Effect of Far-field Radiation on Freely-falling Droplet Burning Behavior

Numerous studies have used ground-based, freely falling, isolated droplets to study the combustion behavior of liquid fuels, fuel emulsions, and liquid-solid slurries (Sangiovanni and Labowsky, Combust. Flame, 1982; Choi et al, 23rd Symp. (Int’l.) on Comb., 1990; Lee and Law, Comb. Sci. Tech., 1992). Droplets of mono-disperse size formed repetitively are projected down a duct flow oriented parallel to the gravity vector. Typically, ignition of the individual droplets is achieved by exposure of the droplet stream to post-combustion gases issued from an inverted flat-flame burner and projected downward through the duct. The isolated droplets are subjected to both Stokes drag and body forces, causing a varying gas-drop relative velocity and local buoyancy disturbances of the flame structure surrounding each droplet. A novel method of further reducing both relative convection and buoyancy effects is to incorporate acceleration of the surrounding gases to match that of gravity, resulting in minimal Reynolds number and Grashof number based upon the characteristic length scales of droplet and flame diameter, respectively. A second consideration is that the post flame gas stream may have heat losses that result in centerline temperature gradients of more than 10 K per cm if the duct surfaces are not thermally buffered to reduce heat losses.