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 Damköhler number limit), and a lower bound of pure evaporation rate (small Damköhler 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.

This content is only available via PDF.
You do not currently have access to this content.