Autoignition and burning rates of fuel droplets under microgravity

This paper presents a mathematical model for the unsteady evaporation, ignition, and combustion of isolated fuel droplets under microgravity. The model consists of a large structured system of differential algebraic equations, where the numerical complexity is due both to the stiff nature of the kinetic mechanism and to the flame structure around the droplet. A very general, detailed kinetic scheme, consisting of ∼200 species and over 5000 reactions, is used to describe the gas-phase combustion of different fuels. Several comparisons with experimental measurements, carried out under various operating conditions, confirm that the proposed model is a useful tool for characterizing low-temperature and high-temperature ignition delay times. The predicted explosion diagrams of n-alkanes, as well as their ignition delay times, agree with the experimental measurements; the various oxidation regions are closely reproduced too. In addition to this, recent experimental results, relating to the influence of the initial diameter on droplet burning rates in cold and hot environments, are also presented and discussed. Lastly, an analysis of the extinction diameters for the combustion of n-heptane droplets allows a discussion of the role of radiative heat transfer, as well as further emphasizing the importance of the low-temperature oxidation mechanisms.