Radial Segregation Research Articles

Characterization of 4 in VGF-GaAs single crystals grown in a heater-magnet module

Silicon-doped 4in VGF-GaAs single crystals were grown under the influence of traveling magnetic fields (TMF). A KRISTMAG˜® heater-magnet module (HMM) was used for the simultaneous generation of heat and TMF through a combination of DC and AC control. In this study, we investigated changing structural and electronical properties as well as micro- and macrosegregation of VGF-GaAs single crystals grown with applied TMF.Striations were observed in crystals grown without or too strong TMF. Almost no micro-inhomogeneities were detected when the magnetic flux densities of the TMF were matched to progression of solidification. With utilized TMF, induced melt flow opposed natural convection driven by buoyancy forces. Axial dopant incorporation was enhanced through a reduction of flow velocities and converging melt flow towards the center of the solid–liquid interface. The radial segregation profiles were flattened through a reduction of the concave deflection. While low frequency TMF bended the interface center more convex, the effect of high frequency TMF was more limited to the melt periphery. Furthermore, with large high frequency TMF current shares the impact of the minor three-dimensional asymmetric magnetic field distribution in the HMM became more relevant. Consequently, the application of double-frequency TMF led to a reduction of etch pit density through reduction of interface concavity.

Simulation of particle flow in a bell-less type charging system of a blast furnace using the discrete element method

A three-dimensional model was established by the discrete element method (DEM) to analyze the flow and segregation of particles in a charging process in detail. The simulation results of the burden falling trajectory obtained by the model were compared with the industrial charging measurements to validate the applicability of the model. The flow behavior of particles from the weighing hopper to the top layer of a blast furnace and the heaping behavior were analyzed using this model. A radial segregation index (RSI) was used to evaluate the extent of the size segregation in the charging process. In addition, the influence of the chute inclination angle on the size segregation and burden profile during the charging process was investigated.

Chromosome positioning from activity-based segregation

Chromosomes within eukaryotic cell nuclei at interphase are not positioned at random, since gene-rich chromosomes are predominantly found towards the interior of the cell nucleus across a number of cell types. The physical mechanisms that could drive and maintain the spatial segregation of chromosomes based on gene density are unknown. Here, we identify a mechanism for such segregation, showing that the territorial organization of chromosomes, another central feature of nuclear organization, emerges naturally from our model. Our computer simulations indicate that gene density-dependent radial segregation of chromosomes arises as a robust consequence of differences in non-equilibrium activity across chromosomes. Arguing that such differences originate in the inhomogeneous distribution of ATP-dependent chromatin remodeling and transcription machinery on each chromosome, we show that a variety of non-random positional distributions emerge through the interplay of such activity, nuclear shape and specific interactions of chromosomes with the nuclear envelope. Results from our model are in reasonable agreement with experimental data and we make a number of predictions that can be tested in experiments.

Emergent Phenomena via Molecular Dynamics

Emergent phenomena are unusual because they are not obvious consequences of the design of the systems in which they appear, a feature no less relevant when they are being simulated. Several systems that exhibit surprisingly rich emergent behavior, each studied by molecular dynamics (MD) simulation, are described: (i) Modeling self-assembly processes associated with virus growth reveals the ability to achieve error-free assembly, where paradoxically, near-maximum yields are due to reversible bond formation. (ii) In fluids studied at the atomistic level, complex hydrodynamic phenomena in rotating and convecting fluids – the Taylor– Couette and Rayleigh–Bénard instabilities – can be reproduced, despite the limited length and time scales accessible by MD. (iii) Segregation studies of granular mixtures in a rotating drum reproduce the expected, but counterintuitive, axial and radial segregation, while for the case of a vertically vibrated layer a novel form of horizontal segregation is revealed.

