Pyrolysis effects during high-temperature vaporization of alkane droplets

Abstract

The temporal evolution of the droplet radius is measured experimentally in high-temperature inert atmospheres for three different alcohols (ethanol, n-butanol, and glycerol) and three alkanes (n-heptane, n-dodecane, and n-hexadecane). It is shown that, while accompanying theoretical predictions of droplet-radius variations show excellent accuracy for the three alcohols, the three alkanes exhibit vaporization rates that are significantly smaller than those predicted theoretically. The accompanying observation of significant soot formation suggests that endothermic fuel pyrolysis may be responsible for the diminished vaporization rate. The quantification of this phenomenon is investigated here using a one-step irreversible reaction with an Arrhenius rate to model the fuel decomposition. It is shown how an analytical description developed on the basis of activation-energy asymptotics can be used in combination with the experimental measurements of the temporal droplet-radius evolution to adjust the fuel-pyrolysis kinetics, embodied at leading order in an effective pyrolysis temperature, which is obtained for n-heptane, n-dodecane, and n-hexadecane.