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International audience; To explore the potential interactions between toluene/benzyl and the common C 3 combustion intermediates, toluene-propylene and toluene-propyne co-pyrolysis is studied in the current work by taking neat toluene pyrolysis as a reference. Experiments are carried out at a nominal pressure of 20 bar over a temperature range of 1050 −1700 K, using a single-pulse shock tube facility coupled to the gas chromatography-mass spectrometry speciation diagnostic technique. Temperature-dependent mole fraction profiles are obtained for numerous species ranging from small-molecule products to three-ring polycyclic aromatic hydrocarbons (PAHs). A kinetic model, which has been under development in our serial works, is extended by including the interplays between toluene/benzyl and propylene/propyne chemistry. The updated model can satisfactorily predict the measurements, regarding the absolute mole fractions as well as the variation trends brought by different initial fuel compositions. Increased reactivity is observed in the conversion of toluene with the presence of propylene or propyne, while the consumption rates of the studied C 3 fuels are barely influenced by toluene. Benzene formation is facilitated by the C 3 + C 3 reactions introduced by the C 3 fuels. The pyrolysis of propylene (or propyne) significantly enhances the level of C 1 −C 3 molecules/radicals that further react with aromatic species. For instance, the reactions of benzyl + propyne result in much higher mole fractions and lower speciation temperature windows of indene and naphthalene in toluene-propylene (or propyne) co-pyrolysis. The reactions with small hydrocarbons result in reduced levels of benzyl and other C 7 radicals in the reaction system in toluene-propylene (or propyne) co-pyrolysis. Consequently, for the PAHs which are mainly formed through C 7 + C 7 reactions, such as bibenzyl and phenanthrene, the mole fractions are lowered by the addition of propylene (or propyne). Propyne has more obvious influences on the species pool of toluene pyrolysis than propylene, because the effective C 7-C 3 interactions are mostly through the reactions between toluene/benzyl and propyne/propargyl in both cases of toluene-propylene and toluene-propyne co-pyrolysis.

Made available in DSpace on 2019-10-04T11:56:28Z (GMT). No. of bitstreams: 0 Previous issue date: 2019-01-01 Solar Turbines Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Flame acceleration experiments were performed in a 10cm inner-diameter tube filled with evenly spaced 0.43 blockage ratio orifice plates. The critical mixture composition required for flame acceleration to a fast-flame was measured for four methane/hydrogen fuel-air mixtures at initial temperatures of 298 K, 423 K, and 573 K. These conditions provide a large range in the Zeldovich number between 12 and 28, where the Zeldovich number was calculated from the laminar burning velocity obtained from 1-D flame simulations. The data collapsed very well when the expansion ratio across the flame (calculated at the critical condition) was plotted versus the Zeldovich number. This is consistent with correlation proposed by Dorofeev [7], that was based on experimental data obtained over a narrower Zeldovich number range. For pure hydrogen fuel, the critical expansion ratio was found to be between 2 and 4, and for pure methane the critical expansion ratio was as high as 8, for an initial temperature of 573 K. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Queens Univ, Kingston, ON, Canada CNRS, INSIS, ICARE, Paris, France Sao Paulo State Univ, Sao Paulo, Brazil Sao Paulo State Univ, Sao Paulo, Brazil FAPESP: 2015/23351-9 FAPESP: 2015/25435-5

