Cool flames are commonly associated with engine-knock phenomenon in spark ignition engines and autoignition in diesel engines, and results from low temperature partial oxidation of the fuel air mixture that eventually leads to hot flame ignition. However, the possibility of a cool flame supporting quasi-steady combustion of a fuel droplet has never been speculated, let alone been experimentally observed. Recent microgravity droplet combustion onboard the International Space Station (ISS) under the Flame Extinguishment (FLEX) Experiment Program showed anomalous combustion of n-heptane droplets; high temperature combustion followed by radiative visible extinction and a transition to a second stage burn characterized by loss of visible flame emission. In the second stage the droplet regression continues eventually resulting in extinction diameters characteristic of diffusive extinction. Experimental examples of the two stage burning and extinction characteristics of isolated n-heptane droplets under microgravity conditions are presented and analyzed numerically. Predictions show that the second stage combustion occurs as a result of chemical kinetics associated with classical premixed "Cool Flame" phenomena. In contrast to the kinetic interactions responsible for premixed cool flame properties, those important to cool flame droplet burning are characteristically associated with the temperature range between the turnover temperature and the hot ignition. Initiation of and continuing second stage combustion involves a dynamic balance of heat generation from diffusively controlled chemical reaction and heat loss from radiation and diffusion. Within the noted temperature range, increasing reaction temperature leads to decreased chemical reaction rate and vice versa. As a result, changes of heat loss rate are dynamically balanced by heat release from chemical reaction rate as the droplet continue to burn and regress in size. Factors leading to initiation of the second stage burning phenomena are also investigated. The chemical kinetics dictating the second stage combustion and extinction process is also discussed.
Farouk, T. (2015). Flameless “Cool” combustion in multi-phase configuration. In Procedia Engineering (Vol. 105, pp. 520–528). Elsevier Ltd. https://doi.org/10.1016/j.proeng.2015.05.085