Christian Vovelle

Chemical Structure of Atmospheric Pressure Premixed n-Decane and Kerosene Flames

Abstract This study was aimed at comparing the chemical structure of two rich premixed flames Ted respectively with a neat fuel: n-decane and a multi-component fuel, kerosene. Both flames were stabilized on a flat-flame burner at atmospheric pressure. Samples were withdrawn along the symmetry axis with a quartz microprobe and analyzed by gas phase chromatography. The main objective was to justify, for a future modelling purpose, the use of a detailed reaction mechanism validated in the n-decane flame to predict the chemical structure of a kerosene flame. A close similitude was observed for the mole fraction profiles of both major species and main intermediates measured in each flame. Relative concentrations of the intermediate species in the n-decane flame can be interpreted qualitatively by β-scission of large alkyl radicals. The only marked difference observed when changing the fuel concerns benzene. The maximum mole fraction measured in the kerosene flame: 1.05·10−3 exceeds by one order of magnitude, t...

Experimental study of 1 atmosphere, rich, premixed n-heptane and iso-octane flames

Temperature and species mole fraction profiles have been measured in laminar premixed n-heptane/O2/N2 and iso-octane/O2/N2 flames. Both flames have been stabilized on a flat-flame burner, at atmospheric pressure. Species identification and concentration measurements have been performed by gas chromatography and GC-MS analysis of samples withdrawn locally by a quartz microprobe. Temperature profiles were measured by Pt-Pt 10%Rh thermocouples with radiative heat losses suppressed by the electrical compensation method. For both flames, the equivalence ratio was equal to 1.9 and a faint yellow luminosity due to soot particles was observed.|The main objective of this work was to provide detailed experimental data on the nature and concentration of the intermediate species formed by consumption of a linear or highly branched fuel molecule. In addition to reactants and major products (CO, CO2, H2, H2O), the mole fraction profiles of C1(CH4), C2(C2H2, C2H4, C2H6), C3(allene, propyne, propene, propane), C4(C4H2, C...

Experimental and computational investigation of the structure of a sooting C2H2-O2-Ar flame

Mole fraction profiles have been measured by molecular beam-mass spectrometer technique in a sooting C2H2-O2-Ar flame (27.5%-27.5%-45%) stabilized under reduced pressure (2.6 kPa) on a flat flame burner. Emphasis was put on the detection and concentration measurement of the intermediate species which play a role in the formation of the first aromatic rings. In addition to the major products and the radicals usually involved in acetylene oxidation mechanisms, C2H, C2H3, C3, C4 species and benzene have been measured. The mole fraction profiles have been compared with predictions from a simulation model. Care was taken to use as much as possible a detailed mechanism known to model acetylene oxidation in a wide range of experimental conditions. The mechanism proposed by Warnatz23, for the oxidation of alkanes and recently checked by Westmoreland38 for the modelling of a rich C2H2/O2 flame was adopted as a starting point. This tested mechanism was complemented by formation and consumption reactions for C4H3, C4H4, C4H5 and benzene. The satisfactory agreement between calculated and measured profiles was turned to account to specify the main steps in the route to benzene.

Kinetic modeling of a rich, atmospheric pressure, premixed n-heptane/O2/N2 flame

Abstract A detailed reaction mechanism has been evaluated by comparison of computed species mole fraction profiles with experimental profiles measured in a rich n-heptane/O 2 /N 2 flame stabilized at atmospheric pressure. A similar study was carried out previously in our laboratory, at low pressure (6 kPa) with molecular beam-mass spectrometer as the analytical tool. In the present work, species mole fractions are measured by gas chromatography so that isomers that could not be distinguished by the mass spectrometer were identified and analyzed separately. Hence, although the main objective of this work was to extend the n-heptane combustion mechanism to atmospheric pressure, it was also to take advantage of the new data on the isomers to refine the mechanism. Modifications to the low-pressure mechanism have been strictly limited to (i) calculation of high pressure values for reactions in the fall-off regime and (ii) distinction of the isomeric forms of heptenes. The reliability of the mechanism was evaluated by comparison of computed mole fraction profiles with those measured in a rich premixed n-heptane flame (equivalence ratio 1.9). Good agreement was obtained for most molecular species, especially intermediate olefins, dienes, alkynes. Computed benzene concentrations are also in reasonable agreement with experimental observation. Analyses of the main reaction pathways show that the main effect of the change of pressure from 6 to 101 kPa is to increase the relative importance of the thermal decomposition reactions, especially for the intermediate olefins.

