Combustion characteristics of small isolated methanol droplets

The paper presents a transient, axisymmetric numerical simulation of combustion of methanol droplets in a nearly quiescent environment (with ambient temperature of 1200 K, ambient pressure of 1 atm., and Reynolds number of 10-2). The focus is on the combustion process of droplet sizes applicable in a practical spray. A gradual transition from kinetically controlled combustion to diffusion controlled combustion is found with the increase in droplet sizes. Droplets smaller than ≈ 60 μm are influenced more by chemical kinetics due to the reaction zone broadening and a concomitant reduction in flame temperatures. In this kinetically controlled regime, the burning rate of an individual droplet first increases to a maximum, and then decreases. In this regime, the average of droplet burning rate over its life time is bounded by an upper bound of thin flame-sheet burning rate (large Damk̈ohler number limit), and a lower bound of pure evaporation rate (small Damk̈ohler number limit). The actual average burning rate lies between the two values, determined by the droplet size. For droplets smaller than ≈ 60 μm, the flame stand-off ratio varies throughout their lifetime. The analysis underlines the importance of accurate chemical kinetics modelling for small droplets. Copyright © 2012 by ASME.