Flame stretch interactions of laminar premixed hydrogen/air flames at normal temperature and pressure

Effects of positive flame stretch on the laminar burning velocities of hydrogen/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements were based on motion picture shadowgraphy, while numerical simulations were based on typical contemporary chemical reaction mechanisms. Flame conditions studied included hydrogen/air flames having fuel-equivalence ratios in the range 0.3-5.0 at normal temperature and pressure. Both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers over the test range (Karlovitz numbers up to 0.4), 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) roughly 0.7. Present stretch-corrected laminar burning velocities were in reasonably good agreement with other determinations of laminar burning velocities at fuel-lean conditions where Markstein numbers, and thus effects of stretch, are small. In contrast, the stretch-corrected laminar burning velocities generally were smaller than other measurements in the literature at fuel-rich conditions, where Markstein numbers, and thus effects of stretch, are large. Finally, predicted unstretched laminar burning velocities and Markstein numbers were in reasonably good agreement with measurements, although additional study to improve the comparison between predictions and measurements at fuel-rich conditions should be considered.