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Combustion and other high-temperature processes frequently result in the emission of aerosols in the form of polydisperse fractal-like aggregates made of condensed-phase nanoparticles (soot for instance). If certain conditions are met, the emitted aerosol particles are known to evolve into important cloud condensation nuclei (CCN) in the atmosphere. In this work, the hygroscopic parameter κ of complex morphology aggregates is calculated from the supersaturation-dependent activated fraction Fa=Fa(SS) in the frame of κ-Köhler theory. The particle size distribution is approximated with the morphology-corrected volume equivalent diameter calculated from the electrical mobility diameter by taking into account the diameter of the primary particle and the fractal dimension of the aggregate experimentally obtained from transmission electron microscopy measurements. Activation experiments are performed in water supersaturation conditions using a commercial CCN-100 condensation nuclei counter. The model is tested in close-to-ideal conditions of size-selected, isolated spherical particles (ammonium sulfate nanoparticles dispersed in nitrogen), then with complex polydisperse fractal-like aggregates (soot particles activated by exposure to ozone with κ as low as 5×10-5) that represent realistic anthropogenic emissions in the atmosphere.

Abstract The French government has supported as part of its “Investments for the Future” program a 9‐year research project, PeaMUST, devoted to pea and to a lesser extent, faba bean improvement. Focusing on the main causes of yield irregularity that limit pea and faba bean cultivation, an integrated approach, including molecular exploitation of the pea genome sequence, was applied to identify and incorporate favorable alleles and allele combinations in prebreeding material.

In recent years, two-photon laser-induced fluorescence (TPLIF) of inert gas krypton (Kr) has received much attention as a potential non-intrusive flow diagnostic, thermometry, and velocimetry technique. However, the interaction between the laser bandwidth and the atomic excitation transition at varying pressures, and the variation of the fluorescence quantum yield due to spatially varying collisional quenching of Kr are not fully understood. The resulting lineshape and quenching corrections become especially important when the temporal scales of excitation laser pulses vary from nanosecond (ns) to femtosecond (fs) timescales. In this study, we report a comprehensive investigation of Kr line broadening and quenching effects, in particular, with respect to TPLIF-based temperature (T) measurements in flames. Experiments were conducted in both low-pressure and atmospheric-pressure flames and in two different burner configurations, using conventional ns and ultrafast fs duration laser sources. When using ns pulses, the results suggest no lineshape corrections are necessary at low pressure; however, a T -dependent empirical model was needed for atmospheric-pressure flames. In contrast, broadband fs pulses have linewidths in excess of 400 cm−1 , which are significantly larger than sub-cm−1 spectral transitions, hence requiring no lineshape corrections at any pressure. Quenching measurements in low-pressure flames were found to be scaled by T −0.5 using previous low-pressure thermometry data and 1D flame model predictions. The Kr TPLIF temperature profiles measured using ns pulses (low and atmospheric pressures) and fs pulses (atmospheric pressures) agreed well with model predictions. In the absence of the necessity for lineshape corrections, fs Kr-TPLIF provides an easy way to extract T simply by taking the inverse square of the fluorescence signal. T -line imaging using fs-pulses are presented for several flame heights. The present study establishes a simplified approach for Kr TPLIF-based thermometry in flames. [ABSTRACT FROM AUTHOR]

