DROPLET FLAMES IN REACTIVE ENVIRONMENTS

A study of the structure and extinction of envelope flames around a porous sphere burning either methanol or n-heptane in a forced flow of air containing premixed methane or acetylene is described. At low ambient velocities and relatively high ambient fuel concentrations, flame photographs and temperature measurements show a structure containing premixed combustion followed by nonpremixed burning. At higher ambient velocities or low ambient fuel concentrations, the two zones appear to merge. However, the thickness of the reaction zone(s) is greater than that corresponding to the nonreactive ambient case. Measurements indicate that the stability of the envelope flames is enhanced by fuel gases in the ambient. The increase in stretch rates at extinction for acetylene-air mixtures is larger than that for methaneair mixtures. Asymptotic analysis based on high activation energy predicts all of the experimental trends reasonably well. The enhancement in extinction velocity due to fuel gases in the ambient ranges from 10% to 60%. © 1990.