Investigating the role of methane on the growth of aromatic hydrocarbons and soot in fundamental combustion processes

Experimental results are presented on the effect of methane content in a non-aromatic fuel mixture on the formation of aromatic hydrocarbons and soot in various fundamental combustion configurations. The systems considered consist of a laminar flow reactor, a laminar co-flow diffusion flame burner, and a laminar, premixed flame burner, all of which operate at atmospheric pressure. In the flow reactor, the experiments are performed at 1430 K, constant C-atom flow rates, 98% nitrogen dilution, C/O ratio = 2, and with fuel mixtures consisting of ethylene and methane. The diffusion flames are performed with fuel mixtures of methane and ethylene diluted in nitrogen to maintain a constant adiabatic flame temperature. The premixed flame experiments are performed with n-heptane and methane mixtures at a C/O ratio = 0.67 with nitrogen-impoverished air. The results show the existence of synergistic chemical effects between methane and other alkanes in the production of aromatics, despite reduced acetylene concentrations. This effect is attributable to the ability of methane to enhance the production of methyl radicals that will then promote production channels of aromatics that rely on odd-carbon-numbered species. Benzene, naphthalene, and pyrene show the strongest sensitivity to the presence of added methane. This synergy on aromatics trickles down to soot via enhanced inception and surface growth rates by polycyclic aromatic hydrocarbon condensation, but the overall effects on soot volume-fraction are smaller due to a compensating reduction in surface growth from acetylene. These results are observed under the very fuel-rich environments existing in the flow reactor and diffusion flames. In the premixed flames, however, instabilities did not permit investigation of conditions with sufficiently high equivalence ratios to perturb the aromatic and soot-growth regions.

Experimental Study of the Chemical Structure of Low-Pressure Premixed n>Heptane-02-Ar and lso-Octane-02-Ar Flames

Temperature and species mole fraction profiles have been measured in laminar premixed H-heptane/02/Ar and iso-octane/02/Ar flames. Both flames have been stabilized on a flat-flame burner at low pressure (6.0 kPa), and species identification and concentration measurement have been performed by mass spectrometric analyses of samples withdrawn locally by molecular beam formation. Temperature profiles were measured by Pt - Pt 10% Rh thermocouples with corrections of the signals to compensate radiative heat losses. A wide range of equivalence ratios extending to 0.7 up to 2.0 has been considered in order to check how the nature of the fuel influences the evolution with this parameter of the species mole fraction profiles Mole fraction profiles of reactants, major products (C02, H20, CO, H2), main active species (H, O, OH), and small intermediate species (CH3, CH4, C2H2, C2H4, C2H5 2 have been obtained with working conditions of the M BMS technique usually adopted to study the structure of small fuel molecules ...

Modeling of the Structure of a Premixed n-Decane Flame

Abstract The chemical structure of a premixed n-decane/O2/N2 flame (equivalence ratio 1.7) stabilized at atmospheric pressure on a flat-flame burner has been computed with two reaction mechanisms. In the first one, the consumption of the fuel molecule is described in detail, The five different n-decyl radicals formed by H atom abstraction from the decane molecule were distinguished and their consumption reactions were considered in a systematic way. This mechanism comprises 78 species involved in 638 elementary reactions. Modeling with this detailed mechanism led to species mole fraction profiles in good agreement with the experimental results. The main. reaction paths for the formation of final and intermediate species have been identified with special emphasis on benzene formation. The second mechanism was derived from the first one by successively removing an increasing number of n-decyl radicals. For most species, it is possible to maintain the reliability of the model with only one n-decyl radical in...

Flame profiles and combustion mechanisms of methanol-air flames under reduced pressure

Abstract If methanol is to be used as a fuel or as an additive to hydrocarbons, more information is required on its combustion kinetics. Three methanol-air flames (9.3%, 12.6%, and 16.9% by volume) have been stabilized on a flat-flame burner and their concentration, and temperature profiles have been measured. Over all rates for stable species have been deduced from mass and species conservation equations. Despite different experimental conditions (atmospheric pressure versus shock tube conditions) results are in agreement with those published by Bowman [8].

Formation of Aromatic Hydrocarbons in Decane and Kerosene Flames at Reduced Pressure

The formation of frequently discussed soot precursors has been compared in decane and kerosene flames. A specific study on the influence of equivalence ratio on the formation of some species (acetylene, benzene, vinyl benzene and phenyl acetylene) showed that in decane flames aromatic hydrocarbons are formed from acetylene whereas in kerosene flames the main source is the aromatic part of the fuel. From this result it seems to be reasonable to represent kerosene as a mixture of decane (90%) and toluene (10%) when developing a detailed kinetic mechanism to predict benzene formation in flames burning kerosene. The mechanism was checked by comparison of computed mole fraction profiles with measured profiles of a sooting kerosene-oxygen-argon flame with an equivalence ratio of 2.2 and of a decane flame with the same equivalence ratio.

Mass Loss Rate Measurements on Solid Materials Under Radiative Heating

Abstract The influence exerted by radiant flux on transient thermal degradation of PMMA and particle board has been investigated. The mass loss rate has been measured continuously during experiments performed on specimens of dimensions equal to 10 x 10 cm in the range 1-4 W/cm2. The effects of oxygen content of the gaseous atmosphere surrounding the material have also been studied by comparing results obtained in air and in nitrogen. Dependence of the mass loss rale on the temperature of the gases has been briefly investigated as well. It has been shown that if the results are expressed in terms of the variables [mdot]” /([Qdot]”-[Qdot]”0) mass loss rate per unit radiant flux, and q” = ([Qdot]”-[Qdot]”0)·t, heat absorbed by the material, comparison of experiments performed at different radiant flux can be readily achieved. For PMMA progress of the reaction is exactly reproduced when radiant flux is modified except in00 air at very low values where an acceleration of the reaction by oxygen is observed. For...