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International audience; The oxidation of di-n-butyl-ether (DBE) was performed in a jet-stirred reactor (JSR) at 1 atm, 520 and 530 K, and from 480 to 670 K at 10 atm, at a residence time of 1 s, an equivalence ratio of 0.5, and an initial fuel concentration of 5000 ppm. Ignition experiments on DBE/air mixtures were also performed in a rapid compression machine (RCM) under stoichiometric conditions, 5 bar, and from 550 to 630 K. Low-temperature products formed in JSR and RCM experiments were characterized. To this end, high-resolution mass spectrometry analyses (HRMS) with flow injection analyses or ultra-high pressure liquid chromatography coupling were used to characterize hydroperoxides and diols (C8H18O3), ketohydroperoxides (C8H16O4), carboxylic acids (C2H4O2, C3H6O2, C4H8O2), di-keto ethers (C8H14O3), and highly oxygenated molecules (C8H14O5, C8H16O6, C8H14O6, C8H14O7, C8H16O8, and C8H14O9) resulting from up to five O2 additions on fuel's radicals. Whereas HOMs are of minor importance in combustion, they are considered key species for the formation of secondary organic aerosols. In addition, cyclic ethers and esters (C8H16O2) were observed. Heated electrospray and atmospheric pressure chemical ionizations (HESI and APCI) were used in positive and negative modes for MS analyses. H/D exchange with D2O was used to confirm the presence of –OH or –OOH groups in the products. The present results show that DBE oxidation proceeds similarly under JSR and RCM conditions. Whereas the CH2 groups neighboring the ether group are the most favorable sites for H-atom abstraction reactions, speciation indicated that other sites can react by metathesis forming a large pool of intermediates and products. Our kinetic reaction mechanism needs to be extended for simulating the formation of newly detected species.

International audience; Shock tube techniques for kinetic studies are continuously evolving driven by advances in kinetic modeling and detection techniques. An innovative category of shock tubes has been recently developed for use at Synchrotron facilities. In this work, a new high-repetition-rate shock tube (HRRST) was constructed to employ synchrotron-based double imaging photoelectron/photoion coincidence spectroscopy (i 2 PEPICO) at the beamline DESIRS of the SOLEIL synchrotron. The shock tube design and performance (pressure profiles, repeatability of operations) are presented for the first time together with the detailed description of the coupling with the molecular beam end-station holding the i 2 PEPICO spectrometer. The first experimental results with the HRRST/i 2 PEPICO on ethanol pyrolysis are grouped based on four different experimental conditions, each highlighting functionality of this novel experimental system. Experiments were performed at temperatures between 1232 K and 1525 K, pressures between 6.2 bar and 7.5 bar, with 2.7% or 0.25% ethanol in argon, and photon energy of 10.0 eV or 11.0 eV. The results are supported by kinetic analyses with the CRECK model. This study shows the potential of the HRRST/i 2 PEPICO combination for obtaining detailed mechanistic and kinetic data for complex chemical systems. Mass spectra, photoelectron spectra and time-resolved species profiles were obtained for a wide variety of species from methyl radicals to large polyaromatic hydrocarbons. The different experimental conditions studied indicate how future experiments can be designed to target key regimes facilitating the elucidation of desired kinetic and mechanistic data.

The oxidation of diethyl ether was studied experimentally in a jet-stirred reactor. Fuel-lean, stoichiometric and fuel-rich mixtures were oxidized at a constant fuel mole fraction (1000 ppm), at temperatures ranging from 450 to 1250 K, pressures of 1 and 10 atm, and constant residence time (70 and 700 ms, respectively). In total, six mixtures were tested at both pressures. Mole fraction profiles were obtained using gas chromatography and Fourier transform infrared spectrometry. The fuel mole fraction profiles, as well as some reaction intermediate and product profiles indicated strong low-temperature chemistry at high pressure. On the other hand, at atmospheric pressure this behavior was observed to a very small extent and only with the lean and stoichiometric mixtures. These data were compared to modeling results using literature mechanisms for diethyl ether oxidation. None of these predicted low-temperature reactivity under present conditions. Therefore, a kinetic mechanism is proposed in this study, based on recently computed kinetic parameters from literature. It shows good performances for representing the present experimental data as well as experimental data found in literature consisting of ignition delay times, laminar flame speeds and flame structure.

