Radial Segregation Research Articles

Influence of size-induced segregation on the biomass gasification in bubbling fluidized bed with continuous lognormal particle size distribution

Via the multiphase particle-in-cell approach, the biomass gasification in a three-dimensional bubbling fluidized bed with sand material of wide particle size distribution (PSD) is numerically simulated. The gas and solid motion are solved by treating the gas as the continuum medium in the Eulerian framework and numerical parcel in the Lagrangian framework, respectively. After validating the numerical results with experimental data, the radial and axial segregation induced by the wide PSD and chemical reactions, combined with the particle-scale information of both particle type (e.g., temperature, heat transfer coefficient, constituent mass fraction, and solid residence time) are discussed. The results demonstrate that obvious radial and axial segregation impact the spatial distribution of temperature and heat transfer of both particle types. The skewed distribution of the residence time of biomass particles is obtained. Biomass particles exiting the bed behave with large carbon content. PSD width exerts an evident influence on the spatial distribution of heat transfer coefficient, mass, and constituent content of biomass particles. Enlarging the PSD width enhances the gasification performance of biomass particles with the reduced mass and increased residence time for biomass particles in the bed, and the reduced char residual in the biomass particles lost. The results obtained in this work provide new insights regarding the presence of the radial and axial segregation induced by the wide size distribution and chemical reactions, which can be beneficial for the scale-up and the in-depth understanding of the fundamental mechanisms for the gasification process in this kind of reactor.

Numerical study of the bi-disperse particles segregation inside a spherical tumbler with Discrete Element Method (DEM)

The segregation of equal volumes of 2-mm and 4-mm particles inside a 14-cm-diameter spherical tumbler is numerically investigated using the Discrete Element Method (DEM). Two types of segregation take place. Initially, radial segregation appears, and then axial segregation occurs. The radial segregation occurs quickly due to percolation, with large particles moving towards the outer layer and forming a shell, while smaller particles remaining in the center and forming a core. Axial segregation is a slow process in which particles segregate into two different patterns and both with three bands. Smaller particles are at the equator and larger particles at two poles (Large–Small–Large, LSL), or vice versa (Small–Large–Small, SLS), depending upon the particles filling level inside the tumbler and/or the rotation speed of the tumbler. The total tangential force between the tumbler wall and the granular particles decreases rapidly as radial segregation occurs and then becomes almost constant as axial segregation occurs. The circulation frequency of the particles is analyzed, and the results show that the average circulation frequency of the smaller particles along the rotation axis is higher than the circulation frequency of the larger particles. Also, the distribution of the average circulation frequency is uneven. The simulations match up well with existing experimental and simulation results. The results are of common interest in applications like process engineering, where polydispersed granular materials are ubiquitous, and drums and tumblers are universally applied.

A computational particle fluid-dynamics simulation of hydrodynamics in a three-dimensional full-loop circulating fluidized bed: Effects of particle-size distribution

A computational particle fluid-dynamics model coupled with an energy-minimization multi-scale (EMMS) drag model was applied to investigate the influence of particle-size distribution on the hydrodynamics of a three-dimensional full-loop circulating fluidized bed. Different particle systems, including one monodisperse and two polydisperse cases, were investigated. The numerical model was validated by comparing its results with the experimental axial voidage distribution and solid mass flux. The EMMS drag model had a high accuracy in the computational particle fluid-dynamics simulation of the three-dimensional full-loop circulating fluidized bed. The total number of parcels in the system (Np) influenced the axial voidage distribution in the riser, especially at the lower part of the riser. Additional numerical simulation results showed that axial segregation by size was predicted in the two polydisperse cases and the segregation size increased with an increase in the number of size classes. The axial voidage distribution at the lower portion of the riser was significantly influenced by particle-size distribution. However, radial segregation could only be correctly predicted in the upper region of the riser in the polydisperse case of three solid species.

Metal@SiO2 Core-Shells with Self-Arrested Migrating Core.

