Flame/stretch interactions of premixed fuel-vapor/O-2/N-2 flames

Unstretched laminar burning velocities and flame response to stretch (Marks tein numbers) were measured for outwardly propagating spherical laminar pre mixed flames involving mixtures of hydrocarbon vapors, oxygen, and nitrogen . Experimental conditions consisted of vapors of several liquid fuels (n-he xane, n-heptane, iso-octane, methyl-alcohol, and ethyl-alcohol), concentrat ions of oxygen in the nonfuel gases of 19-33% by volume, pressures of 0.5-2 .0 atm, fuel-equivalence ratios of 0.80-1.60, and reactant mixture temperat ures of 298 +/- 5 K, The present flames were very sensitive to flame stretc h, yielding ratios of unstretched to stretched laminar burning velocities i n the range 0.4-4.0 for levels of flame stretch well below quenching condit ions (Karlovitz numbers less than 0.2), At low pressures, the present hydro carbon vapor flames had positive Markstein numbers at fuel-lean conditions, which is consistent with classical preferential-diffusion ideas. Increasin g pressures, however reduced Markstein numbers and progressively decreased the fuel-equivalence ratio range where Markstein numbers were positive. Neg ative Markstein numbers were associated with the presence of preferential-d iffusion instability as evidenced by the appearance of chaotically distorte d (wrinkled) flame surfaces early during the frame propagation process.