The possibility to operate current diesel engines in dual-fuel mode with the addition of hydrogen can be limited by the variation in the combustion properties of the fuel mixture. In the present work, n-heptane was selected as a representative fuel to test the effects of hydrogen addition on the laminar flame speeds and ignition delay times. The spherical bomb technique was used to derive the laminar flame speeds of (n-heptane + hydrogen)/air mixtures (0%, 25%, and 50% hydrogen in the fuel) for an initial temperature of 294 K, pressure of 1 bar, and for equivalence ratios between 0.8 and 1.35. The results showed that average increases of 3% and 10% in the flame speeds were obtained with 25% and 50% hydrogen-enrichment, respectively, while a slight decrease of the Markstein length was obtained. Similar laminar flame speed results were predicted numerically with two kinetic models available in the literature with remarkable accuracy, especially for the Cai and Pitsch model [Cai L, Pitsch H. Combust Flame 2015; 162:1623–37]. The kinetic model was subsequently used to perform additional sensitivity and reaction pathway analyses that showed how the chemistry of n-heptane is not substantially influenced by the presence of hydrogen; while the increase in the flame speed is mainly due to the higher concentrations of radical intermediates. The ignition delay times were measured using the reflected shock tube technique for equivalence ratios equal to 0.832, 1.000, and 1.248, initial nominal pressure of 20 bar, temperatures between 730 K and 1200 K, and for different percentages of hydrogen in the fuel (20%, 50%, and 75%). The Cai and Pitsch model once again did a good job of reproducing the experimental data, indicating how at high temperatures the addition of hydrogen does not significantly affect the ignition delay; and in the NTC region (810 K–920 K) the mixtures composed of (50% n-heptane + 50% hydrogen) and (25% n-heptane + 75% hydrogen) are considerably slower than the reference n-heptane case. This is linked to the concentration of the alkane component and the related low temperature chemistry.

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 gasoline and gasoline/alcohol blends is studied in a shock tube and a spherical reactor. A commercial oxy-free gasoline and two alcohols (ethanol and iso-pentanol) were used in this study. The spherical reactor experiments were conducted at an initial temperature of 483 K, initial pressure of 0.1 MPa, and equivalence ratios from 0.65 to 1.36. Ignition delay times were measured behind reflected shock waves. The shock tube experiments were conducted at 2 MPa over a temperature range from 955 to 1284 K and for two equivalence ratios (0.5 and 1). The experimental measurements indicate that replacing ethanol by iso-pentanol on a gasoline/alcohol blend results in flame speeds which are closer to the ones of a commercial gasoline at nearly stoichiometric conditions (an in-engine applications). On the other hand, the ignition delay times are more affected by the presence of iso-pentanol than the ethanol case. Two different surrogate fuels composed of n-heptane, iso-octane, and toluene were also tested against the newly obtained experimental results (oxy-free gasoline and mixtures with ethanol) using kinetic modeling with a reduced model (Cai, L.; Pitsch, H. Combust. Flame 2015 162, 1623-1637). Although a good agreement between real fuel and surrogate properties was observed for the laminar flame speeds, discrepancies were obtained between the measured and calculated ignition delay times especially in the lower temperature range of our study. Additional simulation analyses on the ignition delay times were performed with a different chemical kinetic model (detailed LLNL model for gasoline surrogates) and a more complex four-component surrogate. The results show a considerable improvement in the prediction capabilities of the ignition properties of the oxy-free gasoline and the oxy-free gasoline + ethanol mixtures. The ignition delay time data are also in agreement with the correlations provided in the literature for gasoline fuels and their surrogates.

International audience; Dimethyl Carbonate (DMC) is a carbonate ester that can be produced in an environmentfriendly way from methanol and CO2. DMC is one of the main components of the flammable electrolyte used in Li-ion batteries, and it can also be used as a diesel fuel additive. Studying the combustion chemistry of DMC can therefore improve the use of biofuels and help developing safer Li-ion batteries. The combustion chemistry of DMC has been investigated in a limited number of studies. The aim of this study was to complement the scarce data available for DMC combustion in the literature. Laminar flame speeds at 318 K, 363 K, and 463 K were measured for various equivalence ratios (ranging from 0.7 to 1.5) in a spherical vessel, greatly extending the range of conditions investigated. Shock tubes were used to measure time histories of CO and H2O using tunable laser absorption for the first time for DMC. Characteristic reaction times were also measured through OH* emission. Shock-tube spectroscopic measurements were performed under dilute conditions, at three equivalence ratios (fuel-lean, stoichiometric, and fuel-rich) between 1260 and 1660 K near 1.3±0.2 atm, and under pyrolysis conditions (98%+) ranging from 1230 to 2500 K near 1.3±0.2 atm. Laminar flame speed experiments were performed around atmospheric pressure. Detailed kinetics models from the literature were compared to the data, and it was found that none are capable of predicting the data over the entire range of conditions investigated. A numerical analysis was performed with the most accurate model, underlining the need to revisit at least 3 key reactions involving DMC.

