Investigation of sooting in microgravity droplet combustion

Abstract

This investigation describes experiments performed at the 2.2-s microgravity facility at NASA-Lewis Research Center using n -heptane droplets burning in atmospheric pressure air. The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-field light-extinction technique and subsequent tomographic inversion using Abel transforms. It has been speculated that under microgravity conditions, the absence of buoyancy and the effects of thermophoresis create a situation in which high concentrations of soot accumulate into a soot cloud. This study presents the first quantitative measurements of the degree of sooting for microgravity droplet combustion. Results indicate that the soot concentrations for microgravity heptane droplet flames (with maximum soot volume fractions ≈60 ppm) are significantly higher than corresponding values that are reported for normal-gravity flames (which are typically ∼1 ppm). Since the accumulated soot represents incomplete combustion and can also modify the heat-transfer mechanism by altering the local temperature distributions within the fuel-rich region, sooting effects can significantly influence the burning behavior under microgravity conditions. Experiments were also performed to asses the droplet-size-dependent effects on the sooting behavior. Initial experiments using 1.0 and 1.75 mm initial diameter droplets indicate that while the distribution of soot volume fractions are comparable for the two cases (for the observation times that were available), the ratio of the instantaneous mass of soot contained within the fuel-rich region for the 1.75-mm droplet compared to the 1.0-mm droplet was more than a factor of 3. This ratio is also expected to increase for longer observations.