Measured and predicted properties of laminar premixed methane/air flames at various pressures

Abstract

Effects of positive flame stretch on the laminar burning velocities of methane/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements based on motion picture shadowgraphs, and numerical simulations based on typical contemporary chemical reaction mechanisms, were used to find the sensitivities of the laminar burning velocities to flame stretch, characterized as Markstein numbers, and the fundamental laminar burning velocities of unstretched flames. Reactant conditions included methane/air mixtures having fuel-equivalence ratios of 0.60–1.35 and pressures of 0.5–4.0 atm, at normal temperatures. Both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied significantly from unity (in the range 0.6–2.3) even though present stretch levels did not approach quenching conditions. Absolute values of Markstein numbers increased with increasing pressure, while the transition from unstable to stable preferential-diffusion conditions with increasing fuel-equivalence ratio shifted from an equivalence ratio of 0.6 at 0.5 atm to 1.2 at 4.0 atm, suggesting increased unstable flame behavior due to preferential-diffusion effects at the elevated pressures of interest for many practical applications. Finally, predictions using two contemporary chemical reaction mechanisms were in reasonably good agreement with present measurements of both Markstein numbers and unstretched laminar burning velocities.