Study of chemiluminescence of methane–air flame stabilized on a flat porous burner

In this work, the spatial distribution and spectral characteristics of the chemiluminescence of chemically excited species, OH${}^{\ast }$ and CH${}^{\ast }$, are experimentally and numerically studied by using a stationary premixed methane–air flame stabilized on the surface of a flat porous burner for various equivalence ratio and normal pressure. Numerical simulations are carried out using detailed reaction mechanisms, and the experimental study includes high-resolution spatial and spectral optical measurements. Despite the data reported in the literature, it is found that (i) the rotational degrees of freedom of OH${}^{\ast }$ and CH${}^{\ast }$ are not in thermal equilibrium with the surrounding gas and therefore cannot be used to measure flame temperature; (ii) there is no direct correlation between the heat release rate and the distribution of OH${}^{\ast }$ and CH${}^{\ast }$; (iii) the detailed reaction mechanisms not only quantitatively, and also qualitatively differ in description of the OH${}^{\ast }$ and CH${}^{\ast }$ concentrations. Since the chemically excited species are well localized in a direction normal to the flame surface, they are demonstrated to be a very accurate markers of flame location. The shape of the combustion front can be reconstructed and resolved up to the accuracy of tens of microns, which is very important for estimation of blow-off critical parameters and measurement of the laminar burning velocity.

Novelty and significance statement

Currently, there is a growing interest in the development of sensors for combustion control systems, including active control and suppression of instabilities, in combustion chambers of various devices and engines based on chemiluminescence of excited reaction species. The possibility of non-invasive determination of parameters such as flame temperature, stoichiometry, heat release rate location, etc. using this technique is discussed. We have found that most of these parameters cannot be estimated either due to fundamental limitations or insufficient knowledge of the reaction kinetics involved in the production of these species. Nevertheless, since OH* and CH* are well localized in the direction normal to the flame surface, they can be used as very accurate markers of flame shape and position, allowing us to reconstruct the flame surface to within tens of microns resolution, which is very important for estimating blow-off critical parameters and measuring laminar burning velocity.