Si1−xGex (x≥0.2) crystal growth in the absence of a crucible

In this work we made a first attempt to grow Si1−xGex single crystals by the so called modified float zone technique [1], without contact of container walls, using a modified AHP (axial heat flux close to the phase interface) heater. This method was derived from the “modified Bridgman” technique using a submerged heater [2]. Some advantages of this method are the improvement in crystal quality [3,4], the reduction of the radial temperature gradient and with it the reduced radial segregation during the growth experiment. For comparison another Si1−xGex crystal was grown by the conventional float zone method, using a residual spark plasma sintered feed rod from the initially described growth experiment. Spark plasma sintering is a synthesis and consolidation technique using a pulsed current and an axial pressure to activate the sintering process [5]. Both samples were characterized by EDX, ICM, Raman and Photoluminescence spectroscopy for the purpose of analyzing the crystal quality. The use of spark plasma sintered raw material as feed material was investigated.

Comparison of DEM results and Lagrangian experimental data for the flow and mixing of granules in a rotating drum

This work assesses the accuracy of the discrete element method (DEM) for the simulation of solids mixing using non‐intrusive Lagrangian radioactive particle tracking data, and explains why it may provide physically sound results even when non‐real particle properties are used. The simulation results concern the size segregation of polydisperse granules in a rotating drum operated in rolling mode. Given that the DEM is sensitive to simulation parameters, the granule properties were measured experimentally or extracted from the literature. Several flow phenomena are investigated numerically and experimentally, including the particle residence time, the radial segregation, and the radial variation of the axial dispersion coefficient. An analysis of the DEM model is then presented, with an emphasis on the Young's modulus and friction coefficients. Finally, dimensionless motion equations and corresponding dimensionless numbers are derived to investigate the effect of simulation parameters on particle dynamics. © 2013 American Institute of Chemical Engineers AIChE J, 60: 60–75, 2014

A design heuristic for optimizing segregation avoidance practices in horizontal drum mixers

Segregation is a major problem for many solid processing industries. Differences in particle size or density lead to flow-induced segregation within the surface layer. Here we examine methods of avoiding radial segregation in a horizontal drum mixer. Recently, it has been suggested that segregation in this type of particle mixer can be thwarted if the sheared (surface) regions of the bed are inverted at a rate above some critical frequency. Further, it has been hypothesized that the effectiveness of this technique can be linked to the probability distribution of the number of surface layer “passes” a particle takes per rotation of the drum. In this article, various baffle configurations are numerically and experimentally studied to investigate the efficacy of this measure as a design heuristic for the development of improved drum mixing devices. We choose the horizontal drum geometry as it represents the simplest possible example of a tumbler-type mixer, however, we expect the results found here regarding the efficacy of our design heuristic to be generic for any surface-dominated mixing device.

Radial segregation of multi-component granular media in a rotating tumbler

Segregation and mixing of granular mixtures are important to the minerals, food processing and pharmaceuticals industry to name just a few. It has recently been demonstrated that a rotating tumbler is a suitable device for separating out binary granular mixtures, i.e. mixtures composed of only two different particle types. However, most practical granular mixtures are composed of multi-component particle types. We therefore study the capability of this rotating tumbler to segregate mixtures composed of more than two components where the particles differ either in size or density. The general pattern of segregation involves the formation of an inner core of smallest or densest particles followed, at larger radii, by the next largest or densest particle type and so-on in an onion-like pattern. In the mixtures where particles differ in size we always get relatively pure inner cores of the smallest particles, while the other regions are less segregated. On the other hand for mixtures whose particles differ in density we get a relatively pure outer region (adjacent to cylinder wall) consisting of the least dense particles while the other regions are less segregated. We attempt to relate the simulation results to phenomenological theory and find that size segregation in a specific multi-component mixture can be suitably described by a recent theoretical model.

