Can cool flames support quasi-steady alkane droplet burning?

• Published in: Combustion and flame Vol. 159; no. 12; pp. 3583 - 3588
• Main Authors: Nayagam, Vedha, Dietrich, Daniel L., Ferkul, Paul V., Hicks, Michael C., Williams, Forman A.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01-12-2012; Elsevier
• Subjects: Alkane chemistry; Alkanes; Applied sciences; Burning rate; Combustion; Combustion. Flame; Cool flames; Droplet combustion; Droplets; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Extinction; Mathematical analysis; Microgravity; Theoretical studies. Data and constants. Metering; Vaporization; Alkane chemistry; Cool flames; Droplet combustion; Extinction; Microgravity; Soot; Flame temperature; Ignition; Combustion; Droplet; Flame propagation; Reaction rate; Heptane; Combustion velocity; Heat liberation; Cool flame; Vaporization; Alkane; Flame structure; Activation energy; Low temperature
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2012.07.012

Experimental observations of anomalous combustion of n-heptane droplets burning in microgravity are reported. Following ignition, a relatively large n-heptane droplet first undergoes radiative extinction, that is, the visible flame ceases to exist because of radiant energy loss. But the droplet continues to experience vigorous vaporization for an extended period according to a quasi-steady droplet-burning law, ending in a secondary extinction at a finite droplet diameter, after which a vapor cloud rapidly appears surrounding the droplet. We hypothesize that the second-stage vaporization is sustained by low-temperature, soot-free, “cool-flame” chemical heat release. Measured droplet burning rates and extinction diameters are used to extract an effective heat release, overall activation energy, and pre-exponential factor for this low-temperature chemistry, and the values of the resulting parameters are found to be closer to those of “cool-flame” overall reaction-rate parameters, found in the literature, than to corresponding hot-flame parameters.