Abstract
Chemiluminescence information is of great significance for characterization of flame structure and combustion characteristics. An atmospheric low swirl burner was developed to investigate the chemiluminescence characteristics of OH* and CH* in low swirl flames, with the equivalence ratio varying from 0.8 to 1.2 and the swirl number from 0.2 to 0.6. The chemiluminescence images were captured via ICCD cameras coupled with narrow-bandpass filters, and an Abel inversion method was introduced to transform the line-of-sight-integrated image into two-dimensional radial distributions. The results show that the distribution of CH* is smaller than that of OH* and concentrated more upstream of the flame near the burner. The equivalence ratio has a relatively more direct influence on chemical reactions, while the swirl number has a more evident effect on the flame structure. As the equivalence ratio increases, the peak value of OH* and CH* increases and the peak position moves downstream of the flame, suggesting that the chemical reactions become more intense. In contrast, the height and width of chemiluminescence distribution increase linearly with increasing swirl number. Moreover, it is found that the equivalence ratio and swirl number can be feasibly estimated based on chemiluminescence measurement results, using the correlation between them derived from this study.
Highlights
Chemiluminescence emissions from methane flames are mainly the result of the spontaneous transition of four excited radicals:[1,2] OH∗, CH∗, C2∗, and CO2∗
Chemiluminescence has been widely studied in combustion control[3] as it can characterize the flame structure and reaction characteristics, including the combustion state,[4,5] reaction region,[6–8] equivalence ratio (ER),[9–11] and heat release rate.[12–15]
Deleo et al.[8] obtained the spatial distribution of OH∗ and CH∗ in opposed flow diffusion methane flames, which showed that both OH∗ and CH∗ radicals are generated in the reaction region, and the peak intensity is a good indicator of the methane flame front
Summary
Chemiluminescence emissions from methane flames are mainly the result of the spontaneous transition of four excited radicals:[1,2] OH∗, CH∗, C2∗, and CO2∗. Chemiluminescence has been widely studied in combustion control[3] as it can characterize the flame structure and reaction characteristics, including the combustion state,[4,5] reaction region,[6–8] equivalence ratio (ER),[9–11] and heat release rate.[12–15]. He et al.[5] investigated the laminar methane–oxygen co-flow diffusion flames and found that OH∗ chemiluminescence under different ER can indicate the combustion state, such as the oxygen-deficient or oxygen-enriched state. It is meaningful to study complex flames based on chemiluminescence
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