Developing easy and customizable strategies for the directional structure modulation of multicomponent nanosystems to influence and optimize their properties are a paramount but challenging task in nanoscience. Here, we demonstrate highly controlled eccentric off-center positioning of metal-core in metal@silica core-shells by utilizing an in situ generated biphasic silica-based intraparticle solid-solid interface. In the synthetic strategy, by including Ca2+-ions in silica-shell and successive oxidative and reductive annealing at high temperature, a unique hairline-biphasic interface is evolved via the heat-induced concentric radial segregation of calcium silicate phase at the interior and normal silica phase at the exterior of core-shell, which can effectively arrest the outwardly migrating metal-core within rubbery calcium silicate phase, affording various eccentric core-shells, where core-positions are flexibly controlled by the annealing time and amounts of initially added Ca2+-ions. In the structure-property correlation study, the strategy allows fine-tuning of dipolar interaction-based blocking temperatures and magnetic anisotropies of different eccentric core-shells as the function of variable off-center distance of magnetic core without changing the overall size of nanoparticles. This work demonstrates the discovery and potential application of biphasic solid-solid media interface in controlling the heat-induced migration of metal nanocrystals and opens the avenues for exploiting the rarely studied high-temperature solid-state nanocrystal conversion chemistry and migratory behavior for directional nanostructure engineering.

Morphological and Optical Characteristics of Transition Metal Doped PVT Grown Zinc Selenide Single Crystal

AbstractAn experimental study on the absorption, emission, crystal quality, and morphological characteristics of the transition metal doped zinc selenide (ZnSe) crystals is performed. This study focuses on the effect of dopant and thermal convection on crystal characteristics. Two ZnSe crystals, one doped with Fe2+ and the other with Cr2+, are grown using the physical vapor transport method. The bulk crystal samples are further divided into localized zones and overall crystallinity is evaluated using optical transparency, scanning electron microscopy, and X‐ray diffraction analyses. Gross defects, such as large precipitates, inclusions and voids, are not observed. The radial segregation and its effect on morphology and optical fluorescence and emission are studied for both doped crystals using different spots along the radius for both crystals. The absorption and emission properties are investigated and the results are discussed in terms of the energy levels from which the optical transitions occur.

The Double Blue Straggler Sequence in NGC 2173: Yes, a Field Contamination Artifact!

Li et al. (2018a; Li18a) claimed that the young stellar cluster NGC2173 in the Large Magellanic Cloud (LMC) harbours a bifurcated sequence of blue straggler stars (BSSs), similar to those detected in a few dynamically old globular clusters. However, Dalessandro et al. (2019; D19) re-analyzed the data by taking into account the contamination of the cluster population from LMC field stars, which was completely neglected by L18a. D19 showed that $\sim40\%$ of the selected BSS sample (and especially the population observed along one of the two sequences) is composed of field star interlopers, concluding that the double BSS sequence is most likely a field contamination artefact. In a recent note Li et al. (2018b) argued that the analysis by D19 is affected by two issues related to (1) the use of different HST instruments/filters in the decontamination procedure, and (2) a presumed overestimate of the number of field stars, which show a central radial segregation. Here we demonstrate that the D19 decontamination procedure is completely unaffected by the use of different HST instruments, and that, the field stars removed by D19 have no significant radial segregation toward the cluster center. Hence, we confirm that the claimed double sequence is just a field contamination artefact.

Intrinsic strain-induced segregation in multiply twinned Cu-Pt icosahedra.

