L. Gasnot

Instantaneous temperature measurement in a rapid-compression machine using laser Rayleigh scattering

The instantaneous local temperature is measured in a Rapid-Compression Machine (RCM) after the compression. The technique that we have used is the laser Rayleigh scattering at 532 nm. Despite the important background noise due to the very confined RCM chamber, optimum optical conditions lead to a single-shot temperature accuracy of 30 K at the end of compression. The temperature history is sampled at the laser pulse rate, and it exhibits large temperature fluctuations just after the end of compression. Comparison with the extensively used calculated adiabatic core gas temperature shows excellent agreement, at least in the time interval corresponding to ignition delays ( < 100 ms). This first experimental assessment of core-gas assumption is important for chemical-kinetics numerical predictions in RCMs.

Correction of LIF temperature measurements for laser absorption and fluorescence trapping in a flame

A computational method is described in order to correct OH LIF temperature measurements for absorption of laser energy and trapping of fluorescence. Calculations are performed in a large range of flame conditions and can be used as a correction data base both in case of (0-0) and (1−0) excitations. Comparison of corrected temperatures profiles obtained in a 40 Torr methanol/air flame, for both kinds of Laser-Induced Fluorescence (LIF) excitations shows a very good agreement. This method is applied to measure the temperature profile of a methanol flame perturbed by a sampling probe. The LIF collection volume is located at the actual probe sampled volume using an experimental procedure already described. Spatial resolution and sensitivity of temperature measurements are sufficiently efficient to highlight, for the first time by LIF, an indubitable cooling effect due to the probe presence that induces important OH profile change. According to flame chemical modelling, it is shown that both effects are strongly correlated.

Detailed analysis of low-pressure premixed flames of CH4 + O2 + N2: a study of prompt-NO

Abstract A detailed experimental study of low-pressure premixed CH 4 /O 2 /N 2 flames has been undertaken for equivalence ratios of 0.8–1.2, to provide an experimental data base for testing chemical mechanisms of hydrocarbon combustion and their ability to predict NO formation. The experimental procedure involved microprobe sampling and gas chromatographic analysis (GC), together with laser-induced fluorescence (LIF). The major and intermediate stable species were determined using GC. Concentrations of OH, CH, and NO were measured by one-photon LIF; those of CO, H, and O by a two-photon excitation scheme. All concentrations, except that of CH, were measured absolutely using an appropriate calibration method. Temperature was measured using the LIF excitation technique on the OH radical. Predictions from three chemical kinetic models, based on the Miller and Bowman (MB) and Gas Research Institute (GRI) mechanisms, are compared with the experimental results. In the case of major and reactive species, the experimental results are well reproduced by the modeling. However some discrepancies are observed for the C 2 hydrocarbon intermediates. The measured concentrations of CH and NO vary with equivalence ratio as predicted by the MBGRI 1.2 mechanism (the MB scheme for forming NO has been added to the GRI 1.2 one for the oxidation of CH 4 ). Under our experimental conditions the kinetic analysis shows a preponderance of prompt-NO formation. Trends in the evolution of CH with equivalence ratio are well predicted by the GRI 2.11 mechanism, but important disagreements are pointed out for predictions of NO. Important discrepancies are also observed in the amounts of CH and NO with the MB mechanism. These discrepancies are developed and could be directly linked to uncertainties in the reactions of CH and H 2 .

A comprehensive chemical mechanism for the oxidation of methylethylketone in flame conditions

