Propagation Velocity and Structure of Flames in Droplet-Vapor-Air Mixtures

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Abstract
-Flame speeds of ethanol and n-octane mixtures of mono-sized droplets, vapor and air have been measured to study the fundamental aspects of spray combustion. The state of the mixtures, which were prepared in an apparatus similar to the Wilson cloud chamber, was experimentally verified in detail. A rugged, undulated and thickened flame front with the cellular structure is peculiar to the flame propagation in the mixtures containing large droplets and is in sharp contrast to the smooth and continuous flame front observed with premixed gas mixtures or mixtures containing droplets of much smaller size. This difference in the flame structure, caused by the heterogeneous nature of the unburned mixture, can explain the observation that the burning velocity in a droplet-vapor-air mixture is larger than in a homogeneous mixture of the same overall fuel-air ratio, even on the lean side of the stoichiometric point, provided that the droplet diameter is large enough. Another aspect of the flame propagation in the droplet-vapor-air mixtures can be interpreted in terms of the effective fuel-air ratio, which accounts for the incomplete evaporation and mixing of the liquid fuel.

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