International audience; Pyrolysis of phenylacetylene with and without the presence of C 2 hydrocarbons (acetylene or ethylene) was studied in a single-pulse shock tube coupled to gas chromatography/gas chromatography-mass spec-trometry equipment for speciation diagnostics. Quantitative speciation profiles were probed from each reaction system over the temperature range of 110 0-170 0 K at a nominal pressure of 20 bar. A kinetic model was proposed to interpret how phenylacetylene is consumed under high-pressure pyrolytic conditions and how the resulting intermediates react to form polycyclic aromatic hydrocarbons (PAHs), and furthermore , how the extra acetylene or ethylene alter the reaction schemes. It was found that the bimolec-ular reaction between phenylacetylene and hydrogen atom leading to phenyl and acetylene dominates phenylacetylene decomposition throughout the temperature window. The addition/elimination reactions between phenylacetylene and phenyl not only produce hydrogen atoms to maintain the reactivity of the fuel decay, but also directly lead to the formation of several C 14 H 10 PAH isomers including diphenylacety-lene, 9-methylene-fluorene and phenanthrene. Intermediates pools, regarding both species categories and abundance, are changed by the two C 2 fuels introduced into the reaction system. The added acetylene enables the Hydrogen-Abstraction-Acetylene-Addition (HACA) mechanism starting from the phenylacetylene radical to occur at relatively low temperatures. But the yielded naphthyl core does not stabilize in naph-thalene due to the lack of hydrogen atoms in the reaction system, and instead, it carries on the HACA route by further combining with another acetylene molecule, ending up in acenaphthylene. Differently, the added ethylene intensifies the HACA routes by contributing to the acetylene formation, and more importantly, provides hydrogen atoms participating in the naphthalene formation from naphthyl radical.

International audience; Benzene and toluene were pyrolyzed under highly argon-diluted conditions at a nominal pressure of 20 bar in a single-pulse shock tube coupled to gas chromatography/gas chromatography-mass spectrome-try (GC/GC-MS) diagnostics. Concentration evolutions of polycyclic aromatic hydrocarbon (PAH) inter-mediates were measured in a temperature range of 1100-1800 K by analyzing the post-shock gas mixtures. Different PAH speciation behaviors, regarding types, concentrations and formation temperature windows, were observed in the two reaction systems. A kinetic model was proposed to predict and interpret the measurements. Through a combination of experimental and modeling efforts, PAH formation patterns from species pools of benzene and toluene pyrolysis were illustrated. In both cases, channels leading to PAHs basically originate from the respective fuel radicals, phenyl and benzyl. Due to the higher thermal stability of benzene, the production of phenyl, and thus most PAH species, occur in higher temperature windows, in comparison to the case of toluene. In benzene pyrolysis, benzyne participates in the formation of crucial PAH species such as naphthalene and acenaphthylene. Phenyl self-recombination takes considerable carbon flux into biphenyl, which serves as an important intermediate leading to acenaphthylene through hydrogen loss and ring closure. The resonantly-stabilized benzyl is abundant in toluene pyrolysis, and its decomposition further produces other resonantly-stabilized radicals such as fulvenallenyl and propargyl. Barrierless addition reactions among these radicals are found to be important sources of PAHs. Fuel-specific pathways have pronounced effects on PAH speciation behaviors, particularly at lower temperatures where fuel depletion is not completed within the reaction time of 4.0 ms. Contributions from the commonly existing Hydrogen-Abstraction-Carbon-Addition (HACA) routes increase with the temperature in both cases.

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 aim of this paper is to report new experimental results on the effect of turbulence on the combustion properties of lean to stoichiometric H 2 /air mixtures in a closed, fan-stirred, spherical vessel. To do so, a new experimental setup, spherical bomb, has been used to investigate the effect of a given and well-characterized turbulence intensity on the increase of hydrogen/air flame speed and on the combustion pressure. The initial hydrogen molar percent of H 2 in air was comprised between 16% and 28%. The initial turbulence created in the vessel prior to ignition was varied between 0.56 and 2.81 m/s for an integral length scale of around 50 mm. It was shown that the turbulent speed increases drastically when the turbulence is increased but the maximum combustion pressure remains the same. A correlation was proposed in order to the turbulent speed. The proposed turbulent flame speed correlation was able to predict not only the present data but also the results of the literature (Kitagawa et al., 2008).

