American Scientist

To the Editors:

Thanks to the authors Indrek S. Wichman, Sandra L. Olson, Fletcher J. Miller, and Ashwin Hariharan for their study “Fire in Microgravity” (January–February). I have one small correction to suggest regarding the hottest region of a reduction flame.

The beautiful color photographs of microgravity candle flames reveal interesting details about low-gravity flames. In general, their spherical shape is indicative of uniform inward and outward gas flow through the flame’s surface with no shape distortion resulting from macrogravity-induced convection. In particular, they reveal the energy profile of reducing flames, which suggest that the yellow area of a candle flame is not its hottest region. In both micro- and macro-gravity, the fuel is stationary as oxygen diffuses slowly through the flame boundary. Some oxygen molecules escape back into the flame’s exterior, and the remainder is consumed internally, leaving excess unconsumed carbon. The steady state density of oxygen is highest as it enters the flame, where it produces a thin shell of purple emission at the flame boundary, which is thicker in the lower, purple region in convective flow. The rate of energy release per unit volume is highest in this thin region as inferred by the presence of short-wavelength, high-energy photons, indicating the highest temperature in the flame. The yellow light is generated mostly by blackbody radiation from carbon atom clusters in the upper, cooler part of the flame. The elongated microgravity flame thins the flame’s upper boundary, increasing the flame’s surface-to-volume ratio and making it easier for unconsumed soot particles to escape into the neighboring gas sweeping the microgravity-flame’s surface. Other interesting features are revealed in these photographs as well.

Ed Sickafus
Grosse Ile, MI

Drs. Wichman, Olson, Miller, and Hariharan respond:

We appreciate Dr. Sickafus’s perspicacious comments, which are largely correct. Indeed, the hottest part of the flame, especially for the 1-g flame with “macrogravity-induced convection,” is not the yellow (soot) region. It is the blue to purplish region, where most of the heat-releasing chemical reactions occur, whereas the yellow regions are somewhat cooler blackbody emissions from small soot particulates. (Just to be clear, the soot region is still very hot!)

Soot particulates are normally formed when the local combustion conditions are fuel rich, meaning that there is more fuel than necessary for complete combustion. As a result, the fuel molecules breach the flame only to later congeal and form soot. We have determined that, overall, our spreading simulated microgravity flames are fuel lean, meaning that combustion takes place with an overabundance of oxygen. However, it is possible for combustion to be fuel lean overall but fuel rich locally, as here.

In the localized region between the surface and the flame there is indeed more fuel present than will burn with the available oxidizer in the time required, so some of it is simply heated, remains unburned, and congeals into soot particulates. The oxygen gets its pound of flesh, however, and eventually burns off all of the fuel in the yellow (sooty) flame downstream of the purplish flame. The writer is correct: There is much more to be seen besides, which speaks to the great advances in optical visualization over the past half-century.