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The study aims to visualize droplet behaviors of emulsified fuel in secondary atomization in a flame stabilized in a laminar counterflow field. To study secondary atomization characteristics in the flame, direct photography of the flame using a color high-speed video camera (7,000 fps) and magnified shadow imaging of spray droplets using a monochrome high-speed video camera (180,064 fps) were used. Frequencies of secondary atomization in the flame were also discussed. As a result, "bright spots," which were of similar or higher luminosity than the surrounding area were observed by direct photography around the luminous flame of the emulsified fuel, whereas the frequency of occurrence of "bright spots" was almost negligible when n-dodecane was used. Observations indicated that the droplet flame rapidly expanded. This expansion was linked to secondary atomization phenomena such as puffing (i.e., vapor eruption from the droplet surface), partial micro-explosion (i.e., where a large portion of a droplet bursts), and micro-explosion (i.e., where the entire droplet bursts), which were visualized in the flame by magnified shadow imaging. It was suggested that secondary atomization caused rapid evaporation and spread of fuel vapor. The magnified shadow imaging technique provided a clear description of droplet behavior in the flame. Numerous puffing and micro-explosion were observed regardless of the flame emissions. In addition, it was observed that vapor blowout in secondary atomization accelerated droplet velocity, and led to the random movement of spray droplet. It was shown that the frequencies of secondary atomization increased in the downstream region where the luminous flame was formed. Secondary atomization occurred in droplets of various sizes in the downstream region, whereas it occurred in only small droplets in the upstream region where the luminous flame was not formed.

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