The oxidation of styrene, one of the main stable intermediates from the oxidation of large alkylated aromatic hydrocarbons, has been investigated in the present work both experimentally and numerically. Experiments were performed using two complementary techniques, spherical bomb for laminar flame speed studies and shock tube for ignition delay time measurements. In particular, the laminar flame speeds of styrene/air mixtures were measured at three different initial temperatures (342 K, 373 K, and 405 K), over a wide range of equivalence ratios (0.75–1.45), for an initial pressure of 100 kPa. In addition, the autoignition of styrene/O2 mixtures in argon bath gas (ϕ = 0.5, 1.0, and 1.5) was investigated over a wide range of temperatures (1390–1990 K), at highly diluted conditions (94.3%–99% argon), and for pressures between 110 and 200 kPa. A detailed chemical kinetic model, based on the toluene chemistry by Metcalfe et al. (2011), was developed and validated against the newly obtained experimental results and the flow reactor data available in the literature (Litzinger et al., 1986). Sensitivity and rate of production analyses were performed and showed that, at the conditions studied herein for the flame speed investigation, the main fuel consumption pathways include the reaction of the fuel with H atoms to form phenyl radical and ethylene or benzene and vinyl radical, with O to form benzyl radical + HCO, and the H-abstraction reactions on both the vinyl moiety and the ring. On the other hand, the analyses performed at the high-temperature, highly-diluted conditions encountered in the shock tube study highlighted the importance of the fuel thermal decomposition steps for the simulation of the ignition delay time measurements. The model was also tested against the low-pressure flame and jet stirred reactor data by Yuan et al. (2015). The results highlight the need for future modifications in the benzene chemistry by Metcalfe et al. and inclusion of pressure dependent rate parameters in order to improve the prediction capabilities of the model.

International audience; The knock resistance of gasoline is a key factor to decrease the specific fuel consumption and CO2 emissions of modern turbocharged spark ignition engines. For this purpose, high RON and octane sensitivity (S) are needed. This study shows a relevant synergistic effect on RON and S when formulating a fuel with isooctane, cyclopentane and aromatics, the mixtures reaching RON levels well beyond the ones of individual components. The same is observed when measuring their knock resistance on a boosted single cylinder engine.

The present work reports new experimental and numerical results of the combustion properties of hydrogen based mixtures diluted by nitrogen and steam. Spherical expanding flames have been studied in a spherical bomb over a large domain of equivalence ratios, initial temperatures and dilutions at an initial pressure of 100 kPa (Tini = 296, 363, 413 K; N2/O2 = 3.76, 5.67, 9; %Steam = 0, 20, 30). From these experiments, the laminar flame speed S L 0 , the Markstein length L’, the activation energy Ea and the Zel'dovich β number have been determined. These parameters were also simulated using COSILAB® in order to verify the validity of the Mevel et al. [1] detailed kinetic mechanism. Other parameters as the laminar flame thickness δ and the effective Lewis number Leeff were also simulated. These new results aim at providing an extended database that will be very useful in the hydrogen combustion hazard assessment for nuclear reactor power plant new design.

In the present investigation new experimental data on laminar flame speeds of n-decane, n-butylbenzene, and n-propylcyclohexane single- and multi-component mixtures are presented. The experiments have been conducted in a spherical bomb heated to 403 K and at an initial pressure of 1 bar. The comparison between the results indicates that the flame speeds of n-decane and n-butylbenzene are similar at lean conditions diverging for stoichiometric and rich conditions while the opposite was observed for n-decane and n-propylcyclohexane. The flame speeds of the multi-component mixtures are influenced by the corresponding components for most of the conditions considered, although on the rich side the experimental curves seem to be mainly affected by the compound with the fastest propagation speed. The experimental results were also used to validate a comprehensive kinetic model which extends the chemistry of the JetSurF 2.0 model to include the aromatic compounds up to n-butylbenzene. The simulations well reproduce the experimental measurements over a wide range of conditions. Additional sensitivity and rate of production analyses were performed to clarify how the specific structures of the fuels (one linear alkane, one cyclic alkane, and one aromatic) influence the formation of several intermediate compounds relevant to the flame propagation properties.