International audience; This work is centered on the speciation from propylene and propyne pyrolysis by means of shock tube experiments and detailed kinetic modeling. A wealth of intermediates and products, covering small acyclic hydrocarbons up to four-ring aromatics, are probed from the C 3 fuels pyrolysis at a nominal pressure of 20 bar over 1050-1650 K. With updates in reactions involving C 3 species, our ongoing polycyclic aromatic hydrocarbon (PAH) formation kinetic model can well predict the measurements obtained in the current work as well as relevant literature data. Propyne exhibits a unique two-stage decomposition profile, as the characteristic isomerization to allene dominates in its consumption at moderate temperatures below 1300 K. Overall, propylene pyrolysis results in more diverse small hydrocarbons, but much lower contents of aromatics, in comparison to propyne pyrolysis. In both studied cases, the formation of benzene is dependent upon the propargyl recombination, and since propyne decomposition induces a more rapid and more plentiful propargyl production, benzene mole fractions are much higher in propyne pyrolysis. In both cases, naphthalene is observed as the most abundant PAH species, followed by acenaphthalene. Modeling analyses indicate that similar reaction pathways are responsible for the PAH formation in propylene and propyne pyrolysis. Indene is formed from the interactions between benzene/phenyl and C 3 species, through its non-PAH isomers as intermediates. The subsequent reactions of indenyl radical with methyl and propargyl are essential pathways leading to naphthalene and acenaphthalene, respectively. Naphthyl radical further participates in the formation of different larger PAHs. The methylene-substituted cyclopenta-ring species are deemed as important precursors of their aromatic isomers, as is noted from the fulvene-to-benzene, benzofulvene-to-naphthalene and 9-methylene-fluoreneto-phenanthrene conversions.

The energetic conversion of biomass into syngas or biogas is a more and more important topic. In the framework of these studies, improved understanding of glucose and fructose thermal decomposition and oxidation appears crucial. For this task, thermodynamic data are needed to make possible, for instance, the building of a detailed chemical kinetic model of glucose and fructose reactivity at high temperature. A semitheoretical protocol, presented elsewhere, is used for the estimation of the thermodynamic data of glucose and fructose in the gas phase. Five isomers of glucose and five isomers of fructose are considered and the lowest-energy conformers are found to be β- d -glucopyranose for glucose and β- d -fructopyranose for fructose. The data for all 10 isomers are provided in the CHEMKIN-NASA format.

International audience; An analytical model describing the kinetics of carbon dissolution in burning aluminum droplets has been developed in order to simulate its effects under solid rocket motor conditions. A carbon dissolution rate (k) was introduced in different droplet regression laws and depending on the heterogeneous kinetics between the Al sur-face and the surrounding gases. The model was validated using previous experiments performed by the authors on millimeter-sized Al droplets burning in several CO2-containing atmospheres at atmospheric pressure (P=1atm). It has been shown that the carbon dissolution is affected by the presence of hydrogen due to competition betweenCO and H2 chemisorption. The model was then applied to aluminized propellants (AP/HTPB) at high pressures(P=60 atm) and high temperatures (T=3000 and 3500 K), as well as at various burning rates and adsorption conditions. Though the accuracy of the extrapolation results needs further improvement, it has been shown that the carbon dissolution process should not be neglected in order to achieve global understanding of the combustion of Al particles, particularly agglomerates.

In an effort to understand the oxidation chemistry of new generation biofuels, oxidation of a pentanol isomer (2-methyl-1-butanol) was investigated experimentally in a jet-stirred reactor (JSR) at a pressure of 10 atm, equivalence ratios of 0.5, 1, 2 and 4 and in a temperature range of 700–1200 K. Concentration profiles of the stable species were measured using GC and FTIR. A detailed chemical kinetic mechanism including oxidation of various hydrocarbon and oxygenated fuels was extended to include the oxidation chemistry of 2-methyl-1-butanol, the resulting mechanism was used to simulate the present experiments. In addition to the present data, recent experimental data such as ignition delay times measured in a shock tube and laminar flame speeds were also simulated with this mechanism and satisfactory results were obtained. Reaction path and sensitivity analyses were performed in order to interpret the results.

