Topological transitions of Jet A-1 lean azimuthal flames (LEAF)

Abstract

Prior studies about liquid fuel combustion in a vitiated air environment have shown increased combustion efficiency with reduced NOx, CO, and soot emissions. The concept of lean azimuthal flame (LEAF), which can be associated to the latter combustion mode, is based on opposed injections of air and liquid fuel sprays in an axisymmetric chamber with a central outlet, which can result in a highly turbulent toroidal reaction zone. The mixture of fresh air and hot combustion products of each spray provides a vitiated cross-flow configuration to the next spray distributed along the chamber circumference, leading to ignition and sequential combustion of the sprays by the others. The present paper deals with a LEAF combustor with air-assisted spray atomization, which has not been investigated so far. The combustor is fueled with Jet A-1 and operated from 15 to 25 kW with variations in the atomization-air to liquid mass flow ratio (ALR). This study focuses on the flame topology transitions as a function of atomizer ALR. Experimental results based on flame chemiluminescence and OH planar laser-induced fluorescence show two flame topologies: tubular and LEAF topology for ratio of 2 and 4, respectively (denoted ALR2 and ALR4). The spray Mie scattering indicates a significant presence of unburnt droplets for ALR2, whereas quick evaporation is observed for ALR4 cases. In this paper, we propose and validate a basic model based on the spray droplet size distribution, and the evaporation and convection timescales, which are the prominent factors governing the flame topology. Indeed, for ALR2, the evaporation timescale is longer than the convective timescale, which causes incomplete spray evaporation and insufficient vitiated environment, leading to a tubular flame topology and preventing a LEAF to develop. In contrast, for ALR4, the spray evaporation timescales are smaller than the convective timescales, which aids the LEAF topology.