Experimental and numerical study of chemiluminescent species in low-pressure flames

Chemiluminescence has been observed since the beginning of spectroscopy, nevertheless, important facts still remain unknown. Especially, reaction pathways leading to chemiluminescent species such as OH∗, CH∗, $\mathrm{C}_{2}^{*}$, and $\mathrm{CO}_{2}^{*}$ are still under debate and cannot be modeled with standard codes for flame simulation. In several cases, even the source species of spectral features observed in flames are unknown. In recent years, there has been renewed interest in chemiluminescence, since it has been shown that this radiation can be used to determine flame parameters such as stoichiometry and heat release under some conditions.In this work, we present a reaction mechanism which predicts the OH∗, CH∗ (in A- and B-state), and $\mathrm{C}_{2}^{*}$ emission strength in lean to fuel-rich stoichiometries. Measurements have been performed in a set of low-pressure flames which have already been well characterized by other methods. The flame front is resolved in these measurements, which allows a comparison of shape and position of the observed chemiluminescence with the respective simulated concentrations. To study the effects of varying fuels, methane flame diluted in hydrogen are measured as well. The 14 investigated premixed methane–oxygen–argon and methane–hydrogen–oxygen–argon flames span a wide parameter field of fuel stoichiometry (ϕ=0.5 to 1.6) and hydrogen content (H2 vol%=0 to 50).The relative comparison of measured and simulated excited species concentrations shows good agreement. The detailed and reliable modeling for several chemiluminescent species permits correlating heat release with all of these emissions under a large set of flame conditions. It appears from the present study that the normally used product of formaldehyde and OH concentration may be less well suited for such a prediction in the flames under investigation.