Minor-species structure of premixed cellular tubular flames
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2014 The Combustion Institute. 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 51015 cm-3 and peak OH number densities of 91015 cm-3 are found but remain approximately the same value despite doubling the flame stretch from 200 to 400 s-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.
