Droplet combustion in a turbulent, elevated-pressure environment

Abstract

Despite the enduring popularity of single-droplet vaporization studies, few researchers have systematically examined the influence of turbulence on droplet burning dynamics. Existing investigations have looked exclusively at large droplets or porous spheres while utilizing thermally conductive suspension schemes. To further understand how turbulence affects normal-gravity droplet burning, single droplets of heptane were suspended at the center of a fan-stirred chamber on a horizontal microfiber, rapidly ignited, and burned to completion. The experimental conditions were parametrically varied across 112 unique combinations of initial diameter, ambient pressure, turbulence intensity, and background oxygen content. The primary quantity of interest is the burning rate, and how individual and average burning rates are affected by the various parameters. To help interpret the results, the radiant soot emission was recorded alongside the temporal evolution of the droplet diameter. The burning rates of droplets in the super-millimeter range are up to 32% lower than those collected in otherwise identical conditions but with large fiber suspenders. Turbulence has little effect on the droplet burning rate until the ambient pressure is elevated. In these cases, turbulence initially augments the burning rate until a critical turbulence level is reached, after which the burning rate quickly falls. The reduction in the burning rate corresponds to the reoccurring appearance of temporary luminous extinction (TLE), where the hot incandescent region that normally surrounds the droplet disappears for a short period, thus tempering the overall burning rate. The cause of, and behavior during, TLE is contrasted with similar phenomena from the literature. Smaller, sub-millimeter droplets behave in largely the same manner, but with lower peak burning rates and greater run-to-run variation. Modest increases to the background oxygen content, from the baseline 21% up to 25% and 30%, delay the onset of TLE to higher turbulence levels. At the highest pressures, turbulent droplet burning rates of the oxygen-enriched cases can double their counterparts in ambient oxygen levels—a synergistic effect with turbulence playing a critical role.