Pascale Desgroux

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.

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.

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 .

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.

Time-resolved electric field measurements in nanosecond surface dielectric discharge. Comparison of different polarities. Ignition of combustible mixtures by surface discharge in a rapid compression machine.

Surface nanosecond dielectric barrier discharge has been studied in air and at pressures ranging from 1 to 5 bar, with a coaxial geometry of the electrodes for positive and negative polarities of the high-voltage pulses. Pulses of a 24–55 kV amplitude on the electrode, positive or negative polarity, 20 ns duration, 0.5 ns rise time and 10 Hz repetitive frequency were used to initiate the discharge. ICCD images of the discharge development have been taken with a 2 ns gate. In the case of discharges in nitrogen, the emissions of molecular bands of the first negative and second positive systems of molecular nitrogen have been measured, and the dependence of their ratio versus pressure and distance from the highvoltage electrode has been analyzed. A comparison of the discharge development has been made in the case of negative and positive polarities at the high-voltage electrode. Ignition delay times under the action of a high-voltage nanosecond discharge have been studied and compared with autoignition delays in a rapid compression machine (RCM). The nanosecond Surface Dielectric Barrier Discharge (SDBD) was initiated in a quasi-uniform radial geometry in the proximity of the end plate of the combustion chamber of the RCM. Experiments were performed for methane and n-butane containing mixtures diluted by Ar or N2 for temperatures and pressures at the end of compression respectively ranging from 650 to 1000 K and 6 to 16 bar. A significant decrease of the ignition delay time is observed, when compared to autoignition experiments. The possibility to ignite lean mixtures is demonstrated. Preliminary experiments in the region of negative temperature coefficient for stoichiometric n–butane:oxygen mixture diluted with argon, are performed. The threshold voltage for plasma ignition, over which the ignition delay is decreased, is studied for different mixtures.

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.

Parametric study of polycyclic aromatic hydrocarbon laser desorption

We present the use of a combined laser desorption/multi-photon ionization/time-of-flight mass spectrometry technique for the analysis of polycyclic aromatic hydrocarbon (PAH) solid samples. A thorough characterization of the first step (laser desorption) of this experimental technique has been performed. By varying the energy of the laser pulse, a specific response of each PAH has been evidenced for pure and mixed PAH sample desorption. This behaviour has also been studied with respect to the fragmentation processes. Similar studies on PAHs adsorbed on graphite evidenced the possibility of desorbing molecules from the adsorbed phase only, i.e. without a contribution from the graphite substrate. These findings represent important preliminary steps towards the final goal of setting up a completely characterized analytical method for the investigation of the adsorbed phase of soot particles generated in combustion processes.

NO reburning study based on species quantification obtained by coupling LIF and cavity ring-down spectroscopy

NO reburning is studied in a low pressure (15 hPa) premixed flame of CH4-O2 seeded with 1.8% of NO. Measurements were carried out by using cavity ring-down spectroscopy (CRDS) and laser induced fluorescence (LIF) techniques. The temperature profile was obtained by OH-LIF thermometry in the A-X (0-0) band. The OH profile was determined by LIF and calibrated by single pass absorption. The NO concentration profile was obtained by LIF in the A-X (0-0) band and corrected for Boltzmann fraction and quantum yield variations. The absolute concentration profile was determined in the burned gases by CRDS allowing a direct experimental determination of the NO reburning amount. Finally CH and CN mole fraction profiles were obtained by CRDS by exciting rotational transitions in the B-X (0-0) bands of CH and CN around 387 nm. We found a peak mole fraction of 29 ppm for CH and 3.3 ppm for CN. This last result is in contrast with a previous study of W. Juchmann, H. Latzel, D. L. Shin, G. Peiter, T. Dreier, H. R. Volpp, J. Wolfrum, R. P. Lindstedt and K. M. Leung, XXVIIth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1998, p. 469, performed in a similar flame, which reported much lower levels of CN. In that study the absolute concentration of CN was indirectly obtained by LIF calibrated by Rayleigh scattering. In a second part, experimental species profiles are compared with predictions of the GRI 3.0 mechanism. Comparison between experimental and predicted profiles shows a good agreement particularly for CN and NO species. A qualitative analysis of NO reburning is then performed.

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.