Influence of drop spacing on burning of an emulsified-drop string

Combustion characteristics of a water-in-dodecane emulsion drop string with various initial spacings were studied experimentally by using a free-falling drop burning apparatus. The initial drop spacing of the drop string was considered to be 5, 10, 40, and 70 times the initial drop diameter. The oxygen concentration was fixed at 21%, while two drop sizes, 550 and 450 μm, and three water contents, 5%, 20%, and 30% in volume, were compared. Results showed that the transition of the drop flame occurred for all cases in the experiment. Along the flow direction, the flame of a drop changed from a blue spherical flame to a yellow flame, to a wake flame, and finally was extinguished in the downstream region. Additionally, drop expansion was observed in all cases, while microexplosion only occurred in the downstream region, as d_i = 450 μm with β = 5% and 20%. It was also shown that there existed an optimum circumstance (an optimum spacing) for supporting stronger drop burning for the emulsion drops, similar to previous study on the pure drops. With the aid of microexplosion, the evaporation strength for the emulsion drops could be 2-3 times that for the pure drops at this optimum condition.