132 research outputs found
Nucleus-Electron Model for States Changing from a Liquid Metal to a Plasma and the Saha Equation
We extend the quantal hypernetted-chain (QHNC) method, which has been proved
to yield accurate results for liquid metals, to treat a partially ionized
plasma. In a plasma, the electrons change from a quantum to a classical fluid
gradually with increasing temperature; the QHNC method applied to the electron
gas is in fact able to provide the electron-electron correlation at arbitrary
temperature. As an illustrating example of this approach, we investigate how
liquid rubidium becomes a plasma by increasing the temperature from 0 to 30 eV
at a fixed normal ion-density . The electron-ion
radial distribution function (RDF) in liquid Rb has distinct inner-core and
outer-core parts. Even at a temperature of 1 eV, this clear distinction remains
as a characteristic of a liquid metal. At a temperature of 3 eV, this
distinction disappears, and rubidium becomes a plasma with the ionization 1.21.
The temperature variations of bound levels in each ion and the average
ionization are calculated in Rb plasmas at the same time. Using the
density-functional theory, we also derive the Saha equation applicable even to
a high-density plasma at low temperatures. The QHNC method provides a procedure
to solve this Saha equation with ease by using a recursive formula; the charge
population of differently ionized species are obtained in Rb plasmas at several
temperatures. In this way, it is shown that, with the atomic number as the only
input, the QHNC method produces the average ionization, the electron-ion and
ion-ion RDF's, and the charge population which are consistent with the atomic
structure of each ion for a partially ionized plasma.Comment: 28 pages(TeX) and 11 figures (PS
Negative Electron-electron Drag Between Narrow Quantum Hall Channels
Momentum transfer due to Coulomb interaction between two parallel,
two-dimensional, narrow, and spatially separated layers, when a current
I_{drive} is driven through one layer, is studied in the presence of a
perpendicular magnetic field B. The current induced in the drag layer,
I_{drag}, is evaluated self-consistently with I_{drive} as a parameter.
I_{drag} can be positive or negative depending on the value of the filling
factor \nu of the highest occupied bulk Landau level (LL). For a fully occupied
LL, I_{drag} is negative, i.e., it flows opposite to I_{drive}, whereas it is
positive for a half-filled LL. When the circuit is opened in the drag layer, a
voltage \Delta V_{drag} develops in it; it is negative for a half-filled LL and
positive for a fully occupied LL. This positive \Delta V_{drag}, expressing a
negative Coulomb drag, results from energetically favored near-edge inter-LL
transitions that occur when the highest occupied bulk LL and the LL just above
it become degenerate.Comment: Text file in Latex/Revtex/preprint format, 7 separate PS figures,
Physical Review B, in pres
Analytic structure factors and pair-correlation functions for the unpolarized homogeneous electron gas
We propose a simple and accurate model for the electron static structure
factors (and corresponding pair-correlation functions) of the 3D unpolarized
homogeneous electron gas. Our spin-resolved pair-correlation function is built
up with a combination of analytic constraints and fitting procedures to quantum
Monte Carlo data, and, in comparison to previous attempts (i) fulfills more
known integral and differential properties of the exact pair-correlation
function, (ii) is analytic both in real and in reciprocal space, and (iii)
accurately interpolates the newest, extensive diffusion-Monte Carlo data of
Ortiz, Harris and Ballone [Phys. Rev. Lett. 82, 5317 (1999)]. This can be of
interest for the study of electron correlations of real materials and for the
construction of new exchange and correlation energy density functionals.Comment: 14 pages, 5 figures, submitted to Phys. Rev.
