Cool-flame extinction during n-alkane droplet combustion in microgravity

Recent droplet-combustion experiments onboard the International Space Station (ISS) have revealed that large n-alkane droplets, following radiative extinction of the visible flame, can continue to burn quasi-steadily in a low-temperature regime, characterized by negative-temperature-coefficient (NTC) chemistry. In this study we report experimental observations of n-heptane, n-octane, and n-decane droplets of varying initial size burning in oxygen/nitrogen, oxygen/nitrogen/carbon dioxide, and oxygen/nitrogen/helium environments at pressures from 0.5 to 1.0 atm, with oxygen concentrations from 14% to 25% by volume. These large n-alkane droplets exhibited radiative extinction of the hot flame, followed by quasi-steady low-temperature burning, which terminated with diffusive extinction accompanied by the formation of a vapor cloud, while small droplets did not exhibit radiative extinction but instead burned to completion or disruptively extinguished. Results for droplet burning rates in both the hot-flame and cool-flame regimes, as well as droplet extinction diameters at the end of each stage, are presented. The cool-flame extinction diameters for all three n-alkanes are shown to follow a similar trend as functions of the oxygen concentration, predicted here from a simplified theoretical model that is based on the reaction-rate parameters for the oxygen molecule addition to the alkyl radical and for ketohydroperoxide decomposition.