International audience; The oxidation of a stoichiometric diethyl ether-oxygen-nitrogen mixture containing 5000 ppm of fuel was studied in a jet-stirred reactor at 10 atm and a residence times of 1 s. Experimental temperatures varied stepwise for 440 to 740 K and products were quantified by gas chromatography with TCD and FID, gas chromatography-mass spectrometry, and FTIR. Other experiments in the temperature range 480 to 570 K were performed for characterizing elusive cool flame products. To this end, gas samples were trapped in acetonitrile for flow injection analyses and liquid chromatography-mass spectrometry (Orbitrap Q-Exactive®). For ionization, we used positive and negative atmospheric pressure chemical ionization (APCI). Among fuel-specific products, hydroperoxides and diols (C4H10O3), carbonyl hydroperoxides (C4H8O4), acetic acid, di-keto ethers (C4H6O3), cyclic ethers (C4H8O2) and highly oxygenated molecules, i.e., keto-dihydroperoxides (C4H8O6), keto-trihydroperoxides (C4H8O8), di-keto-hydroperoxides (C4H6O5), and diketo-dihydroperoxides (C4H6O7), were detected. To confirm the presence of –OH or –OOH groups in oxidation products, H/D exchange with D2O was used. DNPH derivatization was used to identify carbonyls present in samples, especially those with a molecular weight below 50 amu which cannot be detected directly by the mass spectrometer. Chemical kinetic modeling using a mechanism taken from the literature was performed. Although reasonable agreement between the data and the simulations was observed for several species, some discrepancies between experimental and computed mole fractions were observed.

International audience; This work studies the influence of phoretic motion (thermophoresis and diffusiophoresis) of fine alumina particles ("smoke") produced during the combustion of aluminum. Direct numerical simulations on a single aluminum droplet burning in a quiescent environment suggest that thermophoresis is the main mechanism driving smoke back to the aluminum surface, hence a major contributor to the oxide lobe development. The presence of this lobe is found to distort the flowfield, which favors hot and smoke-rich regions closer to the lobe, thereby enhancing thermophoresis. This combination of aerodynamic and thermophoretic effects leads to a mass rate of deposited smoke which is consistent with experimental data. A simplified model, deduced from simulation results, is able to predict the size of the final oxide residue in good agreement with measurements. This study supports that aluminum oxide present on the burning aluminum particle is largely due to material formed in the flame, subsequently desposited by thermophoresis.

International audience; A wide variety of high temperature experimental data obtained in this study complement the data on the oxidation of the two di-isobutylene isomers presented in Part I, and offers a basis for an extensive validation of the kinetic model developed in this study. Due to the increasing importance of unimolecular decomposition reactions in high-temperature combustion, we have investigated the di-isobutylene isomers in high dilution utilizing a pyrolysis micro reactor and detected radical intermediates and stable products using vacuum ultraviolet (VUV) synchrotron radiation and photoelectron photoion coincidence (PEPICO) spectroscopy. Additional speciation data at oxidative conditions were also recorcollecdted utilizing a plug flow reactor at atmospheric pressure in the temperature range from 725 K to-1150 K at equivalence ratios of 1.0 and 3.0 and at a residence time of 0.35 s and 0.22 s respectively. Combustion products were analyzed using gas chromatography (GC) and mass spectrometry (MS). Ignition delay time measurements for di-isobutylene were performed at pressures of 15 and 30 bar at equivalence ratios of 0.5, 1.0, and 2.0 diluted in 'air' in the temperature range 900-1400 K using a high-pressure shock-tube facility. New measurements of the laminar burning velocities of di-isobutylene/ + air flames are also presented. Experiments have been performed using the heat flux method at atmospheric pressure and initial temperatures of 298-358 K. Moreover, data consistency was assessed with the help of analysis of the temperature and pressure dependencies of the laminar burning velocity measurements, which was interpreted using an empirical power-law expression. Electronic structure calculations were performed to compute the energy barriers to the formation of many of the product species formed. The predictions of the present mechanism were found to be in good adequate agreement with the wide variety of the experimental measurements performed.

International audience; Di-isobutylene has received significant attention as a promising fuel blendstock, synthesized via biological routes, and is a short-listed molecule for the Co-Optima initiative. Di-isobutylene is also popularly used as an alkene representative in multi-component surrogate models for engine studies of gasoline fuels. However, there is limited experimental data available in the literature for neat diisobutylene under engine-like conditions. Hence, most existing di-isobutylene models have not been extensively validated, particularly at lower temperatures (< 1000 K). Most gasoline surrogate models include the di-isobutylene sub-mechanism published by Metcalfe et al. [1] with little or no modification. The current study is undertaken to develop a detailed kinetic model for di-isobutylene and validate the model using a wide range of relevant experimental data. Part 1 of this study exclusively focuses on the low-to intermediate temperature kinetics of di-isobutylene. An upcoming part 2 discusses the high-temperature model development and validation of the relevant experimental targets. Ignition delay time measurements for di-isobutylene isomers were performed at pressures ranging from 15-30 bar at equivalence ratios of 0.5, 1.0, and 2.0 diluted in air and the temperature range 650-900 K using two independent rapid compression machine facilities. In addition, speciation experiments of these isomers were performed in a jet-stirred reactor and in a rapid compression machine. A detailed kinetic model for di-isobutylene isomers is developed to capture the wide range of new experimental targets. For the first time, a comprehensive low-temperature chemistry submodel has been included. The differences in the important reaction pathways for the accurate prediction of the oxidation of the two DIB isomers are compared using reaction path analysis. The most sensitive reactions controlling the ignition delay time of DIB isomers under pressure and temperature conditions necessary for autoignition in engines are identified.

