Two kinds of experiment have been conducted to study the effect of the initial droplet diameter on the burning behavior of a single free droplet with a spherical flame. One is performed under microgravity, which is realized in a freely falling chamber to establish the combustion of small droplet under normal gravity, whose initial diameter is about 70 m. The small droplet is injected up vertically and ignited by an electric spark after passing through the apex of its trajectory. Even under normal gravity, for such a small droplet, a spherical flame can be observed through the image intensifier. In this study, the fuel used is n-heptane as sooting fuel.
The time dependence of the squared droplet diameter is categorized into two types with the initial droplet diameter. The so-called d2 law is held. This type can be observed for small droplets in the experiment under normal gravity. It is recognized that extinction occurs around the time when the evaporation constant begins to decrease.
Soot formation tends to be enhanced as the initial droplet diameter is increased, and the no-soot formation can be observed for a small droplet of about 70- m initial diameter.
Hara, H., Kumagai, S., The Effect of Initial Diameter on Free Droplet Combustion with Spherical Flame, Twenty-Fifth International Symposium on Combustion, Combustion Institute, Pittsburgh, PA, pp. 423-430, July 31, 1994.