Water-coalescence in an oil-in-water emulsion droplet burning under microgravity

Abstract

An experimental study was performed to obtain the detailed information needed to provide a deep understanding of the combustion process and the secondary atomization of an oil-in-water emulsion droplet. The experiments were conducted by using the drop shaft of JAMIC (Japan Microgravity Center) at Hokkaido. The oil-in-water emulsion, which consisted of n -hexadecane as a base fuel, distilled water, and a trace of surfactant was tested. Photographic observation and temperature measurements were made of the suspended emulsion droplet during the heating-up and combustion processes under microgravity. The primary attention was toward the phase separation in the droplet, and the time histories of droplet temperature and the amount of water in the droplet, during the period of time prior to disruptive microexplosion. The results showed that the separation of the base fuel and water as well as their agglomeration and coalescence occurred with the lapse of time. The increase in the droplet temperature resulted in phase separation, and the formation of a single water droplet enveloped by a shell of the base fuel, prior to the microexplosion. The volumes of the base fuel and the water in the droplet were estimated from the obtained droplet images. After the phase separation, selective evaporation of the base fuel occurred and the volume of the base fuel decreased, while the water volume did not change. The effects of the emulsion properties on the onset rate of microexplosion were also revealed by using statistical analysis.