Effects of properties of atmosphere diluents on cool-flame combustion of normal-alkane droplets

Droplet combustion experiments carried out in the International Space Station using n-alkane fuels have shown that large droplets, when ignited, first burn with visible hot flames and then extinguish, only to be followed later by quasi-steady cool-flame combustion. In a few of these experiments, some of the nitrogen in the nitrogen-oxygen ambient-gas mixture was replaced by helium, carbon dioxide, or xenon, for n-heptane, n-octane, n-decane, or n-dodecane droplets. Different diluents were observed to exert remarkably different influences on the cool-flame combustion. These initially unexpected differences are summarized here, with explanations offered for their causes. In particular, a simplified theoretical description of cool-flame-supported droplet combustion is employed to predict burning-rate constants and droplet diameters at cool-flame extinction, resulting in good agreement with much of the experimental data but disagreement with certain measurements, for reasons that are explained. The comparisons underscore the important influence of the diluent, especially on the Lewis numbers of cool-flame intermediate species derived from the fuel vapor.