International audience; The oxidation of n-heptane was performed in a jet-stirred reactor (JSR) at 10 atm, an equivalence ratio of 0.5, and an initial fuel concentration of 1000 ppm at a residence time of 1 s in the temperature range 580-790 K (from coolflame, negative temperature coefficient NTC, to intermediate temperature oxidation regime), and with 5000 ppm of fuel at 647 K and a residence time of 1.5 s. Low-temperature products formed in JSR were characterized using high-resolution mass spectrometry analyses (HRMS). Atmospheric pressure chemical ionizations (APCI) was used in positive and negative modes for MS analyses. Both flow injection analyses (FIA) or ultra-high-pressure liquid chromatography-Orbitrap® coupling were used to characterize hydroperoxides (C 7 H 16 O 2), keto-hydroperoxides (C 7 H 14 O 3), cyclic ethers (C 7 H 14 O), carboxylic acids (C 2 H 4 O 2 , C 3 H 6 O 2 , C 4 H 8 O 2), ketones (C 3-5 H 6-10 O), diones (C 7 H 12 O 2), and highly oxygenated molecules (C 7 H 14 O 5 , C 7 H 14 O 7 , C 7 H 14 O 9 , C 7 H 14 O 11) resulting from the addition of up to six O 2 molecules on fuel radicals. H/D exchange with D 2 O was used to confirm the presence of-OH or-OOH groups in the products. Several available kinetic reaction mechanisms were tested against the present measurements of keto-hydroperoxides showing significant discrepancies.

International audience

International audience; Previous studies on sampling probe effects in the premixed flat flame showed that the temperature in the preheat zone can be lowered down to low-temperature oxidation regime (e.g., 400–800 K). In order to investigate the contribution of the low-temperature chemistry in flame-sampling experiments, a stoichiometric laminar premixed flat flame of ethylene/oxygen/argon was investigated at 30 Torr using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) in this work. Ethyl hydroperoxide (C2H5OOH) was identified and quantified in the present experiment, providing an experimental evidence for the existence of low-temperature chemistry in flame-sampling experiments. In addition to C2H5OOH, the formation of several other intermediates like ethanol and formaldehyde was also influenced by the low-temperature chemistry in the present flame-sampling experiment. A literature kinetic model (Hashemi et al., 2017) was used for predictions and analyses. The difference between the predicted maximum mole fractions of C2H5OOH with the perturbed and unperturbed temperature profiles can reach up to five orders of magnitude. The great improvement of the predictions with the perturbed temperature profiles indicates that the observed low-temperature chemistry in the present flame sampling experiment originates from the probe-induced perturbations, which lowers down the temperature window of the preheat zone and leads to a temperature drop of more than 400 K compared with the unperturbed temperature profile. Through modeling analysis, the low-temperature oxidation chemistry of ethylene involved in the present flame-sampling experiment was discussed. The influence of low-temperature chemistry in the present experiment has also been demonstrated by comparing the model predictions with/without key reactions at low temperatures. It is concluded that predicted maximum mole fractions of several low-temperature chemistry related intermediates, i.e. C2H5OOH, ethanol and formaldehyde, are strongly reduced without these reactions, while low-temperature chemistry only has negligible influence on the predictions of the fuel and the majority of flame intermediates. Furthermore, preliminary experiments were also conducted in ethane, propene and n-butane flames under similar conditions, where hydroperoxides can also be observed.

