Combustion Processes Laboratories

Measurements of line-of-sight laser extinction in a co-annular ethylene-air laminar inverse diffusion flame (IDF) were made to determine soot concentration. Extinction has frequently been used in the literature to measure soot concentration in normal diffusion flames (NDFs), but it has rarely been applied to IDFs. A coflow IDF contains a primary air flow surrounded by a fuel annulus. Soot particles form on the outside of IDFs, advect upward, and eventually quench without being oxidized. It has been proposed in the literature that IDFs will produce less near-flame soot than NDFs because, for flames of comparable fuel, size and flow rates, movement of soot outward into cool regions of an IDF limits its simultaneous exposure to the high temperatures and fuel pyrolysis products needed for soot growth. A two-dimensional soot concentration map of an IDF using experimental data confirms this hypothesis by showing integrated soot volume fractions to be an order of magnitude lower than those reported for NDFs in the literature. Computer simulations of particle temperature histories in an NDF and IDF of similar height lend support to these results.