This article is addressed to a scientific community familiar to astrophysics, and most researchers are not familiar with combustion science. Thus the goal is to provide a basic understanding of “the flame” and the different steps that lead to soot particle formation in conditions that are generally close to atmospheric pressure and high temperature, i.e. in conditions far from those encountered in most astrophysical media. Some experimental techniques of sooty flame investigations will also be introduced. The main aim is to outline the concepts and the tools of combustion science to facilitate fruitful scientific exchanges between two communities that are motivated by the understanding of the formation and evolution of PAHs and soot-like particles, in completely different environments. It should be noted that the description does not pretend to be exhaustive at all and certain theoretical and experimental approaches are not described nor even mentioned. Résumé. Cet article s’adresse à une communauté « astrophysique » non familière avec la combustion. Il a pour but de donner quelques bases permettant de comprendre « la flamme » et les étapes chimiques principales conduisant à la formation de particules de suie dans un milieu généralement à pression atmosphérique et haute température, c’est-à-dire dans des conditions de pression et température très éloignées de celles rencontrées dans le milieu interstellaire. Certaines méthodes expérimentales d’investigation de flammes suitées sont également présentées. L’objectif étant de pouvoir jeter des ponts entre deux communautés éloignées mais rassemblées par un même intérêt pour les hydrocarbures aromatiques polycycliques et les particules de suie. Il est clair que la présentation qui est faite ici est très succincte et qu’elle fait l’impasse sur certaines théories, approches, méthodes expérimentales…

This work investigates the role of soot particles in the early steps of formation of condensation trails (contrails). Contrails are thin linear clouds that form behind cruising aircrafts and can evolve into persistent clouds, and therefore can contribute to the radiative forcing of the atmosphere. Mitigating their contribution is considered an efficient way for decreasing the radiative forcing. Condensation freezing is proposed as a major pathway leading to the heterogeneous nucleation of ice particles in the aircraft exhausts. Soot particles, freshly produced in the flames and not significantly oxidized in the post-flame, are hydrophobic. Therefore, many theoretical and experimental approaches that investigate contrails formation introduce prior activation by surface adsorption of sulfur compounds issued from kerosene combustion. Even though the role of sulfur content of the fuel is unclear, and may only weakly impacts contrails formation, many simulations still rely on this assumption. In this work, to elucidate the role of sulfur compounds on the activation of soot chemically aged with OH radicals, a comparative study is performed with soot sampled from a turbulent jet flame burning a sulfur-containing fuel (kerosene) and a sulfur-free fuel (diesel). Soot aging experiments are performed in the atmospheric simulation chamber CESAM, with controlled generation of OH radicals. Soot activation (ratio between the number of nucleated droplets and seeding particles) is measured in water supersaturation conditions. OH exposure significantly enhances soot activation regardless of the sulfur presence in the fuel. The possibility for soot oxidation by OH radicals formed in aeronautical engines to be sufficient to promote soot activation is explored. Large eddy simulation in the high pressure turbine is used to model the fluid particle and OH trajectories and their chemical evolution. Residence time in the turbine is found sufficient to activate soot particles, opening a new possible route to explain ice particles formation. [ABSTRACT FROM AUTHOR]

Lignocellulosic tetrahydrofuranic (THF) biofuels have been identified as promising fuel candidates for spark-ignition (SI) engines. To support the potential use as transportation biofuels, fundamental studies of their combustion and emission behavior are highly important. In the present study, the high-temperature (HT) combustion chemistry of tetrahydrofurfuryl alcohol (THFA), a THF based biofuel, was investigated using a comprehensive experimental and numerical approach. Representative chemical species profiles in a stoichiometric premixed methane flame doped with ∼20% (molar) THFA at 5.3 kPa were measured using online gas chromatography. The flame temperature was obtained by NO laser-induced fluorescence (LIF) thermometry. More than 40 chemical products were identified and quantified. Many of them such as ethylene, formaldehyde, acrolein, allyl alcohol, 2,3-dihydrofuran, 3,4-dihydropyran, 4-pentenal, and tetrahydrofuran-2-carbaldehyde are fuel-specific decomposition products formed in rather high concentrations. In the numerical part, as a complement to kinetic modeling, high-level theoretical calculations were performed to identify plausible reaction pathways that lead to the observed products. Furthermore, the rate coefficients of important reactions and the thermochemical properties of the related species were calculated. A detailed kinetic model for high-temperature combustion of THFA was developed, which reasonably predicts the experimental data. Subsequent rate analysis showed that THFA is mainly consumed by H-abstraction reactions yielding several fuel radicals that in turn undergo either β-scission reactions or intramolecular radical addition that effectively leads to ring enlargement. The importance of specific reaction channels generally correlates with bond dissociation energies. Along THFA reaction routes, the derived species with cis configuration were found to be thermodynamically more stable than their corresponding trans configuration, which differs from usual observations for hydrocarbons. [ABSTRACT FROM AUTHOR]

