901 research outputs found
Evaporation and clustering of ammonia droplets in a hot environment
Recent developments in the transition to zero-carbon fuels show that ammonia is a valid candidate for combustion. However, liquid ammonia combustion is difficult to stabilize due to a large latent heat of evaporation, which generates a strong cooling effect that adversely affects the flame stabilization and combustion efficiency. In addition, the slow burning rate of ammonia enhances the undesired production of NOx and N2O. To increase the flame speed, ammonia must be blended with a gaseous fuel having a high burning rate. In this context, a deeper understanding of the droplet dynamics is required to optimize the combustor design. To provide reliable physical insights into diluted ammonia sprays blended with gaseous methane, direct numerical simulations are employed. Three numerical experiments were performed with cold, standard, and hot ambient in nonreactive conditions. The droplet radius and velocity distribution, as well as the mass and heat coupling source terms are compared to study the effects on the evaporation. Since the cooling effect is stronger than the heat convection between the droplet and the environment in each case, ammonia droplets do not experience boiling. On the other hand, the entrainment of dry air into the ammonia-methane mixture moves the saturation level beyond 100% and droplets condense. The aforementioned phenomena are found to strongly affect the droplet evolution. Finally, a three-dimensional Voronoi analysis is performed to characterize the dispersive or clustering behavior of droplets by means of the definition of a clustering index
Study of CO2 desublimation during cryogenic carbon capture using the lattice Boltzmann method
Cryogenic carbon capture (CCC) can preferentially desublimate CO2 out of the flue
gas. A widespread application of CCC requires a comprehensive understanding of CO2
desublimation properties. This is, however, highly challenging due to the multiphysics
behind it. This study proposes a lattice Boltzmann (LB) model to study CO2 desublimation
on a cooled cylinder surface during CCC. In two-dimensional (2-D) simulations,
various CO2 desublimation and capture behaviours are produced in response to different
operation conditions, namely, gas velocity (PĂ©clet number Pe) and cylinder temperature
(subcooling degree Tsub). As Pe increases or Tsub decreases, the desublimation rate
gradually becomes insufficient compared with the CO2 supply via convection/diffusion.
Correspondingly, the desublimated solid CO2 layer (SCL) transforms from a loose
(i.e. cluster-like, dendritic or incomplete) structure to a dense one. Four desublimation
regimes are thus classified as diffusion-controlled, joint-controlled, convection-controlled
and desublimation-controlled regimes. The joint-controlled regime shows quantitatively
a desirable CO2 capture performance: fast desublimation rate, high capture capacity,
and full cylinder utilization. Regime distributions are summarized on a Pe–Tsub
space to determine operation parameters for the joint-controlled regime. Moreover,
three-dimensional simulations demonstrate four similar desublimation regimes, verifying
the reliability of 2-D results. Under regimes with loose SCLs, however, the desublimation
process shows an improved CO2 capture performance in three dimensions. This is
attributed to the enhanced availability of gas–solid interface and flow paths. This
work develops a reliable LB model to study CO2 desublimation, which can facilitate
applications of CCC for mitigating climate change
Strong Ultraviolet Pulse From a Newborn Type Ia Supernova
Type Ia supernovae are destructive explosions of carbon oxygen white dwarfs.
Although they are used empirically to measure cosmological distances, the
nature of their progenitors remains mysterious, One of the leading progenitor
models, called the single degenerate channel, hypothesizes that a white dwarf
accretes matter from a companion star and the resulting increase in its central
pressure and temperature ignites thermonuclear explosion. Here we report
observations of strong but declining ultraviolet emission from a Type Ia
supernova within four days of its explosion. This emission is consistent with
theoretical expectations of collision between material ejected by the supernova
and a companion star, and therefore provides evidence that some Type Ia
supernovae arise from the single degenerate channel.Comment: Accepted for publication on the 21 May 2015 issue of Natur
Strong Eukaryotic IRESs Have Weak Secondary Structure
BACKGROUND: The objective of this work was to investigate the hypothesis that eukaryotic Internal Ribosome Entry Sites (IRES) lack secondary structure and to examine the generality of the hypothesis. METHODOLOGY/PRINCIPAL FINDINGS: IRESs of the yeast and the fruit fly are located in the 5'UTR immediately upstream of the initiation codon. The minimum folding energy (MFE) of 60 nt RNA segments immediately upstream of the initiation codons was calculated as a proxy of secondary structure stability. MFE of the reverse complements of these 60 nt segments was also calculated. The relationship between MFE and empirically determined IRES activity was investigated to test the hypothesis that strong IRES activity is associated with weak secondary structure. We show that IRES activity in the yeast and the fruit fly correlates strongly with the structural stability, with highest IRES activity found in RNA segments that exhibit the weakest secondary structure. CONCLUSIONS: We found that a subset of eukaryotic IRESs exhibits very low secondary structure in the 5'-UTR sequences immediately upstream of the initiation codon. The consistency in results between the yeast and the fruit fly suggests a possible shared mechanism of cap-independent translation initiation that relies on an unstructured RNA segment
DETERMINATION OF THE ELECTROWEAK CHIRAL-LAGRANGIAN PARAMETERS AT THE LHC
In this work we report on the results obtained in a detailed and systematical
study of the possibility to measure the parameters appearing in the electroweak
chiral lagrangian. The main novelty of our approach is that we do not use the
Equivalence Theorem and therefore we work explicitly with all the gauge boson
degrees of freedom.Comment: 59 pages,latex, figures available on reques
On the buildup of massive early-type galaxies at z<~1. I- Reconciling their hierarchical assembly with mass-downsizing
Several studies have tried to ascertain whether or not the increase in
abundance of the early-type galaxies (E-S0a's) with time is mainly due to major
mergers, reaching opposite conclusions. We have tested it directly through
semi-analytical modelling, by studying how the massive early-type galaxies with
log(M_*/Msun)>11 at z~0 (mETGs) would have evolved backwards-in-time, under the
hypothesis that each major merger gives place to an early-type galaxy. The
study was carried out just considering the major mergers strictly reported by
observations at each redshift, and assuming that gas-rich major mergers
experience transitory phases of dust-reddened, star-forming galaxies (DSFs).
The model is able to reproduce the observed evolution of the galaxy LFs at
z<~1, simultaneously for different rest-frame bands (B, I, and K) and for
different selection criteria on color and morphology. It also provides a
framework in which apparently-contradictory results on the recent evolution of
the luminosity function (LF) of massive, red galaxies can be reconciled, just
considering that observational samples of red galaxies can be significantly
contaminated by DSFs. The model proves that it is feasible to build up ~50-60%
of the present-day mETG population at z<~1 and to reproduce the observational
excess by a factor of ~4-5 of late-type galaxies at 0.8<z<1 through the
coordinated action of wet, mixed, and dry major mergers, fulfilling global
trends that are in general agreement with mass-downsizing. The bulk of this
assembly takes place during ~1 Gyr elapsed at 0.8<z<1. The model suggests that
major mergers have been the main driver for the observational migration of mass
from the massive-end of the blue galaxy cloud to that of the red sequence in
the last ~8 Gyr.(Abridged)Comment: Accepted for publication in Astronomy & Astrophysics; 21 pages, 8
figures. Minor corrections included, shortened title. Results and conclusions
unchange
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