Numerical and experimental studies of the NO formation in laminar coflow diffusion flames on their transition to MILD combustion regime

An experimental and numerical study of NO formation in diluted and preheated combustion has been carried out to provide more insight into the ultimate low-NOx potential of MILD combustion. The measured profiles of NO mole fraction were obtained using laser-induced fluorescence (LIF) in the three laminar coflow diffusion flames burning with varying degree of preheating and dilution of coflow and fuel streams. The structure of all these flames was previously extensively studied experimentally and numerically by the authors. Non-preheated and moderately preheated flames show considerable NO formation (on the level of 100 ppm), while the NO formation in the MILD flame appears to be negligible (few ppm). Comparison of the experimental profiles with the results of flame calculations made with detailed chemical mechanisms and multicomponent mixture-averaged transport showed excellent agreement with each other for all flames studied. The rate-of-production analysis demonstrates that Fenimore production is the dominant path of NO formation in all flames studied. Analysis of the computational results also suggests that the reaction reversed to the reaction CH + N2 → NCN + H returns a considerable fraction of NCN back to N2. We studied also the fate of the NO mole fraction seeded to oxidizer coflow in the MILD flame. The experiments display a significant NO reduction at downstream distances. The calculations are in quantitative agreement with measurements and reveal that the reduction is due to the conversion of the NO molecules to the N-containing intermediates which further downstream convert back to NO rather than N2.