DNS of a diffusional jet flame in turbulent cross-flow using a low Mach number solver

Understanding of flame anchoring in a jet in crossflow (JICF) configuration is vital to the design of fuel injectors in combustion devices. The present study numerically investigates a hydrogen rich jet injecting perpendicularly into hot vitiated crossflow using direct numerical simulation (DNS). Development of the reacting flow field and flame shape along the jet trajectory is scrutinised. The flame is found to be anchored around the jet exit, and downstream only on the windward side. Heat release rate and Chemical Explosive Mode Analysis (CEMA) are used to identify combustion modes. Distinct from flames stabilizing in non-vitiated crossflow where combustion is mainly partially premixed, diffusion flame is significant under the current condition, though some premixed or partially premixed regions are found on the leeward side of the jet due to large scale turbulent mixing

On the galactic chemical evolution of sulfur

Sulfur abundances have been determined for ten stars to resolve a debate in the literature on the Galactic chemical evolution of sulfur in the halo phase of the Milky Way. Our analysis is based on observations of the S I lines at 9212.9, 9228.1, and 9237.5 A for stars for which the S abundance was obtained previously from much weaker S I lines at 8694.0 and 8694.6 A. In contrast to the previous results showing [S/Fe] to rise steadily with decreasing [Fe/H], our results show that [S/Fe] is approximately constant for metal-poor stars ([Fe/H] < -1) at [S/Fe] = +0.3. Thus, sulfur behaves in a similar way to the other alpha elements, with an approximately constant [S/Fe] for metallicities lower than [Fe/H] = -1. We suggest that the reason for the earlier claims of a rise of [S/Fe] is partly due to the use of the weak S I 8694.0 and 8694.6 A lines and partly uncertainties in the determination of the metallicity when using Fe I lines. The S I 9212.9, 9228.1, and 9237.5 A lines are preferred for an abundance analysis of sulfur for metal-poor stars.Comment: Accepted by A&A, 12 pages. Full article with figures in A&

Combustion Mode and Mixing Characteristics of a Reacting Jet in Crossflow

Understanding of flame anchoring in a jet in crossflow (JICF) configuration is vital to the design of fuel injectors in combustion devices. The present study numerically investigates a hydrogen-rich jet injecting perpendicularly into hot vitiated crossflow using direct numerical simulation (DNS). The governing equations of low-Mach-number multicomponent reactive flows are solved, with a chemical mechanism for hydrogen-air flames containing 13 species and 35 reactions. The mixture-averaged multispecies transport model is employed to calculate the diffusion terms. Development of the reacting flow field and flame shape along the jet trajectory is depicted. The flame is found to be anchored around the jet exit and downstream only on the windward side. The heat release rate and chemical explosive mode analysis (CEMA) are used to identify combustion modes. Distinct from flames stabilized in nonvitiated crossflow, diffusion flame is dominant under the current conditions, though some premixed or partially premixed regions are found on the leeward side of the jet due to turbulent mixing. The near-field mixing of the reacting JICF is quantified by spatial unmixedness, in both two-dimensional (2D) and three-dimensional (3D) space

High hydrogen content syngas fuel burning in lean premixed spherical flames at elevated pressures: Effects of preferential diffusion

This study addresses the effects of preferential diffusion on flame structure and propagation of high hydrogen content (HHC) turbulent lean premixed hydrogen-carbon monoxide syngas flames at elevated pressures. The direct numerical simulations with detailed chemistry were performed in three-dimensional domain for expanding spherical flame configuration in a constant pressure combustion chamber. To identify the role of preferential diffusion on flame structure and propagation under low and high turbulence levels at elevated pressure, simulations were performed at an initial turbulent Reynolds number of 15 and 150 at a pressure value of 4 bar. The results demonstrate that the thermo-diffusive instability greatly influences the lean premixed syngas cellular flame structure due to strong preferential diffusion effects under low turbulence level at elevated pressure. In contrast, the results reveal that the thermo-diffusive effects are destabilising and preferential diffusion is overwhelmed by turbulent mixing under high turbulence level at elevated pressure. This finding suggests that the development of cellular flame structure is dominated by turbulence with little or no contribution from the thermo-diffusive instability for the lean premixed syngas flame which operates under conditions of high turbulence and elevated pressures. However, results demonstrate that the flame acceleration and species diffusive flux are still influenced by the preferential diffusion for the lean premixed syngas flame which operates under conditions of high turbulence and elevated pressures

