Nitric oxide detection in turbulent premixed methane/air flames

Abstract

Investigations of the nitric oxide formation mechanisms of turbulent premixed methane/air flames, with regard to turbulence properties and equivalence ratio, are presented. An atmospheric burner with perforated turbulence generating grids allowed the realization of different turbulent flow conditions (Re = 8.5 × 103–1.7 × 104, u′/sL = 0.8–8.0), which were measured by PIV. Planar LIF was used to detect the OH radical (as a tracer for the flame front) and to visualize the distribution of NO in a two-dimensional area for the whole flame up to far downstream positions. A chemiluminescence detection (CLD) of the NO concentration in the post-flame gases was employed to quantify the NO-LIF measurements. The influence of the equivalence ratio on the spatial distribution of NO was similar to numerical predictions for laminar flames, while variations of turbulence conditions played a rather minor role. The statistical evaluation of local flame front locations enabled the determination of their fluctuation region and its probability distribution. Comparing ensemble-averaged and distribution width of the flame location along the burner axis with the spatial distribution of NO assisted in the understanding of the relative contributions of prompt- and thermal-NO formation paths for different values of the equivalence ratio and turbulence intensity in turbulent premixed flames. Furthermore, qualitative single shot NO-LIF measurements showed structural features, resembling those of the instantaneous turbulent flame front.