http://en.wikipedia.org/wiki/Flame

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Flame color depends on several factors, the most important typically being blackbody radiation and spectral band emission, with both spectral line emission and spectral line absorption playing smaller roles. In the most common type of flame, hydrocarbon flames, the most important factor determining color is oxygen supply and the extent of fuel-oxygen "pre-mixture", which determines the rate of combustion and thus the temperature and reaction paths, thereby producing different color hues. In a laboratory under normal gravity conditions and with a closed oxygen valve, a Bunsen burner burns with yellow flame (also called a safety flame) at around 1,000 °C. This is due to incandescence of very fine soot particles that are produced in the flame. With increasing oxygen supply, less blackbody-radiating soot is produced due to a more complete combustion and the reaction creates enough energy to excite and ionize gas molecules in the flame, leading to a blue appearance. The spectrum of a premixed (complete combustion) butane flame on the right shows that the blue color arises specifically due to emission of excited molecular radicals in the flame, which emit most of their light well below ~565 nanometers in the blue and green regions of the visible spectrum.

Flame temperatures of common items include a blow torch at 1,300 °C, a candle at 1,400 °C [1], or a much hotter oxyacetylene combustion at 3,000 °C. Cyanogen produces an even hotter flame with a temperature of over 4525 °C (8180 °F) when it burns in oxygen.[5]

Generally speaking, the coolest part of a diffusion (incomplete combustion) flame will be red, transitioning to orange, yellow, and white the temperature increases as evidenced by changes in the blackbody radiation spectrum. For a given flame's region, the closer to white on this scale, the hotter that section of the flame is. The transitions are often apparent in TV pictures of fires, in which the color emitted closest to the fuel is white, with an orange section above it, and reddish flames the highest of all. Beyond the red the temperature is too low to sustain combustion, and black soot escapes. A blue-colored flame only emerges when the amount of soot decreases and the blue emissions from excited molecular radicals become dominant, though the blue can often be seen near the base of candles where airborne soot is less concentrated.

[edit] Flames in microgravity In zero gravity, convection does not carry the hot combustion products away from the fuel source, resulting in a spherical flame front.

In the year 2000 the National Aeronautics and Space Administration (NASA) of the United States discovered that gravity also plays an indirect role in flame formation and composition.[6] The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a flame (such as in a candle in normal gravity conditions), making it yellow. In microgravity or zero gravity environment, such as on a circular orbit , convection no longer occurs and the flame becomes spherical, with a tendency to become bluer and more efficient. There are several possible explanations for this difference, of which the most likely is the hypothesis that the temperature is sufficiently evenly distributed that soot is not formed and complete combustion occurs.[7] Experiments by NASA reveal that diffusion flames in microgravity allow more soot to be completely oxidized after they are produced than do diffusion flames on Earth, because of a series of mechanisms that behave differently in microgravity when compared to normal gravity conditions.[8][9] These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency. A video of a microgravity flame in the NASA Glenn 5 s drop facility is at [2].