Diffusion and its Effects on Soot Production from the Combustion of Emulsified and Nonemulsified Fuel Droplets

Soot characteristics of emulsified and nonemulsified fuels and the effects of microexplosions on soot production were investigated by analyzing the soot extinction coefficient, burning rate, diffusion characteristics, and natural convection of fuels during droplet combustion. schlieren imaging and laser extinction imaging were used synchronously to perform the experiments. The experiments were conducted under normal atmospheric conditions, and the fuel droplets were supported with glass fiber. The hot boundary results revealed that hot boundary distance (i.e., the distance from the droplet surface to the outermost high-concentration region captured by schlieren imaging) increases as viscous force decreases. Furthermore, laser extinction results revealed that soot production is affected by viscous forces and diffusion. Viscous force was analyzed using the Grashof number, which is a function of buoyancy and viscous forces; the viscous forces in the fuel decreased during the microexplosions. Soot extinction, an imaging mechanism developed to calculate soot thickness (KL) or the soot extinction coefficient (K), revealed that the KL values and peak soot values were reduced by the lower viscous forces and higher hot boundary distances. Higher hot boundary distances indicate that fuel diffusion dominates over air diffusion. Thus, soot generation was revealed to be dependent on fuel diffusion and air diffusion dominance. Soot is minimized at a critical hot boundary distance at which air and fuel diffusion are stable, similar to that under an optimized stoichiometry. In microgravity with no buoyant force, the flame would be extinguished and cool flame would dominate. The results also reveal that Stefan flow area depends on the viscous and buoyant forces. The Richardson number results indicate that natural convection is affected by the microexplosion and changes from natural to mixed convection as water is added to the fuels