Experimental study of dense pyroclastic density currents using sustained, gas-fluidized granular flows
© 2014, Springer-Verlag Berlin Heidelberg. We present the results of laboratory experiments on the behaviour of sustained, dense granular flows in a horizontal flume, in which high-gas pore pressure was maintained throughout the flow duration by continuous injection of gas through the flume base. The flows were fed by a sustained (0.5–30 s) supply of fine (75 ± 15 μm) particles from a hopper; the falling particles impacted an impingement surface at concentrations of ~3 to 45 %, where they densified rapidly to generate horizontally moving, dense granular flows. When the gas supplied through the flume base was below the minimum fluidization velocity of the particles (i.e. aerated flow conditions), three flow phases were identified: (i) an initial dilute spray of particles travelling at 1–2 m s−1, followed by (ii) a dense granular flow travelling at 0.5–1 m s−1, then by (iii) sustained aggradation of the deposit by a prolonged succession of thin flow pulses. The maximum runout of the phase 2 flow was linearly dependent on the initial mass flux, and the frontal velocity had a square-root dependence on mass flux. The frontal propagation speed during phase 3 had a linear relationship with mass flux. The total mass of particles released had no significant control on either flow velocity or runout in any of the phases. High-frequency flow unsteadiness during phase 3 generated deposit architectures with progradational and retrogradational packages and multiple internal erosive contacts. When the gas supplied through the flume base was equal to the minimum fluidization velocity of the particles (i.e. fluidized flow conditions), the flows remained within phase 2 for their entire runout, no deposit formed and the particles ran off the end of the flume. Sustained granular flows differ significantly from instantaneous flows generated by lock-exchange mechanisms, in that the sustained flows generate (by prolonged progressive aggradation) deposits that are much thicker than the flowing layer of particles at any given moment. The experiments offer a first attempt to investigate the physics of the sustained pyroclastic flows that generate thick, voluminous ignimbrites
A One-Dimensional (1-D) Three-Region Model for a Bubbling Fluidized-Bed Adsorber
A general one-dimensional (1-D), three-region model for a bubbling fluidized-bed adsorber with internal heat exchangers has been developed. The model can predict the hydrodynamics of the bed and provides axial profiles for all temperatures, concentrations, and velocities. The model is computationally fast and flexible and allows for any system of adsorption and desorption reactions to be modeled, making the model applicable to any adsorption process. The model has been implemented in both gPROMS and Aspen Custom Modeler, and the behavior of the model has been verified
Conservação de café torrado e moído durante o armazenamento. Parte 2: Massa específica e porosidade
The determination of physical properties is an important factor in the design of machinery
and the scaling of post-harvest operations. The present study evaluates the influence of the
level of roasting and the size of grinding on the physical properties of coffee during storage.
The following physical properties were evaluated: true and bulk density, and intergranular
porosity. Raw coffee beans (Coffea canephora and Coffea arabica), hulled and dried, were
roasted to two different levels: medium light (SCAA#65) and moderately dark (SCAA#45).
The beans were then grinded into three different sizes: fine (0.59 mm), medium (0.84 mm)
and coarse (1.19 mm). An additional coffee lot was kept whole. Following grinding, samples
were stored at two different temperatures (10 and 30 ºC) and analyzed after five different
storage durations (0, 30, 60, 120 and 180 days). The medium light roast had higher values
for each of the measured physical properties. Finely ground samples had higher true and
bulk densities, and porosities. It is concluded that the size of grinding, level of roasting and
duration of storage significantly affect the physical properties of coffee.A determinação das propriedades físicas é fator importante na formulação de projetos
de maquinários e dimensionamento de operações pós-colheita. De forma a permitir o
dimensionamento correto e o uso desses maquinários, objetivou-se avaliar e determinar,
durante o armazenamento, as propriedades físicas: massa específica unitária e aparente e
porosidade intergranular, além de avaliar a influência dos níveis de torrefação e moagem
sobre essas propriedades. Grãos de café cru (Coffea canephora e Coffea arabica) foram
utilizados, descascados e secados e só então torrados em dois níveis: média clara (SCAA#65) e
moderadamente escura (SCAA#45). Os grãos foram moídos em três granulometrias: fina (0,59
mm), média (0,84 mm) e grossa (1,19 mm), além do lote de café inteiro. Realizada a moagem as
amostras foram armazenadas em duas temperaturas (10 e 30 ºC) e analisadas em cinco tempos
distintos de armazenamento (0, 30, 60, 120 e 180 dias). A torra média clara permitiu maiores
valores das propriedades físicas; já as amostras de granulometria fina apresentaram aumento
das massas específica unitária e aparente e porosidade. Conclui-se que a granulometria, o
nível de torrefação e o tempo de armazenamento, afetaram as propriedades físicas do café.National Council for Scientificand Technological Development/[CNPq nº 14/2012]/CNPq/BrasilUCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Agroalimentarias::Centro para Investigaciones en Granos y Semillas (CIGRAS
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