Theory of laminar flame propagation in near-stoichiometric dilute sprays

Abstract

The structure and propagation of a steady, one-dimensional planar, low-speed flame in a dilute, monodisperse, near-stoichiometric spray, with bulk gas-phase burning, upstream droplet vaporization and downstream droplet vaporization/combustion, are studied through the classification of burning structures such as slightly lean, slightly rich, and transition from lean to rich sprays, and using the matched asymptotic techniques in the limit of large activation energy. Results show that the flame speed of a slightly rich spray may be less than that of a gaseous premixture if the liquid droplets are large enough, but at decreased droplet sizes, the flame speed of the rich sprays may be increased to a value higher than that of the premized one. The optimum flame speed exists at a special condition of prevaporized burning, which has a stoichiometric gas-phase mixing. Furthermore, the flame propagation rate of a slightly lean spray increases with decreasing liquid fuel loading and decreasing initial droplet size, while the opposite holds for a slightly ric spray. The transition between slightly lean and slightly rich sprays is smoothly achieved through the variations of the initial droplet size and the mass fraction of liquid fuel in the study.