The recent theoretical and experimental investigations on the growth of polycyclic aromatic hydrocarbons in pyrolytic environments highlight the possible role of the 1,4-cycloaddition/fragmentation (1,4-CAF) steps in the formation of PAH intermediates and consequently soot. The present theoretical study explores the possibility to generalize such mechanism to reactions involving various diradical compounds and stable multiring structures. The calculations were performed using the uB3LYP/6-311G(d,p) method and different composite methods, when possible, for more accurate energy estimates. First, the complex potential energy surface for the reactions between o-benzyne and naphthalene was investigated, including the 1,4-CAF mechanism to form anthracene and acetylene through the dibenzobicyclo[2.2.2]octatriene intermediate. Moreover, the products of the addition reactions to the α- and β-carbons and to the ring-junction atoms were determined. The energies for the optimized CAF structures, which constitute the most-favorable pathway from an energetic point of view, were calculated using CBS-QB3, G3(MP2)B3, and G3B3 methods and compared to the corresponding values for the o-benzyne + benzene reactions. Additional calculations were focused on the possible CAF reactions between o-benzyne and larger multiring structures, such as anthracene, phenanthrene, pyrene, and the four-ring PAHs. The results indicate how the energetics of such reactions is influenced by both the size of the PAH compound and the position of the carbon atoms involved. In the second part of the study, the energy barriers necessary to form multiring diradicals from the corresponding radical molecules were analyzed at a G3(MP2)B3 level of theory. Such calculations are preliminary for the subsequent study on the CAF reactions between the different diradical intermediates and benzene. While the size of the diradical does not affect significantly the energy barriers, the position of the diradical site is critical. The concerted Diels-Alder reactions between the naphthynes and naphthalene were also studied in order to further clarify the analogies between the reactions involving different diradicals. Based on these results, kinetic considerations were provided based on the comparison with the simpler o-benzyne + benzene system, although further higher-level calculations and master equation kinetic analyses will be required to derive the general kinetic rules.

In the present work, the autoignition of single-component and binary mixtures composed of n-decane, n-butylbenzene, and/or n-propylcyclohexane has been investigated using shock-tube techniques. In addition, the effects of the presence of the exhaust gas recirculation (EGR) components on the autoignition behavior of a 4:3:3 molar n-decane: n-butylbenzene:n-propylcyclohexane surrogate mixture have been investigated experimentally. The experiments have been performed at highly diluted conditions in argon bath gas, over a wide range of temperatures (1250–1750 K), equivalence ratios (Φ = 0.2–1.5), and nominal pressures of 10–20 bar. A chemical kinetic model was developed to simulate the newly obtained experimental data by blending sub-models from the literature. In particular, the experimental and numerical analyses suggest that the observed increase in the ignition delay times in the presence of EGR is mainly due to the dilution levels and not to the chemistry of the fuel. Additional kinetic analyses were performed to compare the high-temperature autoignition properties of the different components considered herein.

Fuel surrogates are mixtures of a few single aliphatic and aromatic compounds representative of the main classes of hydrocarbons present in the corresponding fuels. Among these hydrocarbon classes, the cycloalkanes constitute one of the key components in several surrogate formulations. In the present investigation, the oxidation of cyclohexane and decalin has been studied based on new experimental results. The experiments were obtained using two different techniques. A heated shock tube was used to conduct ignition delay time measurements at different stoichiometric conditions, nominal pressure of around 10 bar, and temperatures between 1090 and 1860 K. Ignition delay times in the range between 10 and 1300 μs were measured from both the OH* and the CH* emission signals behind reflected shock waves for highly diluted mixtures (99% argon bath gas). Experiments were also conducted varying the dilution from 99.5% to 92.7% for specific stoichiometric conditions. In addition to the ignition studies, flame speed...