We present an atomistic simulation study on the compositional arrangements throughout Cu-Pt icosahedra, with a specific focus on the effects of inherent strain on general segregation trends. The coexistence of radial and site-selective segregation patterns is found in bimetallic nanoparticles for a broad range of sizes and compositions, consistent with prior analytical and atomistic models. Through a thorough comparison between the composition patterns and strain-related patterns, it is suggested that the presence of gradient and site-selective segregation is natural to largely relieve the inherent strain by preferential segregation of big atoms at tensile sites and vice versa, as previously hypothesized in the literature. Analogous to the case of single crystal particles, Cu-rich surface and damped oscillations can also be found in the outer shells of icosahedra, which are dominated by the lowering of both the surface energy and the chemical energy. The thermodynamic stability of segregated icosahedra is similar to segregated cuboctahedra but higher than disordered bulk alloys, validating prior thinking that element segregation driven by strain relief can extend the stability range of multiply-twinned nanoparticles. Our work sheds new light on understanding strain-induced segregation in multiply-twinned nanosystems that have elements with large lattice mismatch and strong alloying ability.

Centrifugal casting of in-situ Al-Mg2Si(1-x)Snx composites: a study of radial segregation

ABSTRACTDuring centrifugal casting of Al-Mg2Si hypereutectic cylinders, the light Mg2Si particles are commonly segregated in centripetal direction towards the inner periphery. In order to change this inevitable centripetal segregation of Mg2Si particles, some amounts of tin up to 8 wt pct were added to Al-15wt.% Mg2Si alloy to enhance the formation of Mg2Si(1−x)Snx particles with variant stoichiometry thus variant density. XRD, EDS, optical metallography and hardness measurements were used to evaluate the stoichiometry and segregation patterns of in situ formed Mg2Si(1−x)Snx particles. According to the results, the cylinder containing 6 wt pct Sn illustrates a rather uniform distribution of particles thus hardness in all radial sections. However, the cylinders with higher amounts of Sn exhibit a radial segregation of Mg2Si(1−x)Snx particles in centrifugal direction towards the outer periphery. The results are interpreted in view of the Stokes’ law and the difference in density of the particles and the melt.

DEM simulation of heat transfer of binary-sized particles in a horizontal rotating drum

A numerical simulation was conducted by using discrete element method and granular heat conduction model to investigate the heat transfer mechanism inside a horizontal rotating drum. A particulate system containing binary-size spherical particles heating up through the wall was considered as the simulation domain. Radial segregation phenomenon and heat transfer of the particulate system were considered simultaneously. The effects of binary-mixture parameters such as volume ratio (the volume ratio of the small particles to all particles) and diameter ratio (the ratio of large to small diameter of particles) on the heat transfer were investigated. It was found that increasing the volume ratio of small particles causes faster heat up of the particulate bed. Conversely, higher diameter ratios led to lower heat transfer. The temperature difference between small and large particles was more obvious in the case of segregation, and the bed became more thermally non-uniform, particularly in the case of high diameter ratios. Moreover, the effects of important parameters such as rotation speed and filling level, on the heat transfer of the bed were also studied. The results showed that heat transfer rate is in direct relation with rotation speed, but it is inversely related to filling level in the binary-size bed.

Influence of Compaction Length on Radial Melt Segregation in Torsionally Deformed Partially Molten Rocks

AbstractTo investigate the influence of compaction length on radial melt segregation during torsional shear deformation of partially molten rocks, experiments were performed on samples composed of olivine plus ∼7 vol.% of either an albite, alkali basalt, or lithium silicate melt. These three melts cover a range of three orders of magnitude in viscosity, yielding samples that vary by approximately two orders of magnitude in compaction length. Samples were deformed in torsion at 1,473 K and 300 MPa in constant strain rate experiments to outer‐radius shear strains of up to 14.3. Radial melt segregation occurred toward the axial center in all three types of samples that were sheared to 4. At the same strain, samples with the largest compaction length exhibited the highest segregation rate, while samples with intermediate and smallest compaction lengths exhibited similar segregation rates. The experimental observations qualitatively agree with previously published results from two‐phase flow theory for base‐state melt segregation with anisotropic viscosity; specifically, the segregation rate for radial melt segregation increases with increasing compaction length. However, quantitatively, the segregation rate in experiments is smaller than the rate predicted by simulations for the same compaction length. This discrepancy may, for example, reflect the difference in rheological behavior between that observed in our experiments (non‐Newtonian, dislocation‐accommodated creep) and that incorporated into the numerical models (Newtonian, diffusion‐accommodated creep). Our results thus provide a baseline for testing current and future models of two‐phase flow, particularly as applied to understanding melt migration, segregation, and extraction from Earth's deeper interior.

