Excited OH kinetics and distribution in H2 premixed flames

Abstract

With the aim of understanding the link between excited OH (OH∗) and ground species, we have investigated its distribution and kinetics in steady planar premixed H2-O2-N2 flames. We find that the OH∗ distribution mimics the two-layer chemical structure of the underlying host H2 flame, with a thin fuel consumption layer where OH∗ is out of equilibrium and a thicker downstream recombination tail where OH∗ relaxes to thermodynamic equilibrium with OH. This distribution can be characterized by only two representative concentrations: (i) the peak concentration at the fuel consumption layer, and (ii) the final downstream equilibrium concentration. A map of their ratio in an equivalence-dilution plane reveals the gradual transition between two limit-flame types: low-dilution flames emit with similar intensities from both regions, whereas highly-diluted flames emit more strongly from the fuel consumption layer. The relevance of these findings to the detection and to the rational, quantitative use of chemiluminescence is discussed.