Quantification of the O-atom concentration is particularly important for testing the detailed kinetic mechanisms in flames. In this work, relative concentrations of O-atoms are measured by using the two-photon Laser-Induced Fluorescence method (TPLIF). The direct calibration for determining the absolute O-atom mole fraction is achieved by comparing alternately the integrated TPLIF signals from the atomic oxygen and from a known quantity of a noble gas (Xe). Both species have close spectral features with a two-photon absorption around 225 nm, while the fluorescence signals are collected around 844 and 834 nm for O-atom and Xe, respectively. Application of this method previously adopted in plasma environments is demonstrated here for the first time in low-pressure hydrocarbon flames. One major achievement in this work is that the O and Xe fluorescence signals are measured in the flame, thus allowing to properly correcting for the quenching rate of both species in this high temperature environment. Due to the fast quenching rates of O-atom and of Xe in the N 2 diluted flames, a specific flame diluted in Ar was stabilized at 2.8 kPa for the calibration procedure. From Stern-Volmer plots obtained in a large pressure range of flames, the quenching rate determination could be extended to nitrogen-diluted flames. Then, the O-atom profiles were measured in absolute mole fraction in three low-pressure (5.3 kPa) premixed flames of CH 4 /O 2 /N 2 with equivalence ratios equal to 0.8, 1.0 and 1.25. The experimental profiles O-atom mole fraction profiles are compared to the simulated ones. Their gradient locations and the absolute mole fractions at 25 mm are found in good agreement. However, the simulated profiles do not capture the experimental continuous decrease of O in the burnt gases. [ABSTRACT FROM AUTHOR]

Abstract NH is a key short-lived radical involved in the prompt-NO formation. Quantification of NH is thus particularly important for testing the NO kinetic mechanisms. However, quantitative measurements of native NH in hydrocarbon/oxygen/nitrogen flames remain very scarce. Therefore, in this work, the mole fractions of native NH were obtained using a combination of laser-based diagnostics; Laser Induced Fluorescence (LIF) and Cavity Ring-Down Spectroscopy (CRDS). The NH species was probed after exciting the transition R 1 (6) in the A3Π-X3Σ− (0-0) system at 333.9 nm. The mole fraction profiles of NH were successfully obtained in premixed low-pressure flames of CH 4 /O 2 /N 2 and C 2 H 2 /O 2 /N 2 at two equivalence ratios of 1.00 and 1.25. The estimated detection limit for the NH radical was around 4.5 × 108 molecule cm−3 (i.e. 2 ppb in mole fraction at 1600 K), which is nearly 2 orders of magnitude lower than previous values reported in the literature. These new experimental results were compared with predictions by a recently developed NO model (namely NOMecha2.0). In the case of the CH 4 flames, a satisfying agreement between the experiment and model was observed. However, in the case of the C 2 H 2 flames, some discrepancies were observed. Model analysis has highlighted the importance of the HCCO radicals in the NH formation through the HCNO→HNCO→NH 2 reactions pathway. Modification of the rate constant values of the reactions C 2 H 2 + O and HCCO + O 2 , which are key reactions for both the acetylene laminar flame speed and the HCCO predictions, has enabled the model to satisfactorily predict the experimental NH and NO profiles also in the C 2 H 2 flames. [ABSTRACT FROM AUTHOR]