End-wall effects on the mixing process of granular assemblies in a short rotating drum

The mixing process of binary granular systems in a short rotating drum was investigated by performing discrete element simulations. The drum was asymmetrically operated with the left end wall being fixed and the right one rotated with the side wall. The modeled granular materials were made of particles with same size and density but different colors (red and yellow). These yellow and red particles were initially arranged side by side axially. We observed that the system inevitably went through a transient radial segregation state before reaching the final homogeneous mixing state. Depending on the filling degree and rotating speed, there existed totally three types of radial segregation pattern: yellow particles were enveloped radially by red particles (R-Y pattern), red particles were besieged radially by yellow particles (Y-R pattern), and the sandwich type R-Y-R pattern. Analyses of the axial flow field show that such transient radial segregating phenomena were induced by different axial transporting characteristics of particles. The average axial motion of particles in the flowing layer and the location of the boundary between flowing layer and fixed bed layer influenced the transient radial configuration.

Transformation of pore structure in consolidated silty clay: New insights from quantitative pore profile analysis

Pore structure provides essential information for studying the behaviors and properties of silty clay in construction projects. This study uses a developed metal intrusion characterization scheme to investigate the transformation of the pore structure of silty clay in consolidation. Clear pore profile images with nanometer-level resolution are produced by BSE imaging. Comparison between metal intrusion and epoxy impregnation suggests minimal alteration of the pore structure of silty clay with the metal intrusion technique. The pore size redistribution and the transformation of porosity indicate that the pores collapse and form during consolidation process. Solidity is found to decrease as consolidation pressure increases, reflecting the consolidation-induced pore deformation. Aspect ratio is found to be independent of the consolidation pressure, indicating that the pores are likely to shrink evenly in consolidation. Two descriptors, box dimension and probability entropy, are found to decrease as consolidation pressure increases, indicating that the overall pore structure becomes homogenized in consolidation. Washburn’s equation is modified based on the area–perimeter relation of pores to provide a more accurate reflection of the pore size measurement by mercury intrusion porosimetry. The results show clear evidence for a distinctive two stages transformation processes during consolidation namely radial compaction and pore segregation.

Small- and large-scale galactic conformity in SDSS DR7

Galactic conformity is the phenomenon whereby galaxy properties exhibit excess correlations across distance than that expected if these properties only depended on halo mass. We perform a comprehensive study of conformity at low redshift using a galaxy group catalogue from the SDSS DR7 spectroscopic sample. We study correlations both between central galaxies and their satellites (1-halo), and between central galaxies in separate haloes (2-halo). We use the quenched fractions and the marked correlation function (MCF), to probe for conformity in three galaxy properties, $(g-r)$ colour, specific star formation rate (sSFR), and morphology. We assess the statistical significance of conformity signals with a suite of mock galaxy catalogues that have no built-in conformity, but contain the same group-finding and mass assignment errors as the real data. In the case of 1-halo conformity, quenched fractions show strong signals at all group masses. However, these signals are equally strong in mock catalogues, indicating that the conformity signal is spurious and likely entirely caused by group-finding systematics, calling into question previous claims of 1-halo conformity detection. The MCF reveals a significant detection of radial segregation within massive groups, but no evidence of conformity. In the case of 2-halo conformity, quenched fractions show no significant evidence of conformity in colour or sSFR once compared with mock catalogues, but a clear signal using morphology. In contrast, the MCF reveals a small, yet highly significant signal for all three properties in low mass groups and scales of $0.8-4\ h^{-1}\textrm{Mpc}$, possibly representing the first robust detection of 2-halo conformity.

