A theoretical and experimental study of preferential-diffusion/stretch interactions of laminar premixed flames.

Abstract

Recent work shows that preferential-diffusion/stretch interactions of laminar premixed flames are sufficiently robust to affect the stability of practical strongly-turbulent flames. In addition, past measurements of laminar burning velocities should be re-assessed because there generally was no attempt to control flame stretch. Finally, the sensitivity of laminar premixed flames to stretch (represented by the Markstein number) should be studied to better understand and model the properties of laminar premixed flames. Motivated by these considerations, an experimental and computational study of preferential-diffusion/stretch interactions for laminar premixed flames, for both alkane/alcohol-fuel-vapor-fueled flames (as practical fuels) and hydrogen-fueled flames (considering diluent-variation effects) was carried out during the present investigation. Considering outwardly-propagating spherical laminar premixed flames, laminar burning velocities of fuel-vapor/oxygen/nitrogen flames and hydrogen/oxygen/diluent (nitrogen, argon or helium) flames were measured for various values of stretch, fuel-equivalence ratios (0.6--4.5) and pressures (0.3--3 atm). The measurements were reduced to find fundamental unstretched laminar burning velocities and Markstein numbers. The measurements were also used to evaluate corresponding numerical simulations of the experimentally-observed flames, based on contemporary detailed H₂/O₂ reaction mechanisms. Both measured and predicted ratios of unstretched to stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding a constant Markstein number for a particular reactant mixture. The present flames were very sensitive to flame stretch (i.e., they had large Markstein numbers with significant ratios of unstretched to stretched laminar burning velocities) for levels of flame stretch well below quenching conditions. Increasing flame temperatures tended to reduce flame sensitivity to stretch while increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior. At low pressure conditions, nitrogen-/argon-diluted hydrogen flames exhibited negative Markstein numbers at fuel-lean conditions, which is consistent with classical preferential-diffusion ideas, whereas helium-diluted hydrogen flames exhibited only positive Markstein numbers over the whole measured fuel-equivalence ratio range due to stabilization of flame properties by strong thermal diffusion effects. Predicted and measured flame properties exhibited encouraging agreement using contemporary reaction mechanisms. Finally, flame structure predictions suggest that H/OH radical production and transport is an important aspect of preferential-diffusion/stretch interactions, reflecting the strong correlation between laminar burning velocities and H+OH radical concentrations for present test conditions.