Plasma-assisted cool flames of n- heptane were generated in the combustion chamber of a rapid compression machine coupled with a nanosecond dielectric barrier discharge, at a pressure of 1.5 bar and temperature T C = 650 K. Increasing of the voltage pulse amplitude at the electrode resulted in a transition from no reactivity to induced cool flame and then to fast ignition. Sampling of the reacting mixture was performed at selected times during the experiments to draw mole fraction profiles of the fuel and major low temperature stable intermediates, showing a gradual increase in the mole fraction of these species after the discharge. Comparison with a spontaneous cool flame case at a slightly higher pressure shows that no new species are formed in the plasma-assisted case, and that after the initiation of reactivity by the discharge at the nanosecond timescale, the distribution and relative importance of the main reaction pathways is conserved at the millisecond timescale. Differences in the shape of the mole fraction and light emission profiles however suggest that the plasma-assisted cool flame is propagative. [ABSTRACT FROM AUTHOR]

Abstract: The present work aims to quantitatively measure HCN in a low pressure premixed CH4/N2O/O2/N2 flame after gas probe sampling. For that purpose, the (003)–(000) band of HCN has been selected for the first time for HCN monitoring. This band was chosen because it lies in a wavelength range (1.036μm) easily reachable by commercial dye laser combined with Raman cell shifter. The line strengths of its main R branches are 30 times lower than the transitions lying at 1.5μm generally used. The sensitivity was achieved using pulsed CRDS technique in a long absorption cell (1m). The required wavelength is generated from the first stokes radiation of hydrogen after 724nm pumping. First, absorption spectra of HCN diluted in N2 have been obtained and compared to simulated spectra of the R-branch. The very good agreement between the experimental and simulated spectra, both in line position and absolute intensity, allows to validate our strategy and the experimental setup. Second the HCN profile in the flame was determined by measuring the integrated spectral absorptivity around the R(7) transition line of HCN. The profile of the HCN mole fraction along the flame has been measured with a peak value equal to 600ppm. The technique implemented allows detecting HCN in the limit of 2.5×1013 moleculecm−3 with an accuracy of 20%. [Copyright &y& Elsevier]

Abstract: Soot particle emission from combustion processes is a significant environmental and health problem. Optical detection is clearly the only viable choice for in situ PAH and soot measurement in turbulent flames. This paper reviews the considerable progress made in optical diagnostics of soot and its precursors formed this last decade. Advances include the spectroscopy of polycyclic aromatic hydrocarbons (PAHs) and optical diagnostics for PAHs in flames, new approaches to identify the structure and composition of incipient soot, and the latest refinements for soot particle detection and quantification. Improvements of selective and quantitative PAH measurements have been made possible with supersonic expansion of flame extracted samples, coupled with laser induced fluorescence (jet-cooled LIF) and mass-spectroscopy combined with resonance enhanced or synchrotron selective photoionisation. More accurate description of all facets of soot particles properties (optical properties, morphology, and volume fraction) has guided recent research. In particular, optical exploration has led to new insight into soot inception, including the identification of particular soot precursors from large PAHs to nanoparticles passing through stacked PAHs, polymeric-like structures and aliphatic-linked PAHs. Despite the various improvements, the ultimate accuracy of the soot diagnostic methods such as laser induced incandescence, light extinction and elastic light scattering relies on our knowledge of the soot optical properties which may vary as a function of soot age in flames. Nonetheless, with well designed experiments, comparative measurement of sooting trends within or between flames can be acquired with accuracies needed for the validation of numerical simulations of soot formation. These optical methods have been successfully adopted to classify the sooting tendencies of a large number of hydrocarbons and to examine the effect of pressure on soot formation. The ultimate objective is certainly to use optical diagnostics to provide useful data for soot formation modelling in turbulent flames and under practical combustion. [Copyright &y& Elsevier]

