Combustion of n-Butanol, Gasoline, and n-Butanol/Gasoline Mixture Droplets

Fuels derived from biofeedstocks are receiving attention for their potential as additives to conventional petroleum-based transportation fuels. Normal butanol, in particular, can enhance performance compared to ethanol because of its higher energy density. To better understand the combustion dynamics of n-butanol in the context of gasoline, experiments are reported here to examine the isolated droplet combustion characteristics of an 87 octane (ethanol-free) gasoline and a mixture of gasoline (0.9, v/v) and n-butanol (0.1, v/v, B10), along with n-butanol. The experiments are performed in an ambience that minimizes convection and promotes spherical droplet flames. The initial droplet diameters range from 0.52 to 0.63 mm, and the experiments are carried out in room-temperature air at normal atmospheric pressure. Measurements of the evolution Of the droplet diameter show that butanol and B10 droplets have burning rates that are almost identical to gasoline, even though other features of the burning process, such as soot formation and the relative position of the droplet and flame, are quite different. With butanol mixed with gasoline, the mixture flames are comparatively closer to the droplet than for gasoline droplets. A scale analysis is developed that expresses the droplet burning rate in terms of temperature-dependent properties. The results support the experimentally observed similarity of burning rates for butanol, gasoline, and their mixtures, even though soot formation is neglected.