The combustion of iso-octane droplets with initial diameters from 0.5 to 5 mm: Effects on burning rate and flame extinction

Abstract

The burning characteristics of iso-octane droplets in the standard atmosphere are reported over a large range of initial droplet diameters (0.5mm<Do< 5mm) for near one-dimensional droplet flames as promoted by low gravity. A ground-based drop tower and a space-based platform (International Space Station) were used to provide an environment to examine the influence of Do that encompasses regimes where radiation does not have an effect on burning to where it does. For Do< 2.7mm, the droplets burned to completion without extinction. Larger droplets evidenced extinction due to radiative emissions from the flames to the ambience that produced a two stage burning process. The burning rate (K) was nearly constant in the first stage and then gradually decreased. Concurrently, radiation emissions dropped by two orders of magnitude and ‘flickering’ flames were noted during the transition. After extinction, burning entered a regime where K gradually decreased with time and approached values commensurate with evaporation in a heated environment (i.e., no combustion). Energy balances based on scale analyses related K and flame temperature (Tf) to Do: a balance at the flame including radiation losses led to the scaling K ∼ Do−n in reasonable agreement with the data for Do> 2mm; an energy balance at the droplet surface showed that Tf quickly dropped from ∼1600K to ∼700K after flame extinction, followed by a gradual reduction of Tf to near ambient temperature. The trends in the data are also discussed for potentially revealing a possible low temperature combustion regime.