1,208 research outputs found
Numerical modelling of the temperature distribution in a two-phase closed thermosyphon
Interest in the use of heat pipe technology for heat recovery and energy saving in a vast range of engineering applications has been on the rise in recent years. Heat pipes are playing a more important role in many industrial applications, particularly in improving the thermal performance of heat exchangers and increasing energy savings in applications with commercial use. In this paper, a comprehensive CFD modelling was built to simulate the details of the two-phase flow and heat transfer phenomena during the operation of a wickless heat pipe or thermosyphon, that otherwise could not be visualised by empirical or experimental work. Water was used as the working fluid. The volume of the fluid (VOF) model in ANSYS FLUENT was used for the simulation. The evaporation, condensation and phase change processes in a thermosyphon were dealt with by adding a user-defined function (UDF) to the FLUENT code. The simulation results were compared with experimental measurements at the same condition. The simulation was successful in reproducing the heat and mass transfer processes in a thermosyphon. Good agreement was observed between CFD predicted temperature profiles and experimental temperature data.The Saudi Cultural Bureau in London, the Ministry of Higher Education and the Mechanical Engineering Department, Umm Al-Qura University
The Intermediate Mass Black Hole Candidate in the Center of NGC 404: New Evidence from Radio Continuum Observations
We present the results of deep, high-resolution, 5 GHz Expanded Very Large
Array (EVLA) observations of the nearby, dwarf lenticular galaxy and
intermediate mass black hole candidate (M ~4.5 x 10^5 M_sun), NGC 404. For the
first time, radio emission at frequencies above 1.4 GHz has been detected in
this galaxy. We found a modestly resolved source in the NGC 404 nucleus with a
total radio luminosity of 7.6 +/- 0.7 x 10^17 W/Hz at 5 GHz and a spectral
index from 5 to 7.45 GHz of alpha = -0.88 +/- 0.30. NGC 404 is only the third
central intermediate mass black hole candidate detected in the radio regime
with subarcsecond resolution. The position of the radio source is consistent
with the optical center of the galaxy and the location of a known, hard X-ray
point source (Lx ~1.2 x 10^37 erg/s). The faint radio and X-ray emission could
conceivably be produced by an X-ray binary, star formation, a supernova remnant
or a low-luminosity AGN powered by an intermediate mass black hole. In light of
our new EVLA observations, we find that the most likely scenario is an
accreting intermediate mass black hole, with other explanations incompatible
with the observed X-ray and/or radio luminosities or statistically unlikely.Comment: Accepted for publication to Ap
A VLBA search for binary black holes in active galactic nuclei with double-peaked optical emission line spectra
We have examined a subset of 11 active galactic nuclei (AGN) drawn from a
sample of 87 objects that possess double-peaked optical emission line spectra,
as put forward by Wang et al. (2009a) and are detectable in the FIRST survey at
radio wavelengths. The double-peaked nature of the optical emission line
spectra has been suggested as evidence for the existence of binary black holes
in these AGN, although this interpretation is controversial. We make a simple
suggestion, that direct evidence of binary black holes in these objects could
be searched for in the form of dual sources of compact radio emission
associated with the AGN. To explore this idea, we have used the Very Long
Baseline Array to observe these 11 objects from the Wang et al. (2009a) sample.
Of the 11 objects, we detect compact radio emission from two, SDSS
J151709+335324 and SDSS J160024+264035. Both objects show single components of
compact radio emission. The morphology of SDSS J151709+335324 is consistent
with a recent comprehensive multi-wavelength study of this object by Rosario et
al. (2010). Assuming that the entire sample consists of binary black holes, we
would expect of order one double radio core to be detected, based on radio
wavelength detection rates from FIRST and VLBI surveys. We have not detected
any double cores, thus this work does not substantially support the idea that
AGN with double-peaked optical emission lines contain binary black holes.
However, the study of larger samples should be undertaken to provide a more
secure statistical result, given the estimated detection rates.Comment: 14 pages, 3 figures. To appear in A
Paper Session III-B - A Lunar Orbiting Node in Support of Missions to Mars
Future Mars missions may use lunar-derived oxygen as a propellant for interplanetary transit. A man-tended platform as a Node in low lunar orbit offers a site for storage and transfer of lunar oxygen to the transport vehicles as well as rendezvous and transfer for lunar-bound cargo and crews. In addition, it could provide an emergency safe-haven for a crew awaiting rescue. A conceptual design study yielded an approximate size for the platform needed to support typical oxygen transfer rates which were based upon NASA studies of Mars missions. The Node consists of a gravity gradient stabilized lunar orbiting tank-farm with a storage capacity of 100,000 kg of lunar oxygen, 3,300 kg of lunar cargo and 9,300 kg of Earth supplied hydrogen. An emergency habitat configuration accomodates 14 persons on-board for 110 days. The Node supports an annual lunar oxygen Ereduction of 106 kg with 220,000 kg of oxygen delivered to Earth orbit for an expenditure of 109,000 g of Earth supplied hydrogen
Digital modulation of the nickel valence state in a cuprate-nickelate heterostructure
Layer-by-layer oxide molecular beam epitaxy has been used to synthesize
cuprate-nickelate multilayer structures of composition
(LaCuO)/LaO/(LaNiO). In a combined experimental and
theoretical study, we show that these structures allow a clean separation of
dopant and doped layers. Specifically, the LaO layer separating cuprate and
nickelate blocks provides an additional charge that, according to density
functional theory calculations, is predominantly accommodated in the
interfacial nickelate layers. This is reflected in an elongation of bond
distances and changes in valence state, as observed by scanning transmission
electron microscopy and x-ray absorption spectroscopy. Moreover, the predicted
charge disproportionation in the nickelate interface layers leads to a
thickness-dependent metal-to-insulator transition for , as observed in
electrical transport measurements. The results exemplify the perspectives of
charge transfer in metal-oxide multilayers to induce doping without introducing
chemical and structural disorder
Quantum effects in linear and non-linear transport of T-shaped ballistic junction
We report low-temperature transport measurements of three-terminal T-shaped
device patterned from GaAs/AlGaAs heterostructure. We demonstrate the mode
branching and bend resistance effects predicted by numerical modeling for
linear conductance data. We show also that the backscattering at the junction
area depends on the wave function parity. We find evidence that in a non-linear
transport regime the voltage of floating electrode always increases as a
function of push-pull polarization. Such anomalous effect occurs for the
symmetric device, provided the applied voltage is less than the Fermi energy in
equilibrium
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