High pressure iso-octane shock tube experiments were conducted to assist in the development of a Jet A surrogate kinetic model. Jet A is a kerosene based jet fuel composed of hundreds of hydrocarbons consisting of paraffins, olefins, aromatics and naphthenes. In the formulation of the surrogate mixture, iso-octane represents the branched paraffin class of hydrocarbons present in aviation fuels like Jet A. The experimental work on iso-octane was performed in a heated high pressure single pulse shock tube. The mole fractions of the stable species were determined using gas chromatography and mass spectroscopy. Experimental data on iso-octane oxidation and pyrolysis were obtained for temperatures from 835 to 1757 K, pressures from 21 to 65 atm, reactions times from 1.11 to 3.66 ms, and equivalence ratios from 0.52 to 1.68, and ∞. Iso-octane oxidation showed that the fuel decays through thermally driven oxygen free decomposition at conditions studied. This observation prompted an experimental and modeling study of iso-octane pyrolysis using an iso-octane sub-model taken from a recently published n-decane/iso-octane/toluene surrogate model. The revised iso-octane sub-model showed improvements in predicting intermediate species profiles from pyrolytic experiments and oxidation experiments. The modifications to the iso-octane sub-model also contributed to better agreement in predicting the formation of carbon monoxide and carbon dioxide when compared to the recently published 1st Generation Surrogate model and a recently published iso-octane oxidation model. Model improvements were also seen in predicting species profiles from flow reactor oxidation experiments and ignition delay times at temperatures above 1000 K at both 10 and 50 atm.

Shock-tube studies at the National Institute of Standards and Technology (NIST) and the University of Illinois at Chicago (UIC) have been used to examine the decomposition of 1-pentyl radicals in argon between (833–1130) K and (100–5000) kPa. High pressure limiting values of the product branching ratios appear to be approached at the highest pressures studied. Results agree well with a ‘best-fit’ model previously developed [1] and are consistent with an energy transfer value 〈ΔEdown〉 = (675 ± 100) cm−1 at 1000 K.

The paper illustrates the methodology developed for unsupervised analysis to be conducted on high-definition, high sampling rate image sequences collected in experiments with a single spark ignition optically accessible spherical bomb. Images recorded along the line-of-sight were first processed to identify the reaction front, and then analyzed by means of a two-dimensional numerical estimation technique. The laminar flame front is detected by making use of the concept of “scalar dissipation rate” basing on flame luminosity data, i.e. the square of the gradient of flame luminosity. The new scalar field is then tracked to derive the time history of the flame radius. In order to extract the Region Of Interest from the images, masking techniques are employed, whereas signal-to-noise ratio is improved by means of data binning. The proposed automatic, non-intrusive method proves effective in providing a fast characterization of the flame propagation phenomenon in terms of apparent velocity.

Long chain alcohols such as 1- and iso- pentanol are foreseen as a suitable replacement for ethanol, due to more favorable physical properties (higher energy density, higher boiling point and lower hygroscopicity). The present study presents high accuracy laminar flame speed measurements for iso- pentanol/air and 1-pentanol/air mixtures, at initial temperatures of 353 K, 433 K and 473 K, 1 bar pressure and equivalence ratios ranging from 0.7 to 1.5. Comparisons with previous measurements from the literature are also presented and the observed deviations are discussed in detail. The updated kinetic mechanism for alcohols combustion from the CRECK group at Politecnico di Milano is discussed and used for modeling purposes. For a more complete validation of the oxidation mechanism at high temperature conditions, modeling results are also compared with shock tube ignition delay times from the literature. This study extends the presently sparse and uncertain experimental database for high molecular weight alcohols oxidation in laminar flames, providing high accuracy and reliable experimental data of use for alcohols oxidation mechanism development and improvement.

An experimental investigation of phenyl radical pyrolysis and the phenyl radical + acetylene reaction has been performed to clarify the role of different reaction mechanisms involved in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) serving as precursors for soot formation. Experiments were conducted using GC/GC-MS diagnostics coupled to the high-pressure single-pulse shock tube present at the University of Illinois at Chicago. For the first time, comprehensive speciation of the major stable products, including small hydrocarbons and large PAH intermediates, was obtained over a wide range of pressures (25-60 atm) and temperatures (900-1800 K) which encompass the typical conditions in modern combustion devices. The experimental results were used to validate a comprehensive chemical kinetic model which provides relevant information on the chemistry associated with the formation of PAH compounds. In particular, the modeling results indicate that the o-benzyne chemistry is a key factor in the formation of multi-ring intermediates in phenyl radical pyrolysis. On the other hand, the PAHs from the phenyl + acetylene reaction are formed mainly through recombination between single-ring aromatics and through the hydrogen abstraction/acetylene addition mechanism. Polymerization is the common dominant process at high temperature conditions.

