Developing Surrogates for Liquid Transportation Fuels: The Role of Spherically Symmetric Droplet Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 385

Fig. 12 Upper row of images are taken from self-illuminated color videos, arranged from highest
to lowest intensity. Numbers denote the initial droplet diameters in mm. The images were selected
from each sequence for maximum qualitative luminosity. Lower row of images are backlighted
video that show soot dynamics; the images were selected for the thickest soot cloud in the burning
sequence. The indicated sooting trend is based on a qualitative assessment of the fl flame image

intensity (Liu and Avedisian 2012 ). The red box is a reference for gasoline

For suf ciently large D o , the fl flame will extinguish owing to radiation (Chao

et al. 1990 ). The ame temperature drops

below that needed to sustain the combustion process. Figure 13 shows an example
of the dynamics of this process for an n-decane droplet studied in the MDCA where
an oscillatory behavior is noted. The mechanism for this process is not fully
understood, though it may be related to droplet size and motion.
As burning progresses, the droplet diameter decreases and the

fl flame diameter becomes so large that the fl

flame is reduced
perhaps for chemical kinetic reasons. Continued reduction of temperature will
extinguish the

fl

flame. When the droplet moves (even slightly), it enters a fresh
oxidizing ambience on the front side, while on the back side, the gas is depleted of
oxygen. The

fl

flame is then hottest on the front, while on the opposite side, it is
colder and extinction can occur locally as manifested by a hole appearing in the
fl

fl

flame that propagates around the droplet as oxygen is used up. As the extinction
boundary moves to the front of the droplet, it enters a region of gradually enriched
oxygen concentration and higher temperature due to the ame recedes

to the back side as combustion is re-established, thereby completing one cycle of an
oscillation. This sort of oscillatory phenomenon has often been associated with
“

fl ow and the fl

cool fl ame ” phenomena (Nayagam et al. 2012 ; Farouk and Dryer 2013 ; Dietrich

et al. 2014 ).