International audience; In order to investigate the pressure-dependent effects of carbon chain growth from methyl radical, pyrolysis experiments of butane-2,3‑dione (also known as diacetyl) were conducted both in a flow reactor over 780–1520 K at low and atmospheric pressures and in a jet-stirred reactor (JSR) over 650–1130 K at 10 bar. Identification and quantification of the intermediates were achieved by synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor and Fourier transform infrared spectrometry and gas chromatography in a JSR. A pyrolysis model of butane-2,3‑dione, incorporating the sub-mechanisms of butane-2,3‑dione, ketene, C1–C4 hydrocarbons and the formation of benzene, was developed from our recently reported C1 model in this work. It was validated against the present pyrolysis data of butane-2,3‑dione and the pyrolysis data of C2–C4 hydrocarbon fuels and ketene in the literature. Generally, the present model could reasonably predict all the experimental targets. Based on the rate of production analysis, the unimolecular decomposition reaction (CH3COCOCH3 (+ M) = 2CH3CO (+ M)) is the dominant pathway to consume the fuel at low pressure. However, at atmospheric and high pressures, both the unimolecular decomposition reaction and the H abstraction reaction of butane-2,3‑dione by methyl radical are important. Ketene, acetone and acetaldehyde were measured in this work and their formation is also pressure dependent. For the methyl-related growth chemistry, addition-elimination reactions play an important role at low pressure. Their contribution becomes less at atmospheric pressure and can be neglected at high pressure. In contrast, the combination reactions are responsible for methyl-related growth at high pressure. For the formation of benzene, the recombination of propargyl radical is the most important pathway at low pressure, while the H-assisted isomerization reaction of fulvene becomes dominant at atmospheric and high pressures.

International audience; The combustion of fossil fuels, which are mostly based on hydrocarbon chains, produces large quanti ties of carbon dioxide which is a major contributor to the global warming. Due to alarming concerns, initiatives are taken to promote the use of ’zero-carbon fuel’. The use of ammonia as an alternative fuel in combustors like gas turbines and spark-ignition engines have already been tested. As high pressures and temperatures are encountered in these combustors, it is essential to find laminar flame speed data for ammonia combustion at these conditions. The present study focuses on performing experiments at elevated conditions for different equivalence ratios in a constant volume spherical chamber. A literature study was performed to select and evaluate the most recent kinetic mechanisms for ammonia combustion under these conditions. A sensitivity analysis highlighting the key reactions for two of the schemes has been performed. A range of exponential factors for temperature and pressure which is used to calculate the flame speeds for a given reference conditions : αand βhave been determined.

This work presents a comprehensive experimental and kinetic modeling study on the combustion of n -propylbenzene. Flow reactor pyrolysis of n -propylbenzene at 0.04, 0.2 and 1 atm and laminar premixed flames of n -propylbenzene at 0.04 atm with equivalence ratios of 0.75 and 1.00 were investigated with synchrotron vacuum ultraviolet photoionization mass spectrometry. Jet stirred reactor (JSR) oxidation of n -propylbenzene at 10 atm with equivalence ratios of 0.5, 1.0, 1.5 and 2.0 was investigated with gas chromatography. A detailed kinetic model for n -propylbenzene combustion with 340 species and 2069 reactions was developed and validated against the data measured in this work. Model analyses such as rate of production analysis and sensitivity analysis were also performed to reveal the key pathways in the consumption of fuel and formation of polycyclic aromatic hydrocarbons (PAHs). The analysis results demonstrate that the benzylic C C bond dissociation reaction is crucial for the decomposition of n -propylbenzene in the pyrolysis and rich flame. Low temperature oxidation reactions play important roles in the high pressure JSR oxidation of n -propylbenzene. In addition, the formation pathways of PAHs are strongly related to the fuel structure, especially for the formation of bicyclic PAHs such as indene and naphthalene. Furthermore, the present model was also validated against previous experimental data of n -propylbenzene combustion under a wide range of conditions, including ignition delay times, laminar flame speeds, extinction strain rates, speciation profiles in atmospheric pressure JSR oxidation, flow reactor oxidation and high pressure shock tube pyrolysis and oxidation.

