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Main article: Flame
A flame is an exothermic
, self-sustaining, oxidizing chemical reaction producing energy
and glowing hot matter, of which a very small portion is plasma
. It consists of reacting gases and solids emitting visible and infrared
light, the frequency spectrum
of which depends on the chemical composition of the burning elements and intermediate reaction products.
In many cases, such as the burning of organic matter
, for example wood, or the incomplete combustion
of gas, incandescent
solid particles called soot
produce the familiar red-orange glow of 'fire'. This light has a continuous spectrum. Complete combustion of gas has a dim blue color due to the emission of single-wavelength radiation from various electron transitions in the excited molecules formed in the flame. For reasons currently unknown by scientists, the flame produced by exposure of zinc to air is a bright green, and produces plumes of zinc oxide
. Usually oxygen is involved, but hydrogen
burning in chlorine
also produces a flame, producing hydrogen chloride
(HCl). Other possible combinations producing flames, amongst many more, are fluorine
, and hydrazine
and nitrogen tetroxide
The glow of a flame is complex. Black-body radiation
is emitted from soot, gas, and fuel particles, though the soot particles are too small to behave like perfect blackbodies. There is also photon
emission by de-excited atoms
in the gases. Much of the radiation is emitted in the visible and infrared
bands. The color depends on temperature for the black-body radiation, and on chemical makeup for the emission spectra
. The dominant color in a flame changes with temperature. The photo of the forest fire is an excellent example of this variation. Near the ground, where most burning is occurring, the fire is white, the hottest color possible for organic material in general, or yellow. Above the yellow region, the color changes to orange, which is cooler, then red, which is cooler still. Above the red region, combustion no longer occurs, and the uncombusted carbon particles are visible as black smoke.
The National Aeronautics and Space Administration
(NASA) of the United States
has recently found that gravity
plays a role. Modifying the gravity causes different flame types.
The common distribution of a flame under normal gravity conditions depends on convection
, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In microgravity or zero gravity
, such as an environment in outer space
, convection no longer occurs, and the flame becomes spherical, with a tendency to become more blue and more efficient (although it will go out if not moved steadily, as the CO2 from combustion does not disperse in microgravity, and tends to smother the flame). There are several possible explanations for this difference, of which the most likely is that the temperature is evenly distributed enough that soot is not formed and complete combustion occurs.
Experiments by NASA reveal that diffusion flames
in microgravity allow more soot to be completely oxidized after they are produced than diffusion flames on Earth, because of a series of mechanisms that behave differently in microgravity when compared to normal gravity conditions.
These discoveries have potential applications in applied science
, especially concerning fuel efficiency
In combustion engines, various steps are taken to eliminate a flame. The method depends mainly on whether the fuel is oil, wood, or a high-energy fuel such as jet fuel