Minor-species structure of premixed cellular tubular flames

Abstract

Quantitative spatially resolved measurements of atomic hydrogen and hydroxyl radicals are reported of cellular phenomena in lean laminar premixed hydrogen–air tubular flames. Femtosecond two-photon laser-induced fluorescence (fs-TPLIF) enables photolytic-interference-free measurement of H atoms. Quenching corrections are calculated from previously measured temperature and major-species concentrations. In the sub-unity Lewis number, highly stretched and curved premixed flame structure, the strong effect of thermal-diffusive imbalance is observed through clear local extinction (dearth of H atoms) and highly curved reaction cells. Peak H-atom number densities of 5*1015cm–3 and peak OH number densities of 9*1015cm−3 are found but remain approximately the same value despite doubling the flame stretch from 200 to 400s–1. Comparisons to non-cellular tubular flame numerical predictions show that the cellular transition increases temperature and local equivalence ratio but lowers peak number densities of H and OH. For cellular instabilities in the tubular flame, the thermal-diffusive imbalance maintains a similar local flame structure (i.e., same curvature and peak values of H, OH, T, and local equivalence ratio) that is insensitive to stretch rate.