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

• 125 μm Droplets oscillate around the stagnation plane, 200 μm droplets do not.
• Both droplets sizes lead to a second fuel vapour flame below the main gas flame.
• Second flame equivalence ratio is 0.8 with 125 μm droplets, 1 with 200 μm droplets.
• 125 μm and 200 μm Droplets burn in different group combustion regimes.

Spray combustion was studied by injecting monodisperse 125 or 200 μm 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, 125 μm 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 125 μm droplets, mean local equivalence ratio of the fuel vapour flame was about 0.8, suggesting lean-premixed combustion; whereas 200 μm droplets led to stoichiometric combustion. “Group Combustion” number was estimated from measurements and suggested that 125 μm and 200 μm droplets burned in different “Group Combustion” regimes.