A large sample of calibration stars for Gaia: log g from Kepler and CoRoT

Asteroseismic data can be used to determine surface gravities with precisions of < 0.05 dex by using the global seismic quantities Deltanu and nu_max along with Teff and [Fe/H]. Surface gravity is also one of the four stellar properties to be derived by automatic analyses for 1 billion stars from Gaia data (workpackage GSP_Phot). We explore seismic data from MS F, G, K stars (solar-like stars) observed by Kepler as a potential calibration source for methods that Gaia will use for object characterisation (log g). We calculate log g for bright nearby stars for which radii and masses are known, and using their global seismic quantities in a grid-based method, we determine an asteroseismic log g to within 0.01 dex of the direct calculation, thus validating the accuracy of our method. We find that errors in Teff and mainly [Fe/H] can cause systematic errors of 0.02 dex. We then apply our method to a list of 40 stars to deliver precise values of surface gravity, i.e. sigma < 0.02 dex, and we find agreement with recent literature values. Finally, we explore the precision we expect in a sample of 400+ Kepler stars which have their global seismic quantities measured. We find a mean uncertainty (precision) on the order of <0.02 dex in log g over the full explored range 3.8 < log g < 4.6, with the mean value varying only with stellar magnitude (0.01 - 0.02 dex). We study sources of systematic errors in log g and find possible biases on the order of 0.04 dex, independent of log g and magnitude, which accounts for errors in the Teff and [Fe/H] measurements, as well as from using a different grid-based method. We conclude that Kepler stars provide a wealth of reliable information that can help to calibrate methods that Gaia will use, in particular, for source characterisation with GSP_Phot where excellent precision (small uncertainties) and accuracy in log g is obtained from seismic data.Comment: Accepted MNRAS, 15 pages (10 figures and 3 tables), v2=some rewording of two sentence

Modeling ice crystal growth using the lattice Boltzmann method

Given the multitude of growth habits, pronounced sensitivity to ambient conditions and wide range of scales involved, snowflake crystals are one of the most challenging systems to model. The present work focuses on the development and validation of a coupled flow/species/phase solver based on the lattice Boltzmann method. It is first shown that the model is able to correctly capture species and phase growth coupling. Furthermore, through a study of crystal growth subject to ventilation effects, it is shown that the model correctly captures hydrodynamics-induced asymmetrical growth. The validated solver is then used to model snowflake growth under different ambient conditions with respect to humidity and temperature in the plate-growth regime section of the Nakaya diagram. The resulting crystal habits are compared to both numerical and experimental reference data available in the literature. The overall agreement with experimental data shows that the proposed algorithm correctly captures both the crystal shape and the onset of primary and secondary branching instabilities. As a final part of the study the effects of forced convection on snowflake growth are studied. It is shown, in agreement with observations in the literature, that under such condition the crystal exhibits non-symmetrical growth. The non-uniform humidity around the crystal due to forced convection can even result in the coexistence of different growth modes on different sides of the same crystal

On the metallicity distribution of classical Cepheids in the Galactic inner disk

We present homogeneous and accurate iron abundances for almost four dozen (47) of Galactic Cepheids using high-spectral resolution (R$\sim$40,000) high signal-to-noise ratio (S/N $\ge$ 100) optical spectra collected with UVES at VLT. A significant fraction of the sample (32) is located in the inner disk (RG $le$ 6.9 kpc) and for half of them we provide new iron abundances. Current findings indicate a steady increase in iron abundance when approaching the innermost regions of the thin disk. The metallicity is super-solar and ranges from 0.2 dex for RG $\sim$ 6.5 kpc to 0.4 dex for RG $\sim$ 5.5 kpc. Moreover, we do not find evidence of correlation between iron abundance and distance from the Galactic plane. We collected similar data available in the literature and ended up with a sample of 420 Cepheids. Current data suggest that the mean metallicity and the metallicity dispersion in the four quadrants of the Galactic disk attain similar values. The first-second quadrants show a more extended metal-poor tail, while the third-fourth quadrants show a more extended metal-rich tail, but the bulk of the sample is at solar iron abundance. Finally, we found a significant difference between the iron abundance of Cepheids located close to the edge of the inner disk ([Fe/H]$\sim$0.4) and young stars located either along the Galactic bar or in the nuclear bulge ([Fe/H]$\sim$0). Thus suggesting that the above regions have had different chemical enrichment histories. The same outcome applies to the metallicity gradient of the Galactic bulge, since mounting empirical evidence indicates that the mean metallicity increases when moving from the outer to the inner bulge regions.Comment: 10 pages, 5 figures; Corrected typos, corrected Table

Response of curved premixed flames to single-frequency and wideband acoustic waves

The dynamic response of a premixed curved flame interacting with sinusoidal acoustic waves has been numerically studied in the present work. Flame/acoustic interactions are particularly important both from a theoretical point of view and for practical purposes, as a possible trigger mechanism for combustion instabilities. Flames found in practical devices show a complex geometry, far from the planar configuration usually considered in theoretical studies. The particular purpose of the current study is to assess quantitatively the effects of acoustic waves on curved premixed flames, considering both single and wideband frequencies in order to mimic the conditions encountered in practical systems. The interaction process is studied by using Direct Numerical Simulation (DNS) including detailed physicochemical processes and differential molecular diffusion. The chemical reactions are modeled by a 25-step skeletal scheme involving 16 species to describe methane oxidation. The numerical results show strong flame front oscillations back and forth during interaction of the wave with the curved premixed flame. Moreover, the results demonstrate that a single-frequency acoustic wave has a magnifying effect on the preexisting wrinkling of the flame. This extending flame front leads to increasing fuel consumption rate. The effect is found to be maximum at an intermediate excitation frequency of 500 Hz. Interestingly, a wideband excitation from 100 to 1000 Hz leads to significant flame oscillation and the fuel consump- tion rate is highly increased in that case. As a whole, this study shows that curved flames are much more sensitive to acoustic excitations compared to planar flames, due to the baroclinic torque in combination with other inherent instabilities. An oblique acoustic wave has a similar but slightly enhanced distur- bance to the premixed flame. Moreover, non-unity Lewis numbers have significant effects on curved flame-acoustic interaction, even in the present stoichiometric methane flame. However, it presented highly sensitive to the interaction