The experimental investigations performed in the 1960s on the o-benzyne + benzene reaction as well as the more recent studies on reactions involving π-electrons highlight the importance of π-bonding for different combustion processes related to PAH's and soot formation. In the present investigation radical/π-bond addition reactions between single-ring aromatic compounds have been proposed and computationally investigated as possible pathways for the formation of two-ring fused compounds, such as naphthalene, which serve as precursors to soot formation. The computationally generated optimized structures for the stationary points were obtained with uB3LYP/6-311+G(d,p) calculations, while the energies of the optimized complexes were refined using the uCCSD(T) method and the cc-pVDZ basis set. The computations have addressed the relevance of a number of radical/π-bond addition reactions including the singlet benzene + o-benzyne reaction, which leads to formation of naphthalene and acetylene through fragmentation of the benzobicyclo[2,2,2]octatriene intermediate. For this reaction, the high-pressure limit rate constants for the individual elementary reactions involved in the overall process were evaluated using transition state theory analysis. Other radical/π-bond addition reactions studied were between benzene and triplet o-benzyne, between benzene and phenyl radical, and between phenyl radicals, for all of which potential energy surfaces were produced. On the basis of the results of these reaction studies, it was found necessary to propose and subsequently confirm additional, alternative pathways for the formation of the types of PAH compounds found in combustion systems. The potential energy surface for one reaction in particular, the phenyl + phenyl addition, is shown to contain a low-energy channel leading to formation of naphthalene that is energetically comparable to the other examined conventional pathways leading to formation of biphenyl compounds. This channel is the first evidence of a reaction which involves an aromatic radical adding to the nonradical π-bond site of another aromatic radical which leads directly to a fused ring structure.

The oxidation of m-xylene has been studied behind reflected shock waves using the single pulse shock tube at University of Illinois at Chicago. Experiments were performed at nominal high pressures of 25 and 50 atm and for a temperature range of 1024–1632 K, at fuel lean, stoichiometric and fuel rich conditions (Φ = 0.53, 1, 2.35). A variety of stable species ranging from small hydrocarbons, to single ring and polycyclic aromatic hydrocarbons (such as naphthalene, anthracene and acenaphthylene) were sampled from the shock tube and analyzed using standard gas chromatographic techniques. Increased amounts of PAH’s were measured for experiments at fuel rich conditions (Φ = 2.35) when compared to experiments at stoichiometric and fuel lean conditions. A detailed chemical kinetic model was developed to simulate the stable species profiles up to the formation of single ring aromatic hydrocarbons from the current high pressure oxidation experiments. The model provides a good fit for the consumption of the fuel, oxygen and the formation of the major intermediates. The model predicts lower but satisfactory consumption of the minor intermediates. The model provides a satisfactory base for further development, which includes the addition of chemistry leading to multi-ring aromatic hydrocarbons from m-xylene.

The high pressure oxidation of dilute CO mixtures doped with 150–200 ppm of H 2 has been studied behind reflected shock waves in the UIC high pressure single pulse shock tube. The experiments were performed over the temperature range from 1000 to 1500 K and pressures spanning 21–500 bars for stoichiometric ( Φ = 1) and fuel lean ( Φ = 0.5) oxidation. Stable species sampled from the shock tube were analyzed by standard GC, GC/MS techniques. The experimental data obtained in this work were simulated using a detailed model for H 2 /CO combustion that was validated against a variety of experimental observables/targets that span a wide range of conditions. These simulations have shown that within experimental error the model is able to capture the experimental trends for the lower pressure data sets (average nominal pressures of 24 and 43 bars). However the model under predicts the CO and O 2 decay and subsequent CO 2 formation for the higher pressure data sets (average nominal pressures of 256 and 450 bars). The current elevated pressure data sets span a previously unmapped regime and have served to probe HO 2 radical reactions which appear to be among the most sensitive reactions in the model under these conditions. With updated rate parameters for a key HO 2 radical reaction OH + HO 2 = H 2 O + O 2 , the model is able to reconcile the elevated pressure data sets thereby extending its capability to an extreme range of conditions.

