COMPARISON OF THE FLAME SPREADING BEHAVIOR OF OIL-IN-WATER AND WATER-IN-OIL EMULSIONS ALONG LINEAR EMULSION DROPLET ARRAYS IN A MICROGRAVITY ENVIRONMENT

We conducted an experimental study of flame spreading behavior along a linear array of emulsion droplets in microgravity using both water-in-oil (W/O) and oil-in-water (O/W) emulsions. The base fuel was n-dodecane. The initial volume fraction of water in the emulsions was 0.2, 0.4, and 0.6. The droplet array consisted of six emulsion droplets and one pure droplet. The initial sizes of the emulsion droplets in the array were identical. Flame stagnation due to burn-out occurred when the droplet spacing was very large. In the O/W emulsion with a higher water content, flame stagnation due to disruptive microexplosions was observed when the initial droplet diameter was very large, and due to extinction when the initial droplet spacing was very small. The flame spread velocity, which was multiplied by the initial droplet diameter, increased with an increase in the droplet spacing, which was normalized by the initial droplet diameter, and it then decreased in each emulsion and at each water content, as observed with pure fuel droplets. The flame spread velocity decreased with an increase in the initial water content. The flame spread velocities of the O/W emulsion were higher than those of the W/O emulsion with a lower water content. The flame spread velocities of the W/O emulsion exceeded those of the O/W emulsion with a higher water content. The normalized droplet spacing, where the flame spread velocity peaked and spread behavior changed, was almost identical in both types of emulsion and at all cases of water content. Flame acceleration in the O/W emulsion was due to the early occurrence of disruptive microexplosions, although the probability of their occurrence was low.