The effect of stellar-mass black holes on the central kinematics of ω Cen: a cautionary tale for IMBH interpretations

The search for intermediate-mass black holes (IMBHs) in the centre of globular clusters is often based on the observation of a central cusp in the surface brightness profile and a rise towards the centre in the velocity dispersion profiles. Similar signatures, however, could result from other effects, that need to be taken into account in order to determine the presence (or the absence) of an IMBH in these stellar systems. Following our previous exploration of the role of radial anisotropy in shaping these observational signatures, we analyse here the effects produced by the presence of a population of centrally concentrated stellar-mass black holes. We fit dynamical models to omega Cen data, and we show that models with ~5% of their mass in black holes (consistent with ~100% retention fraction after natal kicks) can reproduce the data. When simultaneously considering both radial anisotropy and mass segregation, the best-fit model includes a smaller population of remnants, and a less extreme degree of anisotropy with respect to the models that include only one of these features. These results underline that before conclusions about putative IMBHs can be made, the effects of stellar-mass black holes and radial anisotropy need to be properly accounted for.

Particle shape-induced radial segregation of binary mixtures in a rotating drum

Rotating drums are widely used in various processes such as mixing, coating and drying, where particle segregation remains one of the major concerns. In the past, most of the studies focus on the size or density induced segregation with the assumption that particles are spherical. The particle shape-induced segregation is still not well addressed and understood. In this work, DEM is used to study the radial segregation of binary mixtures of ellipsoidal particles in a rotating drum. The results show that for the binary mixture of ellipsoids (aspect ratio AR = 0.5 or AR = 2.0) and spheres, ellipsoids tend to distribute in the core of the bed, while spherical particles are more likely to accumulate in the periphery. When particle shape difference in the mixture increases, inverse segregation occurs with ellipsoids in the periphery and spheres in the centre. This is attributed to the increased flowability of ellipsoids and the formation of large voids in the free surface flowing layer. Spheres can percolate through the large voids and be trapped in the core section. For the mixture of oblate (AR = 0.5) and prolate (AR = 2.0) spheroids, no segregation is observed. The effect of rotating speed on the degree of segregation is also examined, showing that with the rotating speed increasing from 5 to 40 rpm, the segregation degree reduces for all mixtures.

An overabundance of black hole X-ray binaries in the Galactic Centre from tidal captures

A large population of X-ray binaries (XRBs) was recently discovered within the central parsec of the Galaxy by Hailey et al. While the presence of compact objects on this scale due to radial mass segregation is, in itself, unsurprising, the fraction of binaries would naively be expected to be small because of how easily primordial binaries are dissociated in the dynamically hot environment of the nuclear star cluster (NSC). We propose that the formation of XRBs in the central parsec is dominated by the tidal capture of stars by black holes (BHs) and neutron stars (NSs). We model the time-dependent radial density profiles of stars and compact objects in the NSC with a Fokker-Planck approach, using the present-day stellar population and rate of in situ massive star (and thus compact object) formation as observational constraints. Of the ~10,000-40,000 BHs that accumulate in the central parsec over the age of the Galaxy, we predict that ~60 - 200 currently exist as BH-XRBs formed from tidal capture, consistent with the population seen by Hailey et al. A somewhat lower number of tidal capture NS-XRBs is also predicted. We also use our observationally calibrated models for the NSC to predict rates of other exotic dynamical processes, such as the tidal disruption of stars by the central supermassive black hole (~0.0001 per year at z=0).

Manufacturing of functionally graded metal matrix composite materials by segregation

AbstractBy applying vibrations to a granular media differing in size and density, various segregation states can be established. If an additional rotational motion is engaged, the effective force is no longer gravity but a centrifugal one, leading to a radial segregation. In this contribution, an experimental setup is presented which utilizes these effects. This setup is used to produce a cylindrical metal matrix composite consisting of silicon carbide and aluminium having a radial gradient. The influence of different material- and process-specific parameters on the segregation behaviour was investigated. The evaluation of micrographs of the pressed and sintered samples shows that both positive and negative gradients can be achieved. The rotation speed and the grain size ratio were identified as significant factors. The variation of the vibration amplitude leads to opposite effects. On the one hand, the gradient intensity increased. On the other hand, the variances of the SiC distribution in the tangential or radial directions increased as well. In addition, it has been shown that the effects of different grain shapes are marginal.