International audience; This study concerns the oxidation of n-hexane. It was conducted in continuous flow fused-silica jet-stirred reactor (JSR) at 10 atm and an equivalence ratio of 0.5. n-Hexane initial concentrations were (i) 2500 ppm with a mean residence time of 1.5 s and (ii) 1000 ppm with a mean residence time of 0.7 s; we operated in the cool-flame regime for temperatures ranging from 540 to 720K and 530 to 800K, respectively. Products were analyzed and quantified in the gas phase using gas chromatography (with flame ionization, thermal conductivity, and quadrupole mass spectrometry) and Fourier transform infrared spectrometry. Products of low-temperature oxidation were sampled in the JSR and trapped in acetonitrile for characterization using an Orbitrap Q-Exactive®. Flow injection analyses (FIA) and ultra-high pressure liquid chromatography (UHPLC) coupled with atmospheric pressure chemical ionization (APCI +/-modes)-high resolution mass spectrometry (HRMS) analyses were used to characterize hydroperoxides (C6H14O2), keto-hydroperoxides (C6H12O3, C3H6O3, C4H8O3, and C5H10O3), cyclic ethers (C6H12O), carboxylic acids (C2 to C6), ketones (C3 to C6), diones (C6H10O2), unsaturated ketones (C6H10O and C6H8O), unsaturated diones (C6H8O2), and highly oxygenated molecules (C6H12O4-8) produced by addition of three and four oxygen molecules on fuel's radicals. To confirm the presence of hydroxyl or hydroperoxyl groups in the oxidation products we used H/D exchange with D2O. 2,4-Dinitrophenylhydrazine (2,4-DNPH) derivatization was used to characterize and confirm the presence of different carbonyls which can be formed during the low temperature oxidation of n-hexane. An available kinetic reaction mechanism including 3 rd O2 addition on fuel's radicals was used to simulate the formation of the presently detected keto-hydroperoxides (KHP) and highly oxygenated molecules (HOMs).

Periodicity in evolution of premixed methane–air V-shaped flames in the space domain is investigated experimentally. The experiments were performed using the Mie scattering and Particle Image Velocimetry techniques. Three Reynolds numbers of 510, 790, and 1057 along with two fuel–air equivalence ratios of 0.6 and 0.7 were tested in the experiments. The analyses were performed using the Proper Orthogonal Decomposition (POD) technique for the flame front position as well as the velocity data pertaining to non-reacting flow condition. The POD analysis shows that the spectral characteristics of the mode shapes associated with the velocity and the flame front position data feature similarities; however, the corresponding temporal coefficients are significantly different. Specifically, the POD mode shapes pertaining to both velocity and flame front position data feature dominant instabilities. It was shown that the normalized wave number pertaining to these instabilities are similar and equal to the Strouhal number corresponding to non-reacting flow over a circular cylinder. Comparison of the normalized temporal coefficients show that, for the flame front position data, the normalized first and second coefficients are mainly centered close to the origin; however, those associated with the velocity data are positioned around a unity radius circle. This was argued to be linked to the ratio of the corresponding first and second eigenvalues. Specifically, it was shown that, as this ratio approached to unity, the signal energy becomes distributed between the first and the second POD modes. As a result, the normalized temporal coefficients follow a circular pattern.

International audience; In a context of decreasing pollutant emissions, the transport sector has to tackle improvements to the engine concept as well as fuel diversification. The use of these different fuels often has an impact on the combustion performance itself. In the case of Spark Ignition (SI) engines, efficiency is a function of the combustion speed, i.e. the speed at which the fresh air–fuel mixture is consumed by the flame front. Every expanding flame is subject to flame curvature and strain rate, which both contribute to flame stretch. As each air–fuel mixture responds differently to flame stretch, this paper focuses on understanding the impact of flame stretch on fuel performances in SI engines. Different air–fuel mixtures (different fuels or equivalence ratios) with similar unstretched laminar burning speeds and thermodynamic properties but different responses to stretch were selected. The mixtures were studied in a turbulent spherical vessel and in an optical engine using Mie-Scattering tomography. The combustion phasing was also investigated in both optical and all-metal single cylinder engines. Results show that flame stretch sensitivity properties such as Markstein length and Lewis number, determined in laminar combustion conditions, are relevant parameters that need to be taken into consideration to predict the global performance of fuels, either experimentally or for modeling simulation.

© 2015 The Combustion Institute. Chain-branching reactions represent a general motif in chemistry, encountered in atmospheric chemistry, combustion, polymerization, and photochemistry; the nature and amount of radicals generated by chain-branching are decisive for the reaction progress, its energy signature, and the time towards its completion. In this study, experimental evidence for two new types of chain-branching reactions is presented, based upon detection of highly oxidized multifunctional molecules (HOM) formed during the gas-phase low-temperature oxidation of a branched alkane under conditions relevant to combustion. The oxidation of 2,5-dimethylhexane (DMH) in a jet-stirred reactor (JSR) was studied using synchrotron vacuum ultra-violet photoionization molecular beam mass spectrometry (SVUV-PI-MBMS). Specifically, species with four and five oxygen atoms were probed, having molecular formulas of C8H14O4(e.g., diketo-hydroperoxide/keto-hydroperoxy cyclic ether) and C8H16O5(e.g., keto-dihydroperoxide/dihydroperoxy cyclic ether), respectively. The formation of C8H16O5species involves alternative isomerization of OOQOOH radicals via intramolecular H-atom migration, followed by third O2addition, intramolecular isomerization, and OH release; C8H14O4species are proposed to result from subsequent reactions of C8H16O5species. The mechanistic pathways involving these species are related to those proposed as a source of low-volatility highly oxygenated species in Earth's troposphere. At the higher temperatures relevant to auto-ignition, they can result in a net increase of hydroxyl radical production, so these are additional radical chain-branching pathways for ignition. The results presented herein extend the conceptual basis of reaction mechanisms used to predict the reaction behavior of ignition, and have implications on atmospheric gas-phase chemistry and the oxidative stability of organic substances.

