Theory of laminar flame propagation in off-stoichiometric dilute sprays

The structure and propagation of a steady, one-dimensional planar, low-speed flame in a dilute, monodisperse, sufficiently off-stoichiometric and weakly-heterogeneous spray, with bulk gas-phase burning, upstream droplet vaporization and downstream droplet vaporization/combustion, is analyzed using activation energy asymptotics. A prevaporized mode and a partially prevaporized mode of flame propagation are identified. Results show that lean and rich sprays exhibit qualitatively opposite behavior in response to the extent of mixture heterogeneity; specifically, the burning intensities of lean and rich sprays are respectively reduced and enhanced with increasing liquid fuel loading and increasing initial droplet size. Classification of all possible spray burning modes as a function of the mixture stoichiometry and initial droplet size is also presented.