Flame
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A flame is the product of a highly exothermic reaction (for example, combustion, a self-sustaining oxidation reaction). In other words, a flame is the visible (light emitting) part of a fire.
The color and temperature of a flame are dependent on the type of fuel involved in the combustion, for example, when a lighter is held to a candle. This applied heat causes the fuel molecules in the wick to vaporize, in this state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame. The high temperature of the flame tears apart the vaporized fuel molecules, forming various incomplete combustion products and free radicals, these products then react with the oxidizer involved in the reaction. Sufficient energy in the flame will excite the electrons in the pre-combusted products, which results in the emission of visible light (see spectrum below for an explanation of which specific radical species produce which specific colors). As the combustion temperature of a flame increases, (if the flame contains small particles of unburnt carbon or other material) so does the average energy of the electromagnetic radiation given off by the flame (see blackbody).
Other oxidizers besides oxygen can be used to produce a flame. Hydrogen burning in chlorine produces a flame and in the process emits gaseous hydrogen chloride (HCl) as the combustion product. Another of many possible chemical combinations is hydrazine and nitrogen tetroxide which is hypergolic and commonly used in rocket engines.
Flames, or at least portions of them, are often considered to be partial plasmas or a partially ionized gas, but in more general terms a flame is a region of exothermic chemical reaction of high enough temperature to emit visible light. A central region enclosed by such a reaction (but which does not necessarily participate in the reaction itself) is often also considered part of the flame.
There are different methods of distributing the required components of combustion to a flame. In a diffusion flame, oxygen and fuel diffuse into each other; where they meet the flame occurs. In a premixed flame, the oxygen and fuel are premixed beforehand, which results in a different type of flame. Candle flames (a diffusion flame) operate through evaporation of the fuel which rises in a laminar flow of hot gas which then mixes with surrounding oxygen and combusts.
[edit] Flame color
Flame color depends on several factors. Typically the most important 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 which determines 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, different color hues are thereby produced.
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 which 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 the vast majority of their light well below ~565 nanometers in the blue and green regions of the visible spectrum.
Flame temperatures of common items include a blowlamp at 1,300°C, a candle at 1,400°C [1], or a much hotter oxyacetylene combustion at 3,000°C.
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. A blue-colored flame only emerges when the amount of soot decreases and the blue emissions from excited molecular radicals become dominant.
Recent discoveries by the National Aeronautics and Space Administration (NASA) of the United States have found that gravity also indirectly plays a role in flame formation and composition. [1] 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, such as an outer space environment, 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. [2] Experiments by NASA in microgravity 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. [3][4] These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.
[edit] References
- ^ Spiral flames in microgravity, National Aeronautics and Space Administration, 2000.
- ^ CFM-1 experiment results, National Aeronautics and Space Administration, April 2005.
- ^ LSP-1 experiment results, National Aeronautics and Space Administration, April 2005.
- ^ SOFBAL-2 experiment results, National Aeronautics and Space Administration, April 2005.
