An Experimental study was made of the combustion of single suspended droplets a pure light oil and of water-in-light oil emulsions in the high pressure environments.
Variations of droplet size with time and instantaneous temperature in droplet center are measured at pressure range of 1-10 atm.
Droplets were suspended on a quartz fiber filament and ignited using a resistance coil ignitor.
Combustion phenomena were photographed using high speed camera at 100 frames per second and instantaneous temperature was detected by fine thermocouple wire.
The obtained data showed that the emulsified fuel droplets underwent a more disruptive burning, called microexplosion, at atmospheric pressure. However, when the environment pressure increases over 5 atm, the overall microexplosion does not occur, but weak disruption termed puffing phenomena is observed.
Overall-microexplosion occurs when the droplet center temperature is about 250℃ in atmospheric pressure. The limit of superheat of water is 250℃ in same pressure.
The puffing phenomena observed in high pressure is explained as follows; The flame-standoff-distance decreases with increasing pressure. It means that the flame-front is closer to the droplet surface as the environment pressure increases. Therefore, the heat transfer rate from the flame-front to the droplet is larger. It results that water microdroplets near the droplet surface are locally superheated and make relatively weak disruption before overall droplet is superheated totally enough to occur overall microexplosion.
As the water content in the droplet increases, the average of the droplet center temperature is lower. This trend of decreasing droplet temperature with the increasing water content is due to heat loss effect of water. This result therefore gives the decrease in the emission of soot by the suppressed thermal cracking reaction.
