LAMINAR BURNING VELOCITIES AND MARKSTEIN NUMBERS OF HYDROCARBON AIR FLAMES/

Effects of positive flame stretch on the laminar burning velocities of hydrocarbon/air mixtures were studied experimentally using outwardly propagating spherical flames. The test conditions included propane, me thane, ethane, and ethylene-air flames at various fuel-equivalence rat ios and normal temperature and pressure. Karlovitz numbers generally w ere less than 0.3 so that the flames were remote from quenching condit ions. Within this range, the ratio of the unstretched (plane flames) t o stretched laminar burning velocities varied linearly with Karlovitz numbers, yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. In addition, Markstein num bers varied in a roughly linear manner with fuel-equivalence ratios ov er the range of the measurements, which were somewhat removed from fla mmability limits where behavior might differ. Effects of stretch were substantial: Markstein numbers varied from -2.5 to 7.2, yielding corre sponding laminar burning velocity variations of 0.4-2.7 times the valu e for an unstretched (plane) flame over the test range. The ranges of fuel-equivalence ratios for unstable preferential-diffusion conditions (negative Markstein numbers) were as follows: propane, greater than 1 .44; methane, less than 0.74; ethane, greater than 1.68; and ethylene, greater than 1.95. Fuel-equivalence ratios for maximum flame temperat ures and laminar burning velocities are near unity for the present fla mes; therefore, neutral preferential-diffusion conditions are shifted toward fuel-equivalence ratios on the unstable side of unity, in quali tative agreement with recent approximate theories treating the effects of stretch on laminar premixed flames.