Combustion and Flame 143 (2005) 211–226 www.elsevier.com/locate/combustflame Autoignition and burning rates of fuel droplets under microgravity A. Cuoci, M. Mehl, G. Buzzi-Ferraris, T. Faravelli, D. Manca, E. Ranzi ∗ Dipartimento di Chimica, Materiali e Ingegneria Chimica, Politecnico di Milano, 20133 Milano, Italy Received 10 May 2004; received in revised form 2 June 2005; accepted 3 June 2005 Available online 20 July 2005 Abstract 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 equa- tions, 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 oxida- tion 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.  2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Fuel droplets; Microgravity; Burning rates; Detailed kinetics; Autoignition 1. Introduction Because of their high-energy density per unit vol- ume, the combustion of liquid fuels is of great interest in many practical applications from industrial burners to diesel engines. The characterization of this process is complicated by the strong interactions between sev- eral physicochemical processes dominating the igni- tion of fuel droplets. In fact, it is necessary not only to * Corresponding author. Fax: +39 02 7063 8173. E-mail address: eliseo.ranzi@polimi.it (E. Ranzi). account for the heating of the liquid droplet, its evap- oration, and finally the ignition delay time of the fuel vapor, but also for the interactions between the differ- ent droplets of a spray. It is thus useful to take apart the overall system and to study simpler and possibly ideal conditions. The chemistry of the ignition and autoignition of the pure components and their mix- tures is only a first step toward a proper understanding and the optimal design of various combustion devices and chambers. Higher combustion efficiencies offer a way forward for sustainable development and eco- nomic use of fuels, while reducing the production of greenhouse gases, particularly CO 2 . The formation 0010-2180/$ – see front matter  2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2005.06.003