Abstract: The present work has been performed to quantitatively determine the NCN radical concentration profile in a laminar low-pressure CH4/O2/N2 (ϕ =1.25) flame. Laser induced fluorescence (LIF) technique has been implemented in order to record a NCN spectrum in the 326.7–330.1nm range in flame conditions, and secondly to measure the spatially resolved NCN profile in the flame by exciting the main congested branch of NCN around 329.13nm. The LIF profile has been calibrated into absorbance using cavity ring-down spectroscopy (CRDS). The calibration into absolute concentration has been done by using the PGOPHER spectral simulation program. The experimental and the simulated NCN spectra were found to be in very good agreement. It has allowed reconsidering the exact assignment of the transitions involved in the strong bandhead at flame temperature, which actually originate from the contribution of both the origin (000) band and the (010Σ+) component of the (010) band contrary to ambient temperature where only the (000) band contribution is significant around 329.13nm. Thanks to the simulation and using electronic transition dipole moments extracted from available NCN fluorescence lifetimes, we provide a direct determination of the absorption cross-section: σ(λ =329.13nm)=(9.4±1.8)×10−17 cm2/molecule at 1830K. This cross-section was found constant within ±15% in the 1500–1900K temperature range. Its determination is very important for research groups concerned with NCN chemistry at high temperature and prompt-NO formation. From that cross-section the peak mole fraction of the NCN radical has been determined to be equal to 170±90ppb in the low-pressure flame. The experimental uncertainty is mainly linked to the CRDS measurement conditions close to the detection limit and should be reduced in flame conditions yielding higher NCN concentration. [Copyright &y& Elsevier]

Abstract: Soot formation is compared in turbulent diffusion flames burning a commercial Diesel and two Diesel surrogates containing n-decane and α-methylnaphthalene. A burner equipped with a high-efficiency atomisation system has been specially designed and allows the stabilisation of liquid fuels flames with similar hydrodynamics conditions. The initial surrogate composition (70% n-decane, 30% α-methylnaphthalene) was previously used in the literature to simulate combustion in Diesel engines. In this work, a direct comparison of Diesel and surrogates soot tendencies is undertaken and relies on soot and fluorescent species mappings obtained respectively by Laser-Induced Incandescence (LII) at 1064nm and Laser-Induced Fluorescence at 532nm. LIF was assigned to soot precursors and mainly to high-number ring Polycyclic Aromatic Hydrocarbons (PAH). The initial surrogate was found to form 40% more soot than the tested Diesel. Consequently, a second surrogate containing a lower α-methylnaphthalene concentration (20%) has been formulated. That composition which presents a Threshold Soot Index (TSI) very close to Diesel one is also consistent with our Diesel composition that indicates a relatively low PAH content. The spatially resolved measurements of soot and fluorescent soot precursors are quite identical (in shape and intensity) in the Diesel and in the second surrogate flames. Furthermore the concordance of the LII temporal decays suggests that a similar growth of the primary soot particles has occurred for Diesel and surrogates. In addition, the comparison of the LII fluence curves indicates that physical/optical properties of soot contained in the different flames might be similar. The chemical composition present at the surface of soot particles collected in Diesel and surrogate flames has been obtained by laser-desorption ionisation time-of-flight mass spectrometry. An important difference is found between Diesel and surrogate samples indicating the influence of the fuel composition on soot content. [Copyright &y& Elsevier]

