Multi-droplet combustion in the presence of homogeneous and heterogeneous atomizations

The effects of heterogeneous and homogeneous atomizations on combustion dynamics and flame spread rates of multi-droplets are experimentally investigated. The utilized fuel is biodiesel, which is doped with graphene oxide and blended with ethanol. Stable fuel suspensions are prepared. Either one droplet is positioned at the intersection of two thin fibers or an array of three droplets are positioned along a horizontal line and at the intersections of fibers. Simultaneous and time-resolved flame chemiluminescence and shadowgraphy techniques are utilized for imaging the flame and droplets, respectively. It is observed that, for a single droplet combustion of biodiesel, relatively low frequency (about 10 Hz) dynamics appear in the second half of the droplet life-time, which is due to heterogeneous atomizations. Blending with ethanol leads to homogeneous atomizations with dynamics featuring frequencies smaller than 50 Hz and at the beginning of the droplet lifetime. It is reported that the homogeneous atomizations lead to at least three times larger root-mean-square oscillations of the flame chemiluminescence compared to that for heterogeneous atomizations. For all tested multi-droplet configurations, independent of the graphene oxide doping concentration and blending with ethanol, the most dominant frequency of flame chemiluminescence oscillations is about 10-15 Hz. It is observed that the heterogeneous atomizations lead to a flame spread rate of about 6-8 mm2/s, with largest values reported for doping with moderate concentration of graphene oxide. Compared to that for heterogeneous atomizations, the flame spread rate for homogeneous atomizations are significantly larger 10-40 mm2/s, are not sensitive to graphene oxide doping concentration, and feature a positive relation with the root-mean-square of the flame chemiluminescence oscillations.