307 research outputs found
Intensification des transferts dans les échangeurs thermiques et multifonctionnels : techniques, outils d'analyse et optimisation
Intensification des transferts dans les échangeurs thermiques et les échangeurs-réacteurs multifonctionnels
Effect of the angle of attack of a rectangular wing on the heat transfer enhancement in channel flow at low Reynolds number
Convective heat transfer enhancement can be achieved by generating secondary flow structures that are added to the main flow to intensify the fluid exchange between hot and cold regions. One method involves the use of vortex generators to produce streamwise and transverse vortices superimposed to the main flow. This study presents numerical computation results of laminar convection heat transfer in a rectangular channel whose bottom wall is equipped with one row of rectangular wing vortex generators. The governing equations are solved using finite volume method by considering steady state, laminar regime and incompressible flow. Three-dimensional numerical simulations are performed to study the effect of the angle of attack α of the wing on heat transfer and pressure drop. Different values are taken into consideration within the range 0° < α < 30°. For all of these geometrical configurations the Reynolds number is maintained to Re = 456. To assess the effect of the angle of attack on the heat transfer enhancement, Nusselt number and the friction factor are studied on both local and global perspectives. Also, the location of the generated vortices within the channel is studied, as well as their effect on the heat transfer enhancement throughout the channel for all α values. Based on both local and global analysis, our results show that the angle of attack α has a direct impact on the heat transfer enhancement. By increasing its value, it leads to better enhancement until an optimal value is reached, beyond which the thermal performances decrease
The molecular complex associated with the Galactic HII region Sh2-90: a possible site of triggered star formation
We investigate the star formation activity in the molecular complex
associated with the Galactic HII region Sh2-90, using radio-continuum maps
obtained at 1280 MHz and 610 MHz, Herschel Hi-GAL observations at 70 -- 500
microns, and deep near-infrared observation at JHK bands, along with Spitzer
observations. Sh2-90 presents a bubble morphology in the mid-IR (size ~ 0.9 pc
x 1.6 pc). Radio observations suggest it is an evolved HII region with an
electron density ~ 144 cm^-3, emission measure ~ 6.7 x 10^4 cm^-6 pc and a
ionized mass ~ 55 Msun. From Hi-GAL observations it is found that the HII
region is part of an elongated extended molecular cloud (size ~ 5.6 pc x 9.7
pc, H_2 column density >= 3 x 10^21 cm^-2 and dust temperature 18 -- 27 K) of
total mass >= 1 x 10^4 Msun. We identify the ionizing cluster of Sh2-90, the
main exciting star being an O8--O9 V star. Five cold dust clumps (mass ~ 8 --
95 Msun), four mid-IR blobs around B stars, and a compact HII region are found
at the edge of the bubble.The velocity information derived from CO (J=3-2) data
cubes suggests that most of them are associated with the Sh2-90 region. 129
YSOs are identified (Class I, Class II, and near-IR excess sources). The
majority of the YSOs are low mass (<= 3 Msun) sources and they are distributed
mostly in the regions of high column density. Four candidate Class 0/I MYSOs
have been found; they will possibly evolve to stars of mass >= 15 Msun. We
suggest multi-generation star formation is present in the complex. From the
evidences of interaction, the time scales involved and the evolutionary status
of stellar/protostellar sources, we argue that the star formation at the
immediate border/edges of Sh2-90 might have been triggered by the expanding HII
region. However, several young sources in this complex are probably formed by
some other processes.Comment: 22 pages, 22 figures, accepted for publication in Astronomy and
Astrophysic
Echangeurs thermiques et multi-fonctionnels : quelles évolutions pour un développement durable
Physical and Chemical Phenomena in Heat Exchangers and Multifunctional Reactors for Sustainable Technology
Star formation triggered by the Galactic HII region RCW 120: First results from the Herschel Space Observatory
By means of different physical mechanisms, the expansion of HII regions can
promote the formation of new stars of all masses. RCW 120 is a nearby Galactic
HII region where triggered star formation occurs. This region is well-studied -
there being a wealth of existing data - and is nearby. However, it is
surrounded by dense regions for which far infrared data is essential to obtain
an unbiased view of the star formation process and in particular to establish
whether very young protostars are present. We attempt to identify all Young
Stellar Objects (YSOs), especially those previously undetected at shorter
wavelengths, to derive their physical properties and obtain insight into the
star formation history in this region. We use Herschel-PACS and -SPIRE images
to determine the distribution of YSOs observed in the field. We use a spectral
energy distribution fitting tool to derive the YSOs physical properties.
Herschel-PACS and -SPIRE images confirm the existence of a young source and
allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object
(whose emission is dominated by a massive envelope) towards the massive
condensation 1 observed at (sub)-millimeter wavelengths. This source was not
detected at 24 micron and only barely seen in the MISPGAL 70 micron data.
Several other red sources are detected at Herschel wavelengths and coincide
with the peaks of the millimeter condensations. SED fitting results for the
brightest Herschel sources indicate that, apart from the massive Class 0 that
forms in condensation 1, young low mass stars are forming around RCW 120. The
YSOs observed on the borders of RCW 120 are younger than its ionizing star,
which has an age of about 2.5 Myr.Comment: 5 pqges, 3 figures, accepted by A&A (Special issue on the Herschel
first results
The distance and neutral environment of the massive stellar cluster Westerlund 1
The goal of this study is to determine a distance to Westerlund 1 independent
of the characteristics of the stellar population and to study its neutral
environment, using observations of atomic hydrogen. The HI observations are
taken from the Southern Galactic Plane Survey to study HI absorption in the
direction of the HII region created by the members of Westerlund 1 and to
investigate its environment as observed in the HI line emission. A Galactic
rotation curve was derived using the recently revised values for the Galactic
centre distance of kpc, and the velocity of the Sun around the
Galactic centre of km s. The newly determined
rotation model leads us to derive a distance of kpc to Westerlund
1, consistent with a location in the Scutum-Crux Arm. Included in this estimate
is a very careful investigation of possible sources of error for the Galactic
rotation curve. We also report on small expanding HI features around the
cluster with a maximum dynamic age of 600,000 years and a larger bubble which
has a minimum dynamic age of 2.5 million years. Additionally we re-calculated
the kinematic distances to nearby HII regions and supernova remnants based on
our new Galaxic rotation curve. We propose that in the early stages of the
development of Wd 1 a large interstellar bubble of diameter about 50 pc was
created by the cluster members. This bubble has a dynamic age similar to the
age of the cluster. Small expanding bubbles, with dynamical ages Myr
are found around Wd 1, which we suggest consist of recombined material lost by
cluster members through their winds.Comment: 8 pages, accepted for publication in A&
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