63,391 research outputs found
Evacuation of coal from hoppers/silos with low pressure pneumatic blasting systems
The need for an efficient, economical, effective and quiet device for moving coal and other difficult bulk solids was recognized. Thus came the advent of the low pressure pneumatic blasting system - a very efficient means of using a small amount of plant air (up to 125 PSI) to eliminate the most troublesome material hang-ups in storage containers. This simple device has one moving part and uses approximately 3% of the air consumed by a pneumatic vibrator on the same job. The principle of operation is very simple: air stored in the unit's reservoir is expelled directly into the material via a patented quick release valve. The number, size, and placement of the blaster units on the storage vessel is determined by a series of tests to ascertain flowability of the problem material. These tests in conjunction with the hopper or silo configuration determine specification of a low pressure pneumatic blasting system. This concept has often proven effective in solving flow problems when all other means have failed
Studying pion effects on the chiral phase transition
We investigate the chiral phase transition at finite temperatures and zero
chemical potential with Dyson-Schwinger equations. Our truncation for the
quark-gluon interaction includes mesonic degrees of freedom, which allows us to
study the impact of the pions on the nature of the phase transition. Within the
present scheme we find a five percent change of the critical temperature due to
the pion backreaction whereas the mean field character of the transition is not
changed.Comment: 2 pages, 2 figures, talk given by J.A.M. at the 30th International
School of Nuclear Physics, Erice, Sicily from 16 - 24 September 200
Dynamically induced scalar quark confinement
We employ a functional approach to investigate the confinement problem in
quenched Landau gauge QCD. We demonstrate analytically that a linear rising
potential between massive quarks is generated by infrared singularities in the
dressed quark-gluon vertex. The selfconsistent mechanism that generates these
singularities is driven by the scalar Dirac amplitudes of the full vertex and
the quark propagator. These can only be present when chiral symmetry is broken.
We have thus uncovered a novel mechanism that directly links chiral symmetry
breaking with confinement.Comment: 12 pages, 2 figures; v2: clarifications added and typos corrected,
version to be published by MPL
Inflowing gas onto a compact obscured nucleus in Arp 299A: Herschel spectroscopic studies of H2O and OH
Aims. We probe the physical conditions in the core of Arp 299A and try to put
constraints to the nature of its nuclear power source. Methods. We used
Herschel Space Observatory far-infrared and submillimeter observations of H2O
and OH rotational lines in Arp 299A to create a multi-component model of the
galaxy. In doing this, we employed a spherically symmetric radiative transfer
code. Results. Nine H2O lines in absorption and eight in emission as well as
four OH doublets in absorption and one in emission, are detected in Arp 299A.
No lines of the 18O isotopologues, which have been seen in compact obscured
nuclei of other galaxies, are detected. The absorption in the ground state OH
doublet at 119 {\mu}m is found redshifted by ~175 km/s compared to other OH and
H2O lines, suggesting a low excitation inflow. We find that at least two
components are required in order to account for the excited molecular line
spectrum. The inner component has a radius of 20-25 pc, a very high infrared
surface brightness (> 3e13 Lsun/kpc^2), warm dust (Td > 90 K), and a large H2
column density (NH2 > 1e24 cm^-2). The outer component is larger (50-100 pc)
with slightly cooler dust (70-90 K). In addition, a much more extended
inflowing component is required to also account for the OH doublet at 119
{\mu}m. Conclusions. The Compton-thick nature of the core makes it difficult to
determine the nature of the buried power source, but the high surface
brightness indicates that it is either an active galactic nucleus and/or a
dense nuclear starburst. The high OH/H2O ratio in the nucleus indicates that
ion-neutral chemistry induced by X-rays or cosmic-rays is important. Finally we
find a lower limit to the 16O/18O ratio of 400 in the nuclear region, possibly
indicating that the nuclear starburst is in an early evolutionary stage, or
that it is fed through a molecular inflow of, at most, solar metallicity.Comment: 14 pages, 13 figures, Accepted for publication in Astronomy and
Astrophysic
Modeling the H2O submillimeter emission in extragalactic sources
Recent observational studies have shown that H2O emission at (rest)
submillimeter wavelengths is ubiquitous in infrared galaxies, both in the local
and in the early Universe, suggestive of far-infrared pumping of H2O by dust in
warm regions. In this work, models are presented that show that (i) the
highest-lying H2O lines (E_{upper}>400 K) are formed in very warm (T_{dust}>~90
K) regions and require high H2O columns (N_{H2O}>~3x10^{17} cm^{-2}), while
lower lying lines can be efficiently excited with T_{dust}~45-75 K and
N_{H2O}~(0.5-2)x10^{17} cm^{-2}; (ii) significant collisional excitation of the
lowest lying (E_{upper}<200 K) levels, which enhances the overall
L_{H2O}-L_{IR} ratios, is identified in sources where the ground-state para-H2O
1_{11}-0_{00} line is detected in emission; (iii) the H2O-to-infrared (8-1000
um) luminosity ratio is expected to decrease with increasing T_{dust} for all
lines with E_{upper}<~300 K, as has recently been reported in a sample of
LIRGs, but increases with T_{dust} for the highest lying H2O lines
(E_{upper}>400 K); (iv) we find theoretical upper limits for L_{H2O}/L_{IR} in
warm environments, owing to H2O line saturation; (v) individual models are
presented for two very different prototypical galaxies, the Seyfert 2 galaxy
NGC 1068 and the nearest ultraluminous infrared galaxy Arp 220, showing that
the excited submillimeter H2O emission is dominated by far-infrared pumping in
both cases; (vi) the L_{H2O}-L_{IR} correlation previously reported in
observational studies indicates depletion or exhaustion time scales,
t_{dep}=Sigma_{gas}/Sigma_{SFR}, of <~12 Myr for star-forming sources where
lines up to E_{upper}=300 K are detected, in agreement with the values
previously found for (U)LIRGs from HCN millimeter emission...Comment: 13 pages, 13 figure
Computation of unsteady transonic flows through rotating and stationary cascades. 3: Acoustic far-field analysis
A small perturbation type analysis has been developed for the acoustic far field in an infinite duct extending upstream and downstream of an axial turbomachinery stage. The analysis is designed to interface with a numerical solution of the near field of the blade rows and, thereby, to provide the necessary closure condition to complete the statement of infinite duct boundary conditions for the subject problem. The present analysis differs from conventional inlet duct analyses in that a simple harmonic time dependence was not assumed, since a transient signal is generated by the numerical near-field solution and periodicity is attained only asymptotically. A description of the computer code developed to carry out the necessary convolutions numerically is included, as well as the results of a sample application using an impulsively initiated harmonic signal
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