DEM investigation of the axial dispersion behavior of a binary mixture in the rotating drum

The granular motion of binary-size mixture in a three-dimensional rotating drum operating in the rolling regime is numerically tracked via the discrete element method (DEM), with a focus on the axial dispersion behavior of the different particle types in response to the inherent size-segregation. Accordingly, the evolution with time, the frequency distribution and the space-time profiles of the axial dispersion coefficients are evaluated. The results demonstrate that (i) the fast radial segregation gives rise to the sharp increase and decrease of the axial dispersion coefficients of respectively the large and small particles in both the active and passive regions; (ii) the axial dispersion coefficients in the active region is an order-of-magnitude higher than that in the passive region; (iii) after the radial segregation, the frequency distributions of the axial dispersion coefficients of the large particles, the small particles and all the particles are normal distributions; (iv) the greatest axial dispersion coefficients of both particle types are at the upper part of the active region near the bed surface; (v) increasing the rotating speed and particle diameter ratio clearly enhances the axial dispersion intenstity in the active and passive regions; and (vi) axial segregation results in the greatest axial dispersion near the end walls, while most of the rest of the drum exhibit equally low axial dispersion. The results obtained here contribute towards understanding the dynamical response of the axial dispersion of different particle types to the inevitable radial and axial segregation phenomena, and are expected to be valuable for improving operations and advancing theoretical models of such granular systems with constituents of different sizes.

Numerical study on the axial segregation dynamics of a binary-size granular mixture in a three-dimensional rotating drum

Granular materials are ubiquitous in our daily life and inherent in multitudinous industrial processes. Differences in the granular properties such as size and density inevitably induce segregation. By means of the discrete element method, a binary-size mixture in a three-dimensional rotating drum is numerically simulated to explore the segregation dynamics of the granular material along the axial direction. Snapshots of the distribution of the two particle types in the rotating drum are presented with respect to time to illustrate the spatial evolution of the size-induced segregation structure. The space-time plots of various axial characteristics indicate that (i) radial segregation does not affect the axial distribution of total mass and mass fraction, but axial segregation leads to the formation of axial bands; (ii) greater non-dimensionalized collision forces for both the large and small particles develop where the large particles dominate; and (iii) axial segregation gives rise to the variation of the gyration radii of both particle types along the drum length. In addition, axial flow of both particle types in both directions indicates the dynamic axial exchanges, and the effect of the end walls on the axial flow direction is limited to less than 25% of the drum length from the end walls.

Mixed undifferentiated neuroendocrine tumor and exocrine adenocarcinoma of the pancreas in an adolescent

Segregation and mixing of granular materials are complex processes and are not fully understood. Motivated by industrial need, we performed a simulation using the discrete element method to study size segregation of a binary mixture of granular particles in a horizontal rotating drum. Particles of two different sizes were poured into the drum until it was 50% full. Shear-driven segregation was induced by rotating the side-plates of the drum in the opposite direction to that of the cylindrical wall. We found that radial segregation diminished in these systems but did not completely vanish. In an ordinary rotating drum, a radial core of smaller particles is formed in the center of the drum, surrounded by larger revolving particles. In our system, however, the smaller particles were found to migrate toward the side-plates. The shear from anti-spinning side-plates reduces the voidage and increases the bulk density. As such, smaller particles in the mixer tend to move to denser regions. We varied the shear by changing the coefficient of friction on the side-plates to study the influence of shear rate on this migration. We also compared the extent of radial segregation with stationary side-plates and with side-plates moving in different angular directions.