Droplet characteristics and local equivalence ratio of reacting mixture in spray counterflow flames

Abstract

Spray combustion was studied by injecting monodisperse 125 or 200μm ethanol droplets in a premixed natural gas fuel flame flowing against an opposed heated air jet. Phase Doppler anemometry and chemiluminescence measurements allowed to characterise both droplet parameters and the local reacting mixture. Both types of droplets crossed the flow stagnation plane and entered the opposite jet. However, 125μm droplets reversed their motion and oscillated around the stagnation plane, leading to increased droplet residence time in hot regions. The fuel vapour released by droplets close to the stagnation plane, mixed with the surrounding air and led to the ignition of a second fuel vapour flame below the natural gas flame. For 125μm droplets, mean local equivalence ratio of the fuel vapour flame was about 0.8, suggesting lean-premixed combustion; whereas 200μm droplets led to stoichiometric combustion. “Group Combustion” number was estimated from measurements and suggested that 125μm and 200μm droplets burned in different “Group Combustion” regimes.