Burning times of linear fuel droplet arrays: A comparison of experiment and theory

Abstract

To understand better the effects of droplet interaction on the burning of fuel droplet sprays, the combustion history of single streams of monosized droplets was investigated by both experimental and theoretical methods. Continuous monodispersed streams of fuel droplets were injected into a hot environment where the droplet combustion histories were recorded photographically for spacings ranging from a few droplet diameters to about 30 diameters. Theoretical predictions from a model which describes the quasisteady (QS) combustion of fuel droplet arrays in a quiescent ambience were compared with experimental observations to examine the nature and extent of droplet interaction. Both measured and predicted results show a gradual, but substantial, monotonic increase in burning time with decreasing droplet spacing. For droplet spacing of the order of 2 droplet diameters, extremely strong droplet interaction is indicated by a twofold increase in burning time. Consistent with theoretical predictions, a compilation of data for an extensive range of experimental parameters (2 alcohols, 2 pure hydrocarbons, 3 binary hydrocarbon mixtures, No. 2 fuel oil, 100–300-μm droplets, and 4 ambient conditions) demonstrate that the increase in droplet lifetime due to droplet interaction depends universally on droplet spacing and not on fuel type, droplet size, or ambient conditions. These results also show that the classical “D 2-law,” which states that the surface area of an isolated droplet decreases linearly with time, does not apply rigorously for interacting droplets. Using an ad hoc quasi-steady (QS) “Halo model,” in an attempt to represent the transient nature of droplet interaction, the effect of interaction on droplet lifetime is shown to be mildly dependent on ambient oxygen content.