Recent theoretical investigations of the radical/π-bond addition between single-ring aromatic hydrocarbons highlight the importance of this category of reactions for the formation of PAH intermediates and soot. The present investigation extends the theory of the radical/π-bond addition reactions to the o-benzyne + cyclic C(5) hydrocarbons systems. The calculations, performed using the uB3LYP/6-311+G(d,p) method, have addressed the possible role of the reaction between o-benzyne and cyclopentadiene in the formation of indene through the fragmentation of the bicyclo intermediate benzonorbornadiene. The complex potential energy surface for the reaction between o-benzyne and cyclopentadienyl radical was also investigated. In this case, the formation of the bicyclo benzonorbornadienyl radical and its subsequent fragmentation to indenyl radical and acetylene is not the main reaction pathway, although it could be relevant at relatively high temperatures. At lower temperatures, the isomerization reactions, which lead to the formation of a variety of multiring compounds, are dominant.

The implementation of techniques aimed at improving engine performance and reducing particulate matter (PM) pollutant emissions is strongly influenced by the limited understanding of the polycyclic aromatic hydrocarbons (PAH) formation chemistry, in combustion devices, that produces the PM emissions. New experimental results which examine the formation of multi-ring compounds are required. The present investigation focuses on two techniques for such an experimental examination by recovery of PAH compounds from a typical combustion oriented experimental apparatus. The online technique discussed constitutes an optimal solution but not always feasible approach. Nevertheless, a detailed description of a new online sampling system is provided which can serve as reference for future applications to different experimental set-ups. In comparison, an offline technique, which is sometimes more experimentally feasible but not necessarily optimal, has been studied in detail for the recovery of a variety of compounds with different properties, including naphthalene, biphenyl, and iodobenzene. The recovery results from both techniques were excellent with an error in the total carbon balance of around 10% for the online technique and an uncertainty in the measurement of the single species of around 7% for the offline technique. Although both techniques proved to be suitable for measurement of large PAH compounds, the online technique represents the optimal solution in view of the simplicity of the corresponding experimental procedure. On the other hand, the offline technique represents a valuable solution in those cases where the online technique cannot be implemented.

The thermal decomposition of styrene was investigated in a combined experimental, theory and modeling study with particular emphasis placed on the initial dissociation reactions. Two sets of shock tube/time-of-flight mass spectrometry (TOF-MS) experiments were performed to identify reaction products and their order of appearance. One set of experiments was conducted with a miniature high repetition rate shock tube at the Advanced Light Source at Lawrence Berkeley National Laboratory using synchrotron vacuum ultraviolet photoionization. The other set of experiments was performed in a diaphragmless shock tube (DFST) using electron impact ionization. The datasets span 1660–2260 K and 0.5–12 atm. The results show a marked transition from aromatic products at low temperatures to polyacetylenes, up to C8H2, at high temperatures. The TOF-MS experiments were complemented by DFST/LS (laser schlieren densitometry) experiments covering 1800–2250 K and 60–240 Torr. These were particularly sensitive to the initial dissociation reactions. These reactions were investigated theoretically and revealed the dissociation of styrene to be a complex multichannel process with strong pressure and temperature dependencies that were evaluated with multi-well master equation simulations. Simulations of the LS data with a mechanism developed in this work are in excellent agreement with the experimental data. From these simulations, rate coefficients for the dissociation of styrene were obtained that are in good agreement with the theoretical predictions. The simulation results also provide fair predictions of the temperature and pressure dependencies of the products observed in the TOF-MS studies. Prior experimental studies of styrene pyrolysis concluded that the main products were benzene and acetylene. In contrast, this study finds that the majority of styrene dissociates to create five styryl radical isomers. Of these, α-styryl accounts for about 50% with the other isomers consuming approximately 20%. It was also found that C–C bond scission to phenyl and vinyl radicals consumes up to 25% of styrene. Finally the dissociation of styrene to benzene and vinylidene accounts for roughly 5% of styrene consumption. Comments are made on the apparent differences between the results of this work and prior literature., A combined theoretical and experimental study showing styrene primarily decomposes to styryl radicals + H.

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.