Analysis of CH* concentration and flame heat release rate in laminar coflow diffusion flames under microgravity and normal gravity

The chemiluminescence from electronically excited CH (denoted as CH*) is investigated in nitrogen-diluted laminar coflow methane diffusion flames under microgravity and normal gravity conditions. In combustion studies, this radical species is of significant interest since its spatial distribution is indicative of the flame front position; moreover, given the relatively simple diagnostic involved with its measurement, several studies have been done to evaluate the ability of CH* chemiluminescence to predict the total and local flame heat release rate. In this work, a subset of the publicly available NASA Structure and Liftoff in Combustion Experiments (SLICE) microgravity and normal gravity nitrogen-diluted methane flames has been considered, and a method to extract quantitative CH* concentration information from the SLICE raw data is demonstrated. The measured CH* concentration is then discussed and compared with numerical simulations to assess the correlation between CH* chemiluminescence and heat release rate. The spectral characterization of the digital single lens reflex (DSLR) color camera used to acquire the flame images allowed the signal collected by the blue channel to be considered representative of the CH* emission of the A2Δ → X2∏ transition centered around 431nm; the analysis of the spectral emission of a reference nitrogen-diluted laminar diffusion methane flame accounted for the contribution of chemiluminescence from emitting species other than CH*. Due to the axisymmetric flame structure, an Abel deconvolution of the line-of-sight chemiluminescence was used to obtain the two-dimensional intensity profile and, thanks to an absolute light intensity calibration, a quantification of the CH* concentration was possible. Comparisons with numerical results display reasonably good agreement between measured and computed flame shapes, and it is shown that the difference in peak CH* concentration, between micro- and normal gravity cases, is minimal. Independent of the gravity level, the integrated CH* concentration in a cross section scales proportionally to the integrated computed heat release rate. The two-dimensional CH* and heat release rate spatial profiles match in a satisfactory way, but the gradients and intensity distributions are not comparable.