Experimental studies on the flame structure in the wake of a burning droplet

Abstract

Droplets burn in envelope flames at low approach velocities and in wake flames at high velocities. Detailed measurements on pentane burning on a porous sphere have shown that the wake portion of the envelope flame is quite different from the wake flame. The former has a near wake of a few diameters, which resembles a gaseous laminar diffusion flame, followed by a long far wake in which soot burns. The latter has a short near wake which is similar to the flame zone behind a flame holder, except for the lack of a recirculation zone, followed by a short zone of soot combustion. The burning rate for the envelope flame is about three times greater than the wake flame at a slightly higher approach velocity. Peak temperatures are about 1400°–1500°K, located near the droplet in the wake flame, but in the far wake in the envelope flame. Radiation from envelope flames is far greater than from wake flames; peak emittances are in the ratio of about 3.5 to 1. The contribution of soot to droplet flame radiation is negligible in the near wake, and approaches gas radiation in the far wake. Peak soot concentrations in the envelope flame are more than three times greater than in the wake flame.