Cool flames in droplet combustion

Anomalous combustion of n-heptane droplets burning in microgravity conditions was recently experimentally observed by Nayagam et al. [Combustion and Flame, 159 p. 3583–3588 (2012)]. In particular, after a first radiative extinction (i.e. the visible flame ceases to exist), relatively large, spark-ignited n-heptane droplets continue to strongly vaporize (according to the well-known squared-law) for an extended period, ending in a secondary extinction at a finite droplet diameter. The hypothesis proposed by Nayagam et al. was that this second-stage vaporization is sustained by a low-temperature, soot-free, ‘‘cool-flame’’ heat release. In this paper we applied a numerical model describing the combustion of isolated fuel droplets in microgravity conditions to explore the feasibility of such hypothesis. The numerical model solves the unsteady transport equations of mass, momentum species and energy, both for the droplet and the gas phase, assuming a spherically symmetric domain. Detailed transport properties are accounted for both the phases (liquid and gas) and the droplet/gas interface is described assuming thermodynamic equilibrium. Radiative heat transfer is accurately modeled in order to correctly capture the first-stage (radiative extinction), under the hypothesis of gray gases. A detailed kinetic scheme with ∼150 species, accounting for the lowtemperature mechanism, is adopted. The resulting model consists of a large, structured system of differential algebraic equations, with numerical complexity due both to the stiff nature of the kinetic mechanism and to the flame structure around the droplet. The numerical results confirmed the hypothesis proposed by Nayagam et al.: after the first-stage ignition, a cool flame at ~700 K persists around the droplet for a long period of time (depending on the initial diameter of the droplet), resulting in a vigorous vaporization. The comparison with the experimental measurements is very satisfactory: both the extinction diameters are captured with reasonable agreement and the vaporization rate is correctly predicted. A couple of possible long-term implications of these results can be mentioned: i) the possibility to use the cool-mode combustion of individual droplets may lead to entirely different design concepts of spray burners; ii) since the cool-mode persists after the hot-flame extinction, safety procedures based only on considerations of hot flames may become inadequate for assuring safety under all conditions.