Two-way coupled numerical simulation of electrospray with induced gas flow

The gas flow induced by droplet motion has been included in the numerical simulation of electrosprays. Steady state solutions were sought iteratively in a computational scheme that fully couples a 3D Lagrangian model for the droplet dynamics with a steady state 2D axisymmetric Eulerian model for the induced gas flow. To resolve the reactive drag force on the gas by the droplets we employ Gaussian filters with variable kernel widths that depend on the droplet number density. We have applied this scheme to an experimentally characterized spray from the literature comprised of non-evaporating primary and satellite droplets in air. The predicted characteristics of the droplet plume (droplet number density, droplet velocity, droplet size distribution) better match the experimental values when the induced airflow is accounted for than when it is not (assuming still air). It is shown that the induced airflow contributes to faster moving droplets, shrinkage of plume, and a prominent flux about the spray axis (centerline), as compared with the simulations assuming still air. Induced airflow results in an increase of 80% in axial droplets velocity at the centerline at the collection counterplate. The ratio of mass flux at the centerline between the cases of moving and still air equals 2.6 at the counterplate. We have also observed that the radial segregation by size of the satellite droplets is sensitive to the functional relationship between their charge-to-mass ratio and diameter. Induced gas flow is expected to have important implications in the simulation of droplet evaporation and vapor concentration in electrosprays.

Numerical investigation of heat and mass transfer during vertical Bridgman crystal growth under rotational vibrations

The paper deals with the numerical investigation of convective flows and heat and mass transfer in the directional solidification of binary melts in the presence of rotational vibrations of finite amplitude and frequency. The study is performed in the framework of the Boussinesq approximation and unsteady axisymmetric approach, taking into account the existence of the two-phase zone. The simulation is conducted for binary melt with low phase change temperature (succinonitrile with ethanol), using ANSYS Fluent package which realizes the finite volume method. The data on the temporal evolution of velocity, temperature and solute concentration fields in the melt and on the solute distribution in the grown crystal with and without vibrations are obtained. It is demonstrated that the vibrations make strong stabilizing effect; they reduce the radial segregation and prevent the pit formation and solidification front breakdown, which leads to a substantial increase of grown crystal homogeneity.

Vertical settling and radial segregation of large dust grains in the circumstellar disk of the Butterfly Star

Context: Circumstellar disks are considered to be the environment for the formation of planets. The growth of dust grains in these disks is the first step in the core accretion-gas capture planet formation scenario. Indicators and evidence of disk evolution can be traced in spatially resolved images and the spectral energy distribution (SED) of these objects. Aims: We develop a model for the dust phase of the edge-on oriented circumstellar disk of the Butterfly Star which allows one to fit observed multi-wavelength images and the SED simultaneously. Methods: Our model is based on spatially resolved high angular resolution observations at 1.3 mm, 894 micron, 2.07 micron, 1.87 micron, 1.60 micron, and 1.13 micron and an extensively covered SED ranging from 12 micron to 2.7 mm, including a detailed spectrum obtained with the Spitzer Space Telescope in the range from 12 micron to 38 micron. A parameter study based on a grid search method involving the detailed analysis of every parameter was performed to constrain the disk parameters and find the best-fit model for the independent observations. The individual observations were modeled simultaneously, using our continuum radiative transfer code. Results: We derived a model that is capable of reproducing all of the observations of the disk at the same time. We find quantitative evidence for grain growth up to ~100 micron-sized particles, vertical settling of larger dust grains toward the disk midplane, and radial segregation of the latter toward the central star. Within our best-fit model the large grains have a distribution with a scale height of 3.7 AU at 100 AU and a radial extent of 175 AU compared to a hydrostatic scale height of 6.9 AU at 100 AU and an outer disk radius of 300 AU. Our results are consistent with current theoretical models for the evolution of circumstellar disks and the early stages of planet formation.