Abstract The improvement of the thermal oxidizers commonly used in the industry to treat the gaseous effluents and thus to limit the rejection of pollutants in the atmosphere requires knowledge of the oxidation processes of volatile organic compounds (VOCs). Methylethylketone (MEK) is a representative VOC and it has been chosen also because of the lack of kinetic data available in the literature concerning the ketones oxidation. Premixed laminar stoichiometric CH 4 /MEK/O 2 /N 2 flat flames have been studied at low-pressure by varying the percentage of seeded MEK up to 3%. The experimental measurements, obtained by coupling microprobe sampling with gas chromatography—mass spectrometry (GC/MS) analysis, have been compared with the predictions of a detailed mechanism. The developed mechanism includes 29 oxygenated species involved in 140 reversible reactions specific to the MEK thermal degradation: it takes into account the first steps of the MEK oxidation and the oxidation processes of the main oxygenated intermediate compounds as acetone, methanol, ethanol, acetaldehyde and propanal. When the quantity of MEK added to the methane flame varies, the increase of C 2 and C 3 hydrocarbon species and the evolution of oxygenated intermediates are well reproduced by the model. The sensitivity analysis points out the main reactional pathways involved in the thermal degradation of MEK in flame conditions and reveals the important production of methyl and ethyl radicals which lead, by recombination processes, to the formation of the oxygenated intermediate compounds.

CH3 detection in flames using photodissociation-induced fluorescence

This paper demonstrates that CH 3 photodissociation occurs in flames when laser radiation around 205 nm is focused even slightly. This photodissociation leads to the formation of CH in the A 2 Δ electronic state. Fluorescence signal issuing from A 2 Δ CH is shown to be quantitatively representative of CH 3 concentration. Validation is obtained by using chemical kinetic modeling and by comparing 205-nm excited A 2 Δ CH profile with CH 3 profile measured by molecular beam-mass spectrometry in different methane/air flames. A spectroscopic discussion is described. It is shown that the overall photodissociation process may be consistent with the following steps: ( X 2 A 2 ″)CH 3 + hv (205 nm)→( B 2 A ′l)CH 3 ( B 2 A ′l)CH 3 →(l 3 B 1 )CH 2 +H.(l 1 A 1 )CH 2 +H, and ( X 2 II )CH+H 2 (l 3 B 1 )CH 2 + hv (205 nm)→(2 3 B 1)CH 2 →( A 2 Δ )CH+H The present photodissociation-induced fluorescence technique offers the ability of local, nonintrusive, and instantaneous CH 3 detection in flames. Furthermore, opportunity of simultaneous excitation of hydrogen atom and methyl radical at 205 nm in flames may be very attractive.

Disturbance of laser-induced-fluorescence measurements of NO in methane-air flames containing chlorinated species by photochemical effects induced by 225-nm-laser excitation.

Laser-induced fluorescence measurements of NO in CH(4)-air flames seeded with CH(3)Cl and CH(2)Cl(2) are described. The measurements are perturbed by strong photochemical effects characterized by UV emissions. The contribution of these background emissions is taken into account on the basis of an on-line-off-line resonance procedure. First results indicate an important increase of NO in the presence of chlorinated species. Background emissions observed in the range 220-260 nm and at 278 nm are ascribed, respectively, to electronically excited HCl and CCl photofragments. It is shown that C(2)H(3)Cl and CHCl(2) species are responsible for the formation of HCl and CCl, respectively, and a photolytic mechanism for formation of the photofragments is proposed.

CCl, CH, and NO LIF measurements in methane-air flames seeded with chlorinated species: Influence of CH3Cl and CH2Cl2 on CCl and NO formation

In this work, the laser-induced fluorescence (LIF) technique is used to detect minor species (CCl, NO, and CH) in premixed stoichiometric methane-air flames seeded with monochloromethane or dichloromethane. Quenching data are extracted from time-resolved fluorescence lifetime measurements for all the excited species. First quenching measurements of CCl under flame conditions are reported. It is shown that LIF measurements are strongly perturbed by the presence of background emissions issued from the radiative relaxation of photolytic fragments (HCl*, CCl*, CH*, and C2*) formed upon laser excitation. The parent molecules that are partly responsible for these emissions are C2H3Cl (for HCl*, CH*) and CHCl2 (for CCl*). Profiles of both photolytic fragments and species directly measured by LIF are used to study the influence of CH3Cl and CH2Cl2 addition on CCl and NO formation in methane-air flames. CCl radical is found to be formed in the reaction zone of the flames. The reaction path leading to CCl appears to be dependent on the nature of the chlorinated hydrocarbon (CHC) seeded in the flame. The suggested reaction paths may preferentially involve the contribution of CHCl2 in case of CH2Cl2 degradation and CH2Cl in case of CH3Cl degradation. An important increase of NO in presence of CHC is pointed out for the first time. The NO formation in flames containing CHC appears to occur in the reaction zone of the flames, and [NO] is found to be constant in the burned gases: This suggests a predominance of the prompt-NO mechanism in this kind of flame as confirmed experimentally by the observed [CH] increase. Reaction paths involving the degradation of CHCs, particularly CHCl2, should largely contribute to the formation of CH in flames seeded with CHCs.

