Numerical evaluation of equivalence ratio measurement using OH∗ and CH∗ chemiluminescence in premixed and non-premixed methane–air flames

This work presents results from detailed chemical kinetics calculations of electronically excited OH (A2Σ, denoted as OH∗) and CH (A2Δ, denoted as CH∗) chemiluminescent species in laminar premixed and non-premixed counterflow methane–air flames, at atmospheric pressure. Eight different detailed chemistry mechanisms, with added elementary reactions that account for the formation and destruction of the chemiluminescent species OH∗ and CH∗, are studied. The effects of flow strain rate and equivalence ratio on the chemiluminescent intensities of OH∗, CH∗ and their ratio are studied and the results are compared to chemiluminescent intensity ratio measurements from premixed laminar counterflow natural gas–air flames. This is done in order to numerically evaluate the measurement of equivalence ratio using OH∗ and CH∗ chemiluminescence, an experimental practise that is used in the literature. The calculations reproduced the experimental observation that there is no effect of strain rate on the chemiluminescent intensity ratio of OH∗ to CH∗, and that the ratio is a monotonic function of equivalence ratio. In contrast, the strain rate was found to have an effect on both the OH∗ and CH∗ intensities, in agreement with experiment. The calculated OH∗/CH∗ values showed that only five out of the eight mechanisms studied were within the same order of magnitude with the experimental data. A new mechanism, proposed in this work, gave results that agreed with experiment within 30%. It was found that the location of maximum emitted intensity from the excited species OH∗ and CH∗ was displaced by less than 65 and 115 μm, respectively, away from the maximum of the heat release rate, in agreement with experiments, which is small relative to the spatial resolution of experimental methods applied to combustion applications, and, therefore, it is expected that intensity from the OH∗ and CH∗ excited radicals can be used to identify the location of the reaction zone. Calculations of the OH∗/CH∗ intensity ratio for strained non-premixed counterflow methane–air flames showed that the intensity ratio takes different values from those for premixed flames, and therefore has the potential to be used as a criterion to distinguish between premixed and non-premixed reaction in turbulent flames.