20,290 research outputs found
Particle simulation of vibrated gas-fluidized beds of cohesive fine powders
We use three-dimensional particle dynamics simulations, coupled with
volume-averaged gas phase hydrodynamics, to study vertically vibrated
gas-fluidized beds of fine, cohesive powders. The volume-averaged interstitial
gas flow is restricted to be one-dimensional (1D). This simplified model
captures the spontaneous development of 1D traveling waves, which corresponds
to bubble formation in real fluidized beds. We use this model to probe the
manner in which vibration and gas flow combine to influence the dynamics of
cohesive particles. We find that as the gas flow rate increases, cyclic
pressure pulsation produced by vibration becomes more and more significant than
direct impact, and in a fully fluidized bed this pulsation is virtually the
only relevant mechanism. We demonstrate that vibration assists fluidization by
creating large tensile stresses during transient periods, which helps break up
the cohesive assembly into agglomerates.Comment: to appear in I&EC Research, a special issue (Oct. 2006) in honor of
Prof. William B. Russe
Quantum Hall Ferromagnets: Induced Topological term and electromagnetic interactions
The quantum Hall ground state in materials like GaAs is well known
to be ferromagnetic in nature. The exchange part of the Coulomb interaction
provides the necessary attractive force to align the electron spins
spontaneously. The gapless Goldstone modes are the angular deviations of the
magnetisation vector from its fixed ground state orientation. Furthermore, the
system is known to support electrically charged spin skyrmion configurations.
It has been claimed in the literature that these skyrmions are fermionic owing
to an induced topological Hopf term in the effective action governing the
Goldstone modes. However, objections have been raised against the method by
which this term has been obtained from the microscopics of the system. In this
article, we use the technique of the derivative expansion to derive, in an
unambiguous manner, the effective action of the angular degrees of freedom,
including the Hopf term. Furthermore, we have coupled perturbative
electromagnetic fields to the microscopic fermionic system in order to study
their effect on the spin excitations. We have obtained an elegant expression
for the electromagnetic coupling of the angular variables describing these spin
excitations.Comment: 23 pages, Plain TeX, no figure
Selective copper extraction by multi-modified mesoporous silica material, SBA-15
Β© 2018 Selective copper (Cu) recovery from wastewater mitigates environmental pollution and is economically valuable. Mesoporous silica adsorbents, SBA-15, with amine-grafting (SBA-15-NH2) and manganese loading along with amine-grafting (Mn-SBA-15-NH2) were fabricated using KMnO4 and 3-aminopropyltriethoxysilane. The characteristics of the synthesized adsorbents were evaluated in detail in terms of its crystal structure peaks, surface area and pore size distribution, transmission electron microscope and X-ray photoelectron spectroscopy. The results established the 2.08 mmol/g of Cu adsorption capacity on Mn-SBA-15-NH2. Furthermore, in a mixed heavy metal solution, high selective Cu adsorption capacity on Mn-SBA-15-NH2 (2.01 mmol/g) was achieved while maintaining 96% adsorption amount as that of a single Cu solution. Comparatively, Cu adsorption on SBA-15-NH2 decreased by half due to high competition with other heavy metals. Optimal Cu adsorption occurred at pH 5. This pH condition enabled grafted amine group in Mn-SBA-15-NH2 to form strong chelating bonds with Cu, avoiding protonation of amine group (below pH 5) as well as precipitation (above pH 5). The adsorption equilibrium well fitted to Langmuir and Freundlich isotherm models, while kinetic results were represented by models of linear driving force approximation (LDFA) and pore diffusion model (PDM). High regeneration and reuse capacity of Mn-SBA-15-NH2 were well established by its capacity to maintain 90% adsorption capacity in a multiple adsorption-desorption cycle. Cu was selectively extracted from Mn-SBA-15-NH2 with an acid solution
Planar Superconductor-Normal-Superconductor Josephson Junctions in MgB2
Since the discovery of superconductivity in MgB2 considerable progress has
been made in determining the physical properties of the material, which are
promising for bulk conductors. Tunneling studies show that the material is
reasonably isotropic and has a well-developed s-wave energy gap (∆),
implying that electronic devices based on MgB2 could operate close to 30K.
Although a number of groups have reported the formation of thin films by
post-reaction of precursors, heterostructure growth is likely to require
considerable technological development, making single-layer device structures
of most immediate interest. MgB2 is unlike the cuprate superconductors in that
grain boundaries do not form good Josephson junctions, and although a SQUID
based on MgB2 nanobridges has been fabricated, the nanobridges themselves do
not show junction-like properties. Here we report the successful creation of
planar MgB2 junctions by localised ion damage in thin films. The critical
current (IC) of these devices is strongly modulated by applied microwave
radiation and magnetic field. The product of the critical current and normal
state resistance (ICRN) is remarkably high, implying a potential for very high
frequency applications.Comment: 7 pages including 4 figure
Thermodynamic Phase Diagram of the Quantum Hall Skyrmion System
We numerically study the interacting quantum Hall skyrmion system based on
the Chern-Simons action. By noticing that the action is invariant under global
spin rotations in the spin space with respect to the magnetic field direction,
we obtain the low-energy effective action for a many skyrmion system.
Performing extensive molecular dynamics simulations, we establish the
thermodynamic phase diagram for a many skyrmion system.Comment: 4 pages, RevTex, 2 postscript figure
Compaction and dilation rate dependence of stresses in gas-fluidized beds
A particle dynamics-based hybrid model, consisting of monodisperse spherical
solid particles and volume-averaged gas hydrodynamics, is used to study
traveling planar waves (one-dimensional traveling waves) of voids formed in
gas-fluidized beds of narrow cross sectional areas. Through ensemble-averaging
in a co-traveling frame, we compute solid phase continuum variables (local
volume fraction, average velocity, stress tensor, and granular temperature)
across the waves, and examine the relations among them. We probe the
consistency between such computationally obtained relations and constitutive
models in the kinetic theory for granular materials which are widely used in
the two-fluid modeling approach to fluidized beds. We demonstrate that solid
phase continuum variables exhibit appreciable ``path dependence'', which is not
captured by the commonly used kinetic theory-based models. We show that this
path dependence is associated with the large rates of dilation and compaction
that occur in the wave. We also examine the relations among solid phase
continuum variables in beds of cohesive particles, which yield the same path
dependence. Our results both for beds of cohesive and non-cohesive particles
suggest that path-dependent constitutive models need to be developed.Comment: accepted for publication in Physics of Fluids (Burnett-order effect
analysis added
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