J.F. Pauwels

Absolute CH concentration measurements by cavity ring-down spectroscopy in an atmospheric diffusion flame

Abstract Absolute concentrations of CH radical are reported for the first time in an atmospheric diffusion flame. Measurements are performed by cavity ring-down (CRD) spectroscopy by probing the C–X system of CH around 315 nm. We used standard 308 nm coated mirrors also suitable for OH CRD measurements. Absolute concentrations are obtained from integrated absorption measurements after a deconvolution procedure. Peak mole fractions are found to be around 0.6 ppm in satisfying agreement with previously reported predictions issued from flame modelling. The ability of CRD technique to describe very narrow species profiles is demonstrated by comparison with laser-induced fluorescence measurements.

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.

Cavity ring-down measurements of OH radical in atmospheric premixed and diffusion flames.: A comparison with laser-induced fluorescence and direct laser absorption

Abstract Cavity ring-down spectroscopy (CRDS) is tested in two atmospheric burners: a premixed flat flame burner and a Wolfhard–Parker burner. The quantitative nature and the spatial resolution of CRDS are compared with those of laser-absorption and laser-induced fluorescence by recording OH concentration profiles. Losses per pass due to the abundant OH sample in the CRD cavity need to be carefully controlled to obtain an exponential ring-down decay. Index refraction gradients can be responsible for important random off-resonance losses which perturb CRDS measurements. In contrast, line-of-sight CRD measurements performed along the axis of the gradients are found to be very accurate.

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.

Quantitative Features and Sensitivity of Cavity Ring-Down Measurements of Species Concentrations in Flames

Absolute concentrations of minor species can be measured by cavity ring-down spectroscopy (CRDS) by analysing the exponential time decay of the CRDS signal. This paper shows that quantitative concentrations can be measured by CRDS using a moderately narrowband multimode dye laser, even though the ring-down decays exhibit a multi-exponential behavior (nonlinear variation of the losses with the absorbance). A model based on Fabry-Perot theory has been developed to fit the multi-exponential decays by taking into account the convolution of the laser lineshape and of the absorption line. From this model, true absorbances, corrected for nonlinear effects, can be obtained, leading to quantitative measurements of concentrations. Using the model, the dynamic range of CRD measurements is increased by a factor of ten. The sensitivity of the technique is shown to be reduced in the region of the thermal gradient, which induces an important increase of the off-resonance losses/pass. The best fractional absorption/pass we could obtain was estimated to be 10 ppm in the flame front and 5 ppm in the burnt gases of a low-pressure premixed flame of methane and air. The sensitivity is greater when the laser is coupled to the TEM00 mode of the cavity. CRD measurements of (CH) performed in two different spectral ranges in the C-X and B-X bands are compared.

Experimental and modeling studies of a rich H2/CO/N2O/Ar flame

Using molecular beam mass spectrometry, the structure of a rich, premixed, flat flame of H-2 + CO + N2O + Ar (phi = 1.19) has been established at 30 torr. Three different mechanisms have been tested to model the flame, with good agreement between calculated and experimental results. It has been demonstrated that the main reaction consuming N2O is N2O + H --> N-2 + OH, while the dissociation reaction: N2O + M --> N-2 + O + M operates like a branching reaction by generating O atoms. The reaction N2O + H --> NH + NO is the main process producing NO. Two of the mechanisms simulate the concentration profile of NO well. As for the processes CO + OH --> CO2 + H and CO + N2O --> CO2 + N-2, only the first plays a significant role in the conversion of CO

Experimental and Kinetic Analysis of the Thermal Degradation of the Methylethylketone in Methane / Air Flames UMR CNRS 8522 “Physicochimie des Processus de Combustion”.

The experimental study of the thermal degradation of methylethylketone (MEK) in flame conditions is undertaken to provide an experimental data base concerning Volatile Organic Compounds (VOCs) oxidation. The experimental set-up consists of the coupling of microprobe sampling with gas chromatography and mass spectrometry analysis. Temperature measurements are realized by the coated thermocouple technique. The addition of MEK to a reference methane / air flame induces (1) an enhancement of the C2 and C3 hydrocarbon oxidation pathways which points out the main role of CH3 and C2H5 radicals, and (2) the formation of oxygenated intermediate compounds such aldehydes (CH2O, CH3CHO. CH3CH2CHO), alcohols (CH3OH, CH3CH2OH) and ketones (CH3COCH3, CH3COCHCH2, CH3CH2COCH2CH3, CH3COCH2CH2CH3). The peak mole fraction of these intermediate species increases with the amount of MEK added to the methane / air flame. The experimental results are discussed to point out the kinetic processes involved in the high temperature oxidation of MEK which may contribute to the development of a detailed chemical mechanism.

Pyrene measurements in sooting low pressure methane flames by jet-cooled laser-induced fluorescence.

This paper presents in detail the study we carried out concerning the pyrene measurement by jet-cooled laser-induced fluorescence (JCLIF) in different sooting low pressure methane flames. The aim of this paper is both to demonstrate the potentialities of this technique for the measurement of such moderately sized polycyclic aromatic hydrocarbons under sooting flame conditions and to provide new experimental data for the understanding and the development of chemical models of the soot formation processes. Several concentration profiles of pyrene measured in different sooting flame (various pressure and equivalence ratio) are presented. The validation of the JCLIF method for pyrene measurements is explained in detail as well as the calibration procedure, based on the standard addition method, which has been implemented for the quantification of the concentration profiles. Sensitivity lower than 1 ppb was obtained for the measurement of this species under sooting flame conditions.

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.