SIMPLE DESCRIPTION OF THE COMBUSTION STRUCTURES IN THE STABILIZATION STAGE OF A SPRAY JET FLAME

Abstract

The structure of a two-phase flame has been investigated within its stabilization region in the nearfield of a spray jet. The spray is produced by a coaxial air-blast injector fed with liquid methanol. We focus on a specific structure of the two-phase flame, observed experimentally where the flame presents a double structure with a predominant diffusion character for each reaction zone. The analysis is justified from experimental results of phase Doppler anemometry and planar laser-induced fluorescence of OH. The dynamics of the spray obtained from phase Doppler velocimetry are studied in terms of size classes defined from the Stokes number. The size classification shows that, where the flame stabilizes, the spray is composed of two fluids, one with high inertia (high Stokes number), the other characterized by a low Stokes number. The structure of the two-phase flame is analyzed in the low-inertia part of the spray. The emphasis is put on a regime where tau(ch) < tau(vap) < tau(mix); a double structure may develop in what we called the vaporization regime, since droplets carl cross reaction zones. Such a double structure has been predicted by Continillo and Sirignano [25] and by Greenberg and Sarig [26, 27] through numerical modeling of a hue-phase counterflow flame. The present article gives an experimental confirmation of a real occurrence of such a flame structure in turbulent spray jets and proposes a simplified description in the low-inertia part of the spray and for the flame sheet approximation.