Flame
212427
224118285
2008-07-07T11:47:28Z
PhJ
1657063
interwiki:qu
[[Image:Candleburning.jpg|thumb|150px|Laminar Flame generated by the burning of a candle.]]
[[Image:Flame 1.JPG |thumb|200px|Another candle flame]]
{{Otheruses}}
A '''flame''' is often defined as the visible (light-emitting) part of a [[fire]]. Physically, it is caused by a highly [[exothermic]] reaction (for example, [[combustion]], a self-sustaining [[redox|oxidation]] [[chemical reaction|reaction]]) taking place in a thin zone. A flame generally emits light, by two different mechanisms which will be described below.
The [[color]] and [[temperature]] of a flame are dependent on the type of [[fuel]] involved in the combustion, as, for example, when a [[lighter]] is held to a [[candle]]. The 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 [[radical (chemistry)|free radical]]s, and these products then react with each other and with the oxidizer involved in the reaction. Sufficient [[energy]] in the flame will excite the [[electron]]s in some of the transient reaction intermediates such as CH and C2, which results in the emission of visible [[light]] as these substances release their excess energy (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.<ref>[http://genchem.chem.wisc.edu/demonstrations/Inorganic/pages/Group67/chlorine_and_hydrogen.htm Reaction of Chlorine with Hydrogen]</ref> Another of many possible chemical combinations is [[hydrazine]] and [[nitrogen tetroxide]] which is [[hypergolic]] and commonly used in [[rocket engine]]s.
The [[chemical kinetics]] occurring in the flame is very complex and involves typically a large number of chemical reactions and intermediate species, most of them [[radical (chemistry)|radicals]]. For instance, a well-known chemical kinetics scheme, GRI-Mech <ref>{{Citation
| last = Gregory P. Smith
| first =
| author-link =
| last2 = David M. Golden, Michael Frenklach, Nigel W. Moriarty, Boris Eiteneer, Mikhail Goldenberg, C. Thomas Bowman, Ronald K. Hanson, Soonho Song, William C. Gardiner, Jr., Vitali V. Lissianski, and Zhiwei Qin
| first2 =
| author2-link =
| title = GRI-Mech 3.0
| journal =
| volume =
| issue =
| pages =
| date =
| year =
| url =http://www.me.berkeley.edu/gri_mech/
| doi =
| id = }}</ref>
, uses 53 species and 325 elementary reactions to describe combustion of [[natural gas]].
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.
== Flame color ==
[[Image:Bunsen_burner_flame_types_.jpg|thumb|left|Different flame types of a [[Bunsen burner]] depend on oxygen supply. On the left a rich fuel with no premixed oxygen produces a yellow sooty diffusion flame; on the right a lean fully oxygen premixed flame produces no soot and the flame color is produced by molecular radicals, especially CH and C2 [[band emission]]. The purple color is an artifact of the photographic process]]
[[Image:Spectrum_of_blue_flame.png|thumb|right|200px|Spectrum of the blue (premixed, i.e., complete combustion) flame from a [[butane]] torch showing molecular [[Radical (chemistry)|radical]] band emission and [[Swan bands]]. Note that virtually all the light produced is in the blue to green region of the spectrum below about 565 nanometers, accounting for the bluish color of sootless hydrocarbon flames.]]
[[Image:Flametest--Na.swn.jpg|thumb|right|150px|A [[flame test]] for [[sodium]]. Note that the yellow color in this gas flame does not arise from the blackbody emission of soot particles (as the flame is clearly a blue premixed complete combustion flame) but instead comes from the [[spectral line]] emission of sodium atoms, specifically the very intense "sodium D lines".]]
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 [http://www.doctorfire.com/flametmp.html], or a much hotter [[oxyacetylene]] combustion at 3,000°C. [[Cyanogen]] produces an ever-hotter flame with a temperature of over 4525°C (8180°F) when it burns in oxygen.<ref>{{Citation
| last = Thomas
| first =N.
| author-link =
| last2 =Gaydon, A. G.; Brewer, L.
| first2 =
| author2-link =
| title = Cyanogen Flames and the Dissociation Energy of N2
| journal =The Journal of Chemical Physics
| volume =20
| issue =3
| pages =369-374
| date =March 1952
| year =
| url =http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JCPSA6000020000003000369000001&idtype=cvips&gifs=yes
| doi =
| id = }}</ref>
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.
== Flames in microgravity ==
[[Image:Candlespace.jpg|thumb|left|In [[Weightlessness|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. <ref> [http://science.nasa.gov/headlines/y2000/ast12may_1.htm Spiral flames in microgravity], [[National Aeronautics and Space Administration]], 2000.</ref>
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. <ref> [http://microgravity.grc.nasa.gov/combustion/cfm/usml-1_results.htm CFM-1 experiment results], National Aeronautics and Space Administration, April 2005.</ref> Experiments by [[NASA]] reveal that [[diffusion flame]]s 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. <ref>[http://microgravity.grc.nasa.gov/combustion/lsp/lsp1_results.htm LSP-1 experiment results], National Aeronautics and Space Administration, April 2005.</ref><ref>[http://microgravity.grc.nasa.gov/combustion/lsp/lsp1_results.htm SOFBAL-2 experiment results], National Aeronautics and Space Administration, April 2005.</ref> 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 [http://www.youtube.com/watch?v=SZTl7oi05dQ].
{{-}}
== References ==
{{Reflist}}
[[Category:Fire]]
[[ar:لهب]]
[[cy:Fflam]]
[[de:Flamme]]
[[es:Llama (química)]]
[[fa:شعله]]
[[fr:Flamme (combustion)]]
[[it:Fiamma]]
[[lt:Liepsna]]
[[nl:Vlam]]
[[ja:炎]]
[[nap:Fiamma]]
[[no:Flamme]]
[[pl:Płomień]]
[[qu:Rapa]]
[[scn:Ciamma]]
[[simple:Flame]]
[[su:Seuneu]]
[[zh:焰]]