Flame/stretch interactions of premixed hydrogen-fueled flames: Measurements and predictions

Fundamental unstretched laminar burning velocities, and flame response to s tretch (represented by the Markstein number) were considered both experimen tally and computationally for laminar premixed flames. Mixtures of hydrogen and oxygen with nitrogen, argon and helium as diluents were considered to modify flame transport properties for computationally tractable reactant mi xtures. Freely (outwardly)-propagating spherical laminar premixed flames we re considered for fuel-equivalence ratios of 0.6 to 4.5, pressures of 0.3 t o 3.0 atm, volumetric oxygen concentrations in the nonfuel gases of 0.21 to 0.36, and Karlovitz numbers of 0 to 0.5, at normal temperatures. For these conditions, both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding constant Markstein numbers for particul ar reactant conditions. The present flames were very sensitive to flame str etch, exhibiting ratios of unstretched-to-stretched laminar burning velocit ies in the range 0.6 to 3.0 for levels of flame stretch well below quenchin g conditions. At fuel-lean conditions, increasing flame temperatures (by di lution with argon rather than nitrogen) tended to reduce flame sensitivity to stretch whereas increasing pressures tended to increase tendencies towar d preferential-diffusion instability behavior. At low pressures, helium-dil uted flames had reduced tendencies toward preferential-diffusion instabilit y behavior compared to nitrogen- and argon-diluted flames due to stabilizat ion of flame properties by strong effects of preferential diffusion of heat . Predicted and measured flame properties exhibited encouraging agreement u sing contemporary reaction mechanisms. Finally, flame structure predictions suggest that H and OH radical production and transport are important aspec ts of preferential-diffusion/stretch interactions, reflecting the strong co rrelation between laminar burning velocities and H+OH radical concentration s for present test conditions. (C) 2001 by The Combustion Institute.