Periodic flow structures in vertical gas-particle flows

Abstract Simulations of kinetic theory based two-fluid model for gas-particle flows in vertical riser segments have been performed. In two-dimensional (2D) simulations of flows in a vertical channel segment and in three-dimensional (3D) simulations of flows in a vertical cylindrical riser segment, periodic boundary conditions were employed in the axial direction. These were supplemented by simpler quasi-one-dimensional (1D) simulations of channel flow and quasi-2D simulations in cylindrical geometry, where it was postulated that the flow variables did not change axially. Both 2D and quasi-1D simulations of channel flow were found to manifest in a robust manner quasi-periodic solutions consisting of laterally traveling waves. Similarly, quasi-2D and 3D simulations of cylindrical riser flow manifested spinning flow states, which in the case of high aspect ratios showed signs of transition to swirling flow states. The period of the oscillating/spinning flow could be captured by a rather simple correlation, whose structure suggests a microscopic origin for the flow structure. The centrifugal force associated with the spin appears to be sufficient to impart an influence on the extent of radial segregation of particles.

The effect of liquids on radial segregation of granular mixtures in rotating drums

The presence of liquids can significantly affect the dynamics of granular flow. This paper investigates the effect of liquids on radial segregation of granular mixture in a rotating drum using the discrete element method. The wet granular mixture, due to differences in particle size and density, segregates in a similar way to that of dry particles: lighter/larger particles move to the periphery of the bed while heavier/smaller particles stay in the centre. An index based on the variance of local concentration of one type of particles was proposed to measure the degree of segregation. While the liquid induced capillary force slows down the segregation process, its effect on the final state is more complicated: small cohesion shows no or even positive effect on segregation while high cohesion significantly reduces particle segregation. The effect can be explained by the change of flow regimes and the competing effects of mixing and segregation (un-mixing) in particle flow which are both reduced by the interparticle cohesion. A diagram is generated to describe the combined effect of particle size and density on segregation of wet particles. A theory is adopted to predict the segregation of particles under different density/size ratios.

Characterization of Mixing and Size Segregation in a Rotating Drum by a Particle Tracking Method

The mechanisms of segregation in solids mixing, even in simple rotating drums, are not clearly understood. Although most past studies have focused on binary mixtures, this work investigates the effect of polydispersity on granular flow, mixing, and segregation in a rotating drum operated in rolling regime through particle trajectories obtained from the radioactive particle tracking technique. Velocity profiles, radial segregation, and axial dispersion coefficients for monodisperse and polydisperse systems of glass beads are analyzed with respect to rotational speed and particle size. A model is introduced to predict the residence times along streamlines and evaluate the rate at which the material renews at the free surface and within the inner layers of the bed. Our results reveal similar velocity profiles and residence times for monodisperse and polydisperse systems. They also indicate that the particles distribute along the radial direction of the drum, although not necessarily in a core/shell configuration. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1894–1905, 2013

The influence of end walls on the segregation pattern in a horizontal rotating drum

The influence of end walls on segregation of bidisperse granular beds in a short rotating horizontal drum is studied by a discrete element method. Whereas non-closed periodically continued drums segregate radially, all simulations of drums with end walls resulted in axial segregation with two bands at low friction between the particles and the end-wall, and three bands at high friction. Various simulations show irregular transitions between two approximately equally stable states, with rapid oscillations preceding the conversions. The formation of two axial bands decreases the energy dissipation by the bed, whereas neither radial segregation nor axial segregation into three bands reduced the power absorption at constant angular velocity. Roughening up the end-walls also increased the rate of axial segregation.

Investigation of relationships between high strength self consolidating concrete compressive strength and macroscopic internal structure

Nine high strength self-consolidating concrete (HSSCC) mixes were prepared using high volume of cementitious materials utilizing up to 70% of slag, fly ash, and silica fume to replace cement content. The specimens were prepared using typical cylinders (100×200mm) and were tested under pure compression. All specimens were cured under a constant temperature of 43.3°C to simulate concrete structures produced in hot weather regions. 2-D digital images of HSSCC cylinders internal structure were obtained using a flatbed digital scanner and analyzed using iPas software. The digital image along with sample dimensions, aggregate gradation, air void content are the inputs for the software, the image analysis main output, among others, are number of contact points between aggregate, radial segregation of aggregate, and horizontal aggregate orientation. The results showed good correlations between HSSCC macroscopic structure and compressive strength, correspondence equations were introduced to predict the number of contact points and the horizontal orientation of aggregate in terms of the compressive strength.