Original ArticlesDetailed analysis of low-pressure premixed flames of CH4 + O2 + N2: a study of prompt-NO

Abstract A detailed experimental study of low-pressure premixed CH 4 /O 2 /N 2 flames has been undertaken for equivalence ratios of 0.8–1.2, to provide an experimental data base for testing chemical mechanisms of hydrocarbon combustion and their ability to predict NO formation. The experimental procedure involved microprobe sampling and gas chromatographic analysis (GC), together with laser-induced fluorescence (LIF). The major and intermediate stable species were determined using GC. Concentrations of OH, CH, and NO were measured by one-photon LIF; those of CO, H, and O by a two-photon excitation scheme. All concentrations, except that of CH, were measured absolutely using an appropriate calibration method. Temperature was measured using the LIF excitation technique on the OH radical. Predictions from three chemical kinetic models, based on the Miller and Bowman (MB) and Gas Research Institute (GRI) mechanisms, are compared with the experimental results. In the case of major and reactive species, the experimental results are well reproduced by the modeling. However some discrepancies are observed for the C 2 hydrocarbon intermediates. The measured concentrations of CH and NO vary with equivalence ratio as predicted by the MBGRI 1.2 mechanism (the MB scheme for forming NO has been added to the GRI 1.2 one for the oxidation of CH 4 ). Under our experimental conditions the kinetic analysis shows a preponderance of prompt-NO formation. Trends in the evolution of CH with equivalence ratio are well predicted by the GRI 2.11 mechanism, but important disagreements are pointed out for predictions of NO. Important discrepancies are also observed in the amounts of CH and NO with the MB mechanism. These discrepancies are developed and could be directly linked to uncertainties in the reactions of CH and H 2 .

Detailed analysis of low-pressure premixed flames of CH{sub 4} + O{sub 2} + N{sub 2}: A study of prompt-NO

Abstract A detailed experimental study of low-pressure premixed CH 4 /O 2 /N 2 flames has been undertaken for equivalence ratios of 0.8–1.2, to provide an experimental data base for testing chemical mechanisms of hydrocarbon combustion and their ability to predict NO formation. The experimental procedure involved microprobe sampling and gas chromatographic analysis (GC), together with laser-induced fluorescence (LIF). The major and intermediate stable species were determined using GC. Concentrations of OH, CH, and NO were measured by one-photon LIF; those of CO, H, and O by a two-photon excitation scheme. All concentrations, except that of CH, were measured absolutely using an appropriate calibration method. Temperature was measured using the LIF excitation technique on the OH radical. Predictions from three chemical kinetic models, based on the Miller and Bowman (MB) and Gas Research Institute (GRI) mechanisms, are compared with the experimental results. In the case of major and reactive species, the experimental results are well reproduced by the modeling. However some discrepancies are observed for the C 2 hydrocarbon intermediates. The measured concentrations of CH and NO vary with equivalence ratio as predicted by the MBGRI 1.2 mechanism (the MB scheme for forming NO has been added to the GRI 1.2 one for the oxidation of CH 4 ). Under our experimental conditions the kinetic analysis shows a preponderance of prompt-NO formation. Trends in the evolution of CH with equivalence ratio are well predicted by the GRI 2.11 mechanism, but important disagreements are pointed out for predictions of NO. Important discrepancies are also observed in the amounts of CH and NO with the MB mechanism. These discrepancies are developed and could be directly linked to uncertainties in the reactions of CH and H 2 .