3,067 research outputs found
Taylor dispersion in arbitrarily shaped axisymmetric channels
Advective dispersion of solutes in long thin axisymmetric channels is
important to the analysis and design of a wide range of devices, including
chemical separation systems and microfluidic chips. Despite extensive analysis
of Taylor dispersion in various scenarios, most studies focused on long-term
dispersion behavior and cannot capture the transient evolution of solute zone
across the spatial variations in the channel. In the current study, we analyze
the Taylor-Aris dispersion for arbitrarily shaped axisymmetric channels. We
derive an expression for solute dynamics in terms of two coupled ordinary
differential equations (ODEs). These two ODEs allow prediction of the time
evolution of the mean location and axial (standard deviation) width of the
solute zone as a function of the channel geometry. We compare and benchmark our
predictions with Brownian dynamics simulations for a variety of cases including
linearly expanding/converging channels and periodic channels. We also present
an analytical description of the physical regimes of transient positive versus
negative axial growth of solute width. Finally, to further demonstrate the
utility of the analysis, we demonstrate a method to engineer channel geometries
to achieve desired solute width distributions over space and time. We apply the
latter analysis to generate a geometry that results in a constant axial width
and a second geometry that results in a sinusoidal axial variance in space.Comment: main file: 26 pages, 9 figures. supplementary: 7 pages, 1 figur
The Properties of Galaxies in Voids
We present a comparison of the properties of galaxies in the most underdense
regions of the Universe, where the galaxy number density is less than 10% of
the mean density, with galaxies from more typical regions. We have compiled a
sample of galaxies in 46 large nearby voids that were identified using the
Sloan Digital Sky Survey DR4, which provides the largest coverage of the sky.
We study the u-r color distribution, morphology, specific star formation rate,
and radial number density profiles for a total of 495 galaxies fainter than
M_r=-20.4 +5logh located inside the voids and compare these properties with a
control sample of field galaxies. We show that there is an excess of blue
galaxies inside the voids. However, inspecting the properties of blue and red
galaxies separately, we find that galaxy properties such as color distribution,
bulge-to-total ratios, and concentrations are remarkably similar between the
void and overall sample. The void galaxies also show the same specific star
formation rate at fixed color as the control galaxies. We compare our results
with the predictions of cosmological simulations of galaxy formation using the
Millennium Run semi-analytic galaxy catalog. We show that the properties of the
simulated galaxies in large voids are in reasonably good agreement with those
found in similar environments in the real Universe. To summarize, in spite of
the fact that galaxies in voids live in the least dense large-scale
environment, this environment makes very little impact on properties of
galaxies.Comment: 11 pages, 15 figures, Submitted to MNRA
Thermodynamics of Ion Separation by Electrosorption
We present a simple, top-down approach for the calculation of minimum energy
consumption of electrosorptive ion separation using variational form of the
(Gibbs) free energy. We focus and expand on the case of electrostatic
capacitive deionization (CDI), and the theoretical framework is independent of
details of the double-layer charge distribution and is applicable to any
thermodynamically consistent model, such as the Gouy-Chapman-Stern (GCS) and
modified Donnan (mD) models. We demonstrate that, under certain assumptions,
the minimum required electric work energy is indeed equivalent to the free
energy of separation. Using the theory, we define the thermodynamic efficiency
of CDI. We explore the thermodynamic efficiency of current experimental CDI
systems and show that these are currently very low, less than 1% for most
existing systems. We applied this knowledge and constructed and operated a CDI
cell to show that judicious selection of the materials, geometry, and process
parameters can be used to achieve a 9% thermodynamic efficiency (4.6 kT energy
per removed ion). This relatively high value is, to our knowledge, by far the
highest thermodynamic efficiency ever demonstrated for CDI. We hypothesize that
efficiency can be further improved by further reduction of CDI cell series
resistances and optimization of operational parameters
Performance Metrics for the Objective Assessment of Capacitive Deionization Systems
In the growing field of capacitive deionization (CDI), a number of
performance metrics have emerged to describe the desalination process.
Unfortunately, the separation conditions under which these metrics are measured
are often not specified, resulting in optimal performance at minimal removal.
Here we outline a system of performance metrics and reporting conditions that
resolves this issue. Our proposed system is based on volumetric energy
consumption (Wh/m) and throughput productivity (L/h/m) reported for a
specific average concentration reduction, water recovery, and feed salinity. To
facilitate and rationalize comparisons between devices, materials, and
operation modes, we propose a nominal standard testing condition of removing 5
mM from a 20 mM NaCl feed solution at 50% water recovery for CDI research.
Using this separation, we compare the desalination performance of a
flow-through electrode (fte-CDI) cell and a flow between membrane (fb-MCDI)
device, showing how significantly different systems can be compared in terms of
generally desirable desalination characteristics. In general, we find that
performance analysis must be considered carefully so to not allow for ambiguous
separation conditions or the maximization of one metric at the expense of
another. Additionally, for context we discuss a number of important underlying
performance indicators and cell characteristics that are not performance
measures in and of themselves but can be examined to better understand
differences in performance
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