An experimental study on flame propagation and combustion characteristics of multiple emulsified diesel and biodiesel fuel droplets

To unravel the interaction mechanism between droplets that is relevant for practical liquid fueled burners, the flame propagation characteristics of multi-droplet of emulsified diesel and biodiesel (FAME) fuels were studied using a suspended line droplet array. It was found that when S/d0 ≤ 3 (ratio of droplet spacing to initial droplet diameter), the double droplets were ignited simultaneously and surrounded by a single flame, and as the droplet spacing increased, two independent flames were formed around each droplet. The highest flame propagation rate was attained at S/d0 = 3, which was attributed to the consistent values of flame standoff ratio limited by the dominant heat conduction and finite evaporating rate. The droplet interaction, such as oxygen competition effect, enhanced with smaller droplet distance, can result in the reduction in the droplet combustion rate and flame propagation rate; the latter one, however, showed an increasing trend when small droplet or the size non-uniformity of droplet cloud was considered. In addition, the child droplet ejected from the emulsified droplets showed three burning modes and was an important factor promoting the flame spread, and the results showed that compared with the FAME emulsified fuels, the child droplets in diesel cases were more prone to ignition, expanding the burning regime.