Numerical studies of particle segregation in a rotating drum based on Eulerian continuum approach

Solid–solid–gas three-phase particle segregation in a half-filled rotating drum is simulated using Eulerian continuum approach coupling the kinetic theory of granular flow. A dynamic angle of repose fitting (DARF) method is proposed to determine granular kinetic viscosities of particles of six different sizes moving in the drum rotating at 10rpm, 20rpm or 30rpm. The DARF granular kinetic viscosity increases and decreases with the increasing of particle size and drum rotational speed, respectively. The determined DARF granular viscosity values are used to simulate size-induced particle segregation in a rotating drum. The simulated small-particle-rich segregation structure shows a central small-particle-rich band together with two small-particle-rich side wings. The size of the wings decreases with the increasing of the drum rotational speed. The formation of radial segregation core and axial segregation bands qualitatively agree with the experimental observations.

Discrete particle simulation of radial segregation in horizontally rotating drum: Effects of drum-length and non-rotating end-plates

The radial segregation phenomena of a mixture of two different size grains in a horizontal rotating drum are studied by DEM simulations. The grano-dynamics of radial segregation phenomena is examined as a function of the axial length and the friction between grains and not-rotating end-plates of the drum. The results indicate that, in the longer drums, the radial segregation ratio is higher and the friction on the end-plates shows little effect. Whereas in the shorter drums, the radial segregation is very slow or negligible; however, decreasing the friction on non-rotating end-plates increases the segregation ratio. If we increase the friction further (greater than the frictions between the grain-grain and the grains and the inner wall), the segregation ratio drops in the longer drums while in the shorter drums mixing is seen instead. The cause of these phenomena lies in the mechanism of diffusion in granular flows due to shearing strain by the end-plates. For more roughened end-plates, this shearing activity increases the granular temperature of the system and only the mixing can be observed instead of the segregation.

Magic Pairs and Structural Transitions in Binary Metallic Clusters

Structures and binding energies for bimetallic clusters consisting of a large variety of atomic species are obtained for all atomic sizes N≤40 and all concentrations, using an interatomic potential derived within a quasi-classical description. It is found that increasing the difference between the two types of atoms leads to a gradual disappearance of the well-known homo-atomic geometric magic numbers and the appearance of magic pairs corresponding to the number of atoms of each atomic species in binary nanostructures with higher stability. This change is accompanied by structural transitions and ground-state↔isomer inversions, induced by changes in composition or concentration. There is a clear tendency towards phase separation, the core-shell radial segregation being predominant (energetically favored) in this model.

Experimental study of density segregation at end walls in a horizontal rotating cylinder saturated with fluid: friction to lubrication transition

What is the effect of interstitial fluid viscosity on granular density segregation in a horizontal rotating cylinder? We conducted experiments in the rolling regime with equal amounts of equal sized high and low density, nearly spherical granular particles saturated with air, water, and water-glycerin mixtures. We held particle density, rotation rate and characteristic length scale constant to highlight differences due purely to the interstitial fluid. Images of the granular flow at an end wall were used to determine radial and axial density segregation rates and patterns. Over a four decade change in viscosity, segregation rates varied by only a factor of two. However, for ratios of lubrication to frictional stresses above one, segregation rates decreased by about 30%, and we observed several notable phenomena in the segregation pattern formation. These were a creeping mode of radial density segregation, a change in shape of the granular bed to kidney shaped from flat, and for cylinders more than half full the typically reported unsheared central portion of the granular bed (often referred to in the literature as a core region) was disrupted by a wavy instability where the rate of disappearance of the core region decreased as the fill level increased.