Journal article Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer-Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been simulated with a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The oxidation behavior of 2,5-dimethylhexane is compared with oxidation of other octane isomers to confirm the effects of branching on low and intermediate temperature fuel reactivity. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved. European Research Council - (FP7/2007-2013)/ERC Grant agreement n° 291049 − 2G-CSafe. peer-reviewed

International audience; The main objectives of this research consist in achieving both experimental and numerical studies of the combustion and oxidation of ethanol. Experimental mole fraction profiles of chemical species (stable, radical and intermediates) were measured in three C2H5OH/O2/Ar flat premixed flames stabilized at low pressure (50 mbar) and with equivalence ratios equal to 0.75, 1, and 1.25, respectively. The experimental setup used to determine the structure of one-dimensional laminar premixed flames consists of a molecular beam mass spectrometer system (MBMS) combined with electron impact ionization (EI). The oxidation of ethanol was also experimentally studied using a fused silica jet-stirred reactor (JSR). Experiments were performed in the temperature range 890-1250K, at 1 atm, at four equivalence ratios equal to 0.25, 0.5, 1 and 2 and with an initial fuel concentration of 0.2 mol %. A kinetic study was conducted in order to simulate all experimental data measured. It enabled building a kinetic mechanism by thoroughly reviewing the available literature and by taking into account specificities of the two kinds of experiments performed. Validity of the mechanism was also checked against experimental results previously published (ethanol oxidation in a JSR at 10 atm, ignition in a shock-tube, combustion in premixed, partially-premixed and non-premixed flames)). This mechanism ensures a reasonably good modelling of the combustion and oxidation of ethanol in the wide range of experimental conditions investigated.

The burning rate in the vicinity of a piston is estimated from a fractal analysis. The fractal parameters are determined from laser sheet tomography flame images for methane–air mixtures with three equivalence ratios (1, 0.9, 0.8) in a transparent spark-ignition engine. Two imaging configurations were used: five horizontal planes placed at different distances from the piston (0, 1, 2, 3, and 5 mm) and a vertical one passed through the center of the combustion chamber. The methodology proposed by Foucher et al. [F. Foucher, S. Burnel, C. Mounaim-Rousselle, Proc. Combust. Inst. 29 (2002) 751–757] allows the effect of cyclic variations to be avoided. The fractal formulation is modified to take into account the flame–piston distance and flame quenching. Far from the piston, evolution of the fractal dimension versus q ′ / S L 0 is found to be in good agreement with literature results. Near the piston, the fractal dimension evolves significantly when the distance is about twice the integral length scale and tends toward 2, the fractal dimension of a laminar flame front. The quenching ratio parameter Q R is introduced to consider the quenching of the flame by the piston. Finally, the burning rate is determined as a function of the distance between the wall and the mean flame contour and compared to a flame density approach, and similar results are found.

Ignition delays have been investigated for mixtures of monomethylhydrazine (MMH) + O 2 + Ar at high temperatures (850–1440 K) in the pressure range 160–400 kPa, for equivalence ratios of 1–4.3 and in the Ar dilution range 82–96 mol%. The replacement of argon by nitrogen leads to a twofold increase of the delay. The addition of 0.88–2.4 mol% hydrogen to the MMH + O 2 + Ar mixture shows that hydrogen has, globally, a promoting effect on the ignition in MMH + O 2 + (Ar). Moreover, it is shown, with the help of a kinetic model that the addition of MMH by forming radicals during its oxidative decomposition promotes hydrogen oxidation in the temperature range 900–1000 K.