Abstract: Recent advances in the field of laser desorption/laser ionization mass spectrometry (LD/LI/MS) have renewed interest in these separation methods for fast analysis of chemical species adsorbed on soot particles. These techniques provide mass-separation of the desorbed phase with high selectivity and sensitivity and require very small soot samples. Combining LD/LI/MS with in situ measurements of soot and gaseous species is very promising for a better understanding of the early stage of soot growth in flames. In this work, three lightly sooting laminar jet flames (a methane diffusion flame and two premixed acetylene flames of equivalence ratio (ϕ)=2.9 and 3.5) were investigated by combining prompt and 50ns-delayed laser-induced incandescence (LII) for spatially resolved measurements of soot volume fraction (f v) and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH). Soot and PAH calibration is performed by two-colour cavity ring-down spectroscopy (CRDS) at 1064 and 532nm. Soot particles were sampled in the flames and analysed by LD/LI/Time-of-flight- MS. Soot samples are cooled to −170°C to avoid adsorbed phase sublimation (under high vacuum in the TOF-MS). Our set-up is novel because of its ability to measure very low concentration of soot and PAH together with the ability to identify a large mass range of PAHs adsorbed on soot, especially volatile two-rings and three-rings PAHs. Studied flames exhibited a peak f v ranging from 15ppb (acetylene, ϕ =2.9) to 470ppb (acetylene, ϕ =3.5). Different mass spectra were found in the three flames, each exhibiting one predominant PAH mass; 202amu (4-rings) in methane, 178amu (3-rings) in acetylene,ϕ =2.9 and 128amu (2-rings) in acetylene, ϕ =3.5. These variations with flame condition contrasts with other recent studies and is discussed. The other PAH masses ranged from 102 (C8H6) to 424amu (C34H16) and are well predicted by the stabilomer grid of Stein and Farr. [Copyright &y& Elsevier]

Abstract: Coupling between cavity ring down spectroscopy (CRDS) and laser induced fluorescence (LIF) methods has been implemented to measure nascent C2 (a3Πu) in both non-sooting and sooting flames stabilised on a Wolfhard-Parker burner at atmospheric pressure. In the non-sooting flame, relative concentration profiles have been recorded by LIF measurements and calibrated by CRDS in absolute concentration (maximum of 3.4×10−9 in mole fraction). In the sooting flame, it is shown that LIF is unusable to measure nascent C2 in the sooting regions due to the creation of additional C2 from laser soot vaporisation, even by using extremely low laser energies. It is besides demonstrated that, although CRDS measurements were also affected by PAH/soot interference, relative profile determination and absolute quantification could be satisfactorily obtained. Complete C2 profiles in the sooting flame are thus obtained by combining LIF and CRDS. Hence, LIF can be advantageously used to measure relative profiles of C2 in the vicinity of the burner (non-sooting zone) and CRDS can then provide determination of C2 profiles in the presence of soot. Absolute concentration has been determined by CRDS, providing a peak mole fraction of 6.8×10−9. Based on a comparison with PAH and soot profiles issued from a previous work, the role of C2 in soot formation mechanism is discussed. A comparison with earlier profiles of CH is also proposed, showing a good correlation between C2 and CH radical profiles. [Copyright &y& Elsevier]

Abstract: This study reports CH and NO mole fraction profiles measured in 12 premixed flames representative of natural gas combustion. Four mixtures CH4/O2/N2, CH4/C2H6/O2/N2, CH4/C3H8/O2/N2, and CH4/C2H6/C3H8/O2/N2 have been investigated at three equivalence ratios (0.7, 1, and 1.25). The aim of this work was to study the influence of C2 and C3 compounds of natural gas on NO formation in flames. Flames were stabilised on a flat flame burner at low pressure (33torr). The experimental approach was exclusively based on non-intrusive techniques: laser induced fluorescence (LIF) for OH-LIF thermometry and species concentration profiles determination, and cavity ring-down spectroscopy (CRDS) for absolute calibration. Thus, an extended database including a large dynamic of combined CH and NO mole fraction profiles (ranging, respectively, from 0.7 to 10ppm and from 2.6 to 24ppm) has been obtained and will serve for further improvement of chemical mechanisms of NO formation in natural gas flames. It is shown that, for a given equivalence ratio, NO quantity remains unchanged when substituting methane by ethane and/or propane, while CH mole fraction exhibits a slight decrease. Predominance of prompt-NO mechanism is shown to be sensitive to the equivalence ratio. These new results have been modelled by using the GRI3.0 mechanism, and a reaction path analysis has been performed in the stoichiometric methane flame to highlight the different routes of NO formation in the different zones of the flame, i.e., the preheat zone, the reaction zone, and the burned gases. [Copyright &y& Elsevier]