Cool Flames

Cool flames remained as a laboratory curiosity, mainly associated with its periodic oscillations in a closed container, for almost two centuries since its original discovery by Sir Humphrey Davies in 1817. A series of droplet combustion experiments (FLEX, FLEX-2, and CFI) carried out by NASA in the International Space Station in the Combustion Integrated Rack (CIR) facility over the past several years have revealed for the first time that the cool flames can support steadily burning flames, though invisible to the naked eye, controlled by low temperature (600 to 900 K) chemical kinetics of certain normal alkane fuels. These n-alkane fuels, namely n-heptane, n-octane, n-decane, and n-dodecane are major components in the commercially available transportation fuels, such as gasoline, diesel, and jet-fuels. In these experiments a small (1 to 5 mm) fuel droplet is free floated in microgravity, and ignited under a variety of ambient conditions involving pressures (0.5 to 5 atm), oxygen concentration, and inert diluents (carbon dioxide, nitrogen, helium, and xenon gases). Initially the droplet burns with a visible hot flame; then, it extinguishes through excessive radiative heat loss. Following radiative extinction, the droplet continues to burn vigorously but without any detected visible flame, in a stage termed cool-flame combustion, which also ends abruptly at a finite droplet radius and is followed by pure evaporation without heat release. Since the cool flame consumes only part of the vaporized fuel, a condensation cloud is also found to form, surrounding the droplet toward the end of the cool-flame combustion process. Cool flame burning rates, flame dynamics, radiant energy output, and extinction conditions are measured during these experiments.

The experimental finding that cool flames can support steady combustion of liquid fuels have many potential applications on earth. The next generation engine technologies, such as the homogeneous charge compression ignition (HCCI) engine, can benefit from the improved understanding brought about by this discovery. Since the measurements of droplet diameter at cool flame extinction provides a stringent check for cool flame chemical times, these measurements can be used to validate chemical kinetic mechanisms. Validated chemical kinetic mechanisms can then be used in computational simulation of new generation engines. Other areas of application include fuel reformulation and production of hydrogen from gasoline for use in fuel cells. Another area of importance lies in questions about fire safety in space vehicles. Since the cool mode of droplet combustion persists after hot-flame extinction, safety procedures based only on considerations of hot flames may be inadequate for assuring safety under all conditions.

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