International audience; This work presents a comparative study on the pyrolysis of C 8-C 10 linear alkylbenzenes including ethyl-benzene, n-propylbenzene and n-butylbenzene. Experiments were performed with highly diluted mixtures in argon containing respectively the three fuels under nearly identical conditions in a single-pulse shock tube, at a nominal pressure of 20 bar and over a temperature range of 950-1700 K. Post-shock gas mixtures were sampled and analyzed with the gas chromatographic technique so that species concentration evolutions as function of temperature were obtained for the pyrolysis of each fuel. A kinetic model was developed to interpret the similarities and differences regarding the fuel decomposition and species formation behaviors observed in the experiments. The fuel conversion of n-propylbenzene and n-butylbenzene proceeds along a similar curve, which is much faster than that of ethylbenzene. All three fuels are consumed mainly through the bond fission producing benzyl radical. The simultaneously formed C 1-C 3 alkyl radicals in separate cases significantly impact the fuel reactivity and the formation of small C 1-C 4 and monocyclic aromatic hydrocarbons. Specifically, in n-propylbenzene pyrolysis, the decomposition of ethyl radicals produces a considerable amount of hydrogen atoms, which enhances the reactivity of the reaction system and meanwhile results in relatively high production of benzene during the fuel consumption. The formation of other monocyclic aromatic hydrocarbon intermediates, such as toluene and styrene, is also influenced by fuel-related pathways. Concerning PAH formation, the kinetic schemes in the pyrolysis of linear C 8-C 10 alkylbenzenes are very similar, which are dominated by the reactions of benzyl and other resonantly-stabilized radicals produced from benzyl decomposition. The major PAH formation reactions are barely influenced by the fuel chemistry. The only notable fuel-specific pathway is the indene formation from 1-phenyl-2-propenyl in n-propylbenzene and n-butylbenzene pyrolysis at relatively low temperatures. Styrene is an abundant product and its reaction with phenyl is found to be an important channel of phenanthrene formation.

International audience; A combined experimental and kinetic modeling study is carried out to explore the influences of added acetylene, ethylene, propylene, and propyne on the reaction schemes of benzene pyrolysis. Pyrolysis of benzene with and without the presence of the C 2-or C 3-unsaturated hydrocarbons (HCs) is conducted in a single pulse shock tube coupled to gas chromatography-mass spectrometry technique. A minimum of 30 species is detected in each reaction system, and their mole fraction profiles are obtained over the temperature range 1030-1800 K at a nominal pressure of 20 bar and a nominal reaction time of 4 ms. With updates based on recent theoretical studies, our ongoing detailed kinetic model with 552 species and 4958 reactions can successfully reproduce the decomposition reactivity of the fuels, formation of decomposition products, and the growth of aromatics in the pyrolysis of different fuel mixtures. Various considerations apply to all studied binary mixtures. The addition of C 2-and C 3-HCs to the reaction system leads to C 2 H 2 formation, and consequently promotes the HACA mechanism starting from phenyl radical (C 6 H 5) at relatively low temperatures. The resulted phenylacetylene, formed through C 6 H 5 + C 2 H 2 reaction, promotes the addition-elimination reaction C 6 H 5 C 2 H + C 6 H 5 leading to the enhanced and early formation of all C 14 H 10 PAH isomers including ethynyl biphenyl, methylene-fluorene, diphenylacetylene, phenanthrene, and anthracene. It is also noteworthy that the existence of C 2 H 2 as fuel or its production from C 2 H 4 , C 3 H 6 , and C 3 H 4-P decomposition results in numerous compounds with ethynyl branches such as ethynyl biphenyl, diethynyl naphthalene, and ethynyl acenaphthylene. Considering the specific features of the different fuel mixtures, the addition of acetylene intensifies the HACA route leading to greater acenaphthylene formation, while naphthalene formation remains similar to the pure benzene case due to the limited H-atoms. Differently, in benzene-C 2 H 4 co-pyrolysis, naphthalene and acenaphthylene are mainly formed through reactions between PAH radicals (phenylacetylene radical and naphthyl radical, respectively) and ethylene. Concerning benzene-C 3 co-pyrolysis, indene is the major C 9 species resulting from the reaction of benzene/phenyl with C 3 fuels. This is an important theoretical pathway to indene which is experimentally probed here for the first time. The high concentration of indene leads to the enhanced formation of naphthalene and acenaphthylene through the reactions of indenyl radical with methyl and propargyl (C 3 H 3) radicals, respectively. Afterwards, the naphthyl radicals participate in the formation of several larger C11-13 PAHs such as methylnaphthalene, ethynylnaphthalene and fluorene through their reactions with CH 3 , C 2 H 2 and C 3 H 4-P/C 3 H 3 , respectively.