Measurement of absolute concentrations of minor reactive species in flames by Cavity Ring Down Spectroscopy (CRDS)
Mesure de concentrations absolues d'espèces réactives minoritaires dans les flammes par la technique d'absorption Cavity Ring Down Spectroscopy (CRDS)
Résumé
Combustion processes, which represent our main source of energy today, arouse still numerous questioning. It likes essentially the complexity of the involved chemical mechanisms as well as in the inherent difficulty to the study of an environment which is the field of several thousand simultaneous reactions. Now, even if powerful models exist, allowing the simulation of complex chemical systems, they can not predict any process of combustion and the experimental approach of these ones is still essential for the improvement of the existing models. In particular, the quantitative measure of minor species in flames constitutes a fundamental stage in the validation of the chemical mechanisms with high temperature.
It is in this optics that we developed a new technique for flames study, the Cavity Ring-Down Spectroscopy (CRDS). This technique (appeared to the end of the 80s (O' Keefe and Deacon [1988]) within the framework of a spectroscopic study) is similar to a very high sensibility absorption method. The principle of the CRDS technique is based on the measure of the lifetime of an laser pulse injected in an optical cavity within which is an absorbing sample.
In this report, we show the interest and the potentialities of the CRDS for the study of homogeneous flames. To do it, we clarify in detail the principle of the CRDS and the care to be taken for the measure of absolute concentrations. Besides, a comparison of the absolute concentrations profiles obtained by CRDS (of CN and CH notably) in a CH4 /O2 flame seeded with NO, with those stemming from the modelling by means of the software PREMIX is also presented. The very good agreement which reveals this comparison tends to show that the CRDS, because of its high sensibility and its direct quantitative character, is one of the most efficient methods for the measure of minor species absolute concentrations in homogeneous flames.
Les processus de combustion, qui representent aujourd'hui notre principal source d'energie, suscitent encore de nombreuses interrogations. Cela tient essentiellement a la complexite des mecanismes chimiques mis en jeu ainsi qu'a la difficulte inherente a l'etude d'un milieu qui est le sein de plusieurs milliers de reactions simultanees. Or, meme si des modeles performants permettent la simulation de systemes chimiques complexes, ils ne peuvent predire n'importe quels processus de combustion et l'approche experimentale de ceux-ci reste essentielle pour l'amelioration des modeles existant. En particulier, la mesure quantitative d'especes minoritaires dans les flammes constitue une etape fondamentale dans la validation des mecanismes chimiques a haute temperature. C'est dans cette optique que nous avons developpe une nouvelle technique pour l'etude de flamme, le cavity ring-down spectroscopy (crds). Cette technique, qui s'apparente a une methode d'absorption de tres haute sensibilite et dont le principe est base sur la mesure du temps de vie d'une impulsion laser injectee dans une cavite optique dans laquelle se trouve un echantillon absorbant, est apparue a la fin des annees 80 (o'keefe et deacon 1988) dans le cadre d'une etude spectroscopique.
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