Measurement and simulation of rotationally-resolved chemiluminescence spectra in flames

Abstract

In recent years, there has been renewed interest in chemiluminescence, since it has been shown that these emissions can be used to determine flame parameters such as stoichiometry and heat release under some conditions. Even though the origin of these emissions has been known for a long time, little attention has been paid to the detailed analysis of the spectral structure. In this contribution, we present rotationally-resolved spectra of all important chemiluminescent emissions OH A-X, CH B-X, CH A-X, and C2 d-a in CH4/air flames. A numerical model based on the LASKINν 2 code has been developed that allows, for the first time, to accurately predict the shape of the measured spectra for all of these transitions. Reabsorption of chemiluminescence within the emitting flame is shown to be a major factor, affecting both intensity and structure of OH∗ spectra. Even in lab-scale flames, it might change the intensity of individual lines by a factor of 5. The shape of chemiluminescence spectra depends on several processes including initial state distribution and rotational and vibrational energy transfer (which, in turn, depend on the collisional environment and the temperature). It is shown that chemical reactions form OH∗ in highly excited states and that the number of collisions is not sufficient to equilibrate the initial distribution. Therefore, high apparent temperatures are necessary to describe the shape of the measured spectra. In contrast, CH∗ is formed with less excess energy and the spectral shape is very close to thermal. The rotational structure of $\mathrm{C}_{2}^{*}$ is close to thermal equilibrium as well. Vibrational temperatures are, however, significantly higher than the flame temperature. Implications and perspectives for flame measurements are discussed.