Markstein numbers and unstretched laminar burning velocities of hydrogen-air flames

Effects of positive flame stretch on the laminar burning velocities of hydrogen-air flames at atmospheric pressure were studied both experimentally and computationally. Measurements and numerical simulations considered outwardly-propagating spherical laminar premixed flames at both stable and unstable preferential-diffusion conditions with fuel-equivalence ratios in the range 0.3-5.0, Karlovitz numbers in the range 0-0.5 and laminar burning velocities in the range 100-2700 mm/s. For these conditions, both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. Markstein numbers were in the range -1 to 6, with unstable/stable preferential-diffusion conditions observed at fuel-equivalence ratios below/above 0.7, respectively. Present unstretched laminar burning velocities were in reasonably good agreement with other stretch-corrected determinations of laminar burning velocities, but these values generally are lower than other measurements in the literature, particularly for fuel-rich conditions where Markstein numbers are relatively large. Predicted and measured unstretched laminar burning velocities and Markstein numbers were in fair agreement, using chemical reaction mechanisms and rates due to either Yetter et al. (1991) or Wang and Rogg (1993); nevertheless, additional development of the mechanisms is needed, particularly in order to improve the agreement between predictions and measurements at fuel-rich conditions.