upstream, but the cloud was broken up in downstream when it passed the flame surface. The characteristics of particle movement and influence of combustion were discussed with the analyses of particle velocity and momentum transfer. The flame structure was discussed with the results of gaseous mole fraction, and NOx formation was numerically investigated in this study.

Particle percolation in a poly(lactic acid)/calcium carbonate nanocomposite with a small amount of a secondary phase and its influence on the mechanical properties

In this study, poly(lactic acid) (PLA) is blended with natural rubber (NR) to improve a brittleness of PLA. Surface‐modified inorganic particles of calcium carbonate are incorporated to this polymer blend not only to mitigate the strength loss but also to induce toughening of the PLA. Specifically, in order to toughen the PLA, the influence of NR as a second phase on the particle dispersion characteristics is examined while varying the concentration of the particles or the second phase and undertaking rheological measurements and morphological observations. With the surface‐modified particles, the secondary phase was found to induce the aggregation of particles along the interface as opposed to creating a homogeneous dispersion of particles. In addition, it enhanced the particle percolation throughout the blend and improved the brittleness of the PLA. With regard to elongation at break of the PLA/particle, it was increased from 12 to 86% by the addition of NR (19 vol%). On the other hand, for a composite with uncoated particles, the second phase of NR did not induce particle aggregation. Elongation at break of this composite decreased as the particle concentration increased. POLYM. COMPOS., 40:4023–4032, 2019. © 2019 Society of Plastics Engineers

Effect of Silane Capping on the Dispersion and Combustion Characteristics of Sub-micrometer Boron Particles Loaded in Jet A-1

The present investigation deals with the surface modification of sub-micrometer boron particles with octadecyltrimethoxysilane (OTMS, a silane compound) to improve the dispersion stability of boron...

Dispersion and migration of agricultural particles in a variable-amplitude screen box based on the discrete element method

Many relevant studies about screening of agriculture materials have been conducted from different perspectives such as mechanism design, optimization of parameters, and particle motion. Some studies suggest that a screen driven by parallel mechanisms is more adaptable than a traditional one while the traditional reciprocating screen still plays a positive role in agricultural production. Based on previous research regarding the variable-amplitude screening method, 4 indices of particle movement have been defined and computed in this paper to investigate the quantitative dispersion and migration characteristics of agricultural particles by using DEM (Discrete Element Method) simulation data. It shows that the turning angle of the front swing bar has a significant effect on the horizontal expansion coefficient δx(t), and the particles will be thin quickly at first and be stabilize within the next process. It also shows that the increase in the turning angle of the front swing bar leads to a negative stratification effect, but results in a positive migration effect of all particles on the screen. This research could provide a useful reference for solving the retention problem of agricultural particles in any position on the screen.