Discussion of likely causes of possible variation of the effective ambient environment during quasi-steady droplet combustion supported by cool-flame chemistry

Droplet-combustion experiments with normal-alkane fuels, carried out onboard the International Space Station, have shown that, after radiative extinction of a droplet burning with an ordinary hot flame, often quasi-steady burning is initiated and sustained by a chemical-kinetic mechanism associated with cool flames. This second, cool-flame combustion stage occurs in a partial-burning regime in which both fuel and oxygen leak through a reaction zone that is controlled by the low-temperature chemistry of cool flames. This second stage persists for an appreciable period of time, after which it is terminated by cool-flame extinction, often at a measurable non-zero droplet diameter. This extinction diameter depends on the fuel and on the composition of the ambient atmosphere. Experiments involving large fuel droplets of different initial diameters indicate that the value of this cool-flame extinction diameter may vary with the initial droplet diameter and with the ignition energy imparted by the hot-wire igniters, even though the cool flame lies well within the region described as the inner quasi-steady zone, where flame structures are independent of initial conditions. These observations imply that the effective ambient atmosphere during the second stage likely depends on the previous combustion history. In the present contribution, evaluations are reported concerning possible influences of the ignition process, of the first-stage hot-flame combustion, and of prior cool-flame combustion on the effective atmosphere in which the second-stage combustion and extinction occur. It is concluded that analysis of influences of fuel leakage and condensation may be needed.