Editing Flame

{{Short description|Visible, gaseous part of a fire}}
{{Other uses}}
{{Use dmy dates|date=March 2020}}
[[File:DancingFlames.jpg|thumb|Flames of [[charcoal]]]]

A '''flame''' ({{etymology|la|{{Wikt-lang|la|flamma}}|}}) is the visible, gaseous part of a [[fire]]. It is caused by a highly [[exothermic]] chemical reaction made in a thin zone.<ref>{{cite book|last=Law|first=C. K.|title=Combustion physics|publisher=Cambridge University Press|location=Cambridge, England|year=2006|page=300|chapter=Laminar premixed flames|isbn=0-521-87052-6|chapter-url=https://books.google.com/books?id=vWgJvKMXwQ8C&pg=RA300}}</ref> When flames are hot enough to have [[ionize]]d gaseous components of sufficient density, they are then considered [[Plasma (physics)|plasma]].{{vague |date=November 2018}}<ref>{{Cite web |title=Do flames contain plasma? |url=https://wtamu.edu/~cbaird/sq/2014/05/28/do-flames-contain-plasma/ |access-date=2022-06-26 |website=Science Questions with Surprising Answers |language=en-US}}</ref>

==Mechanism==
[[File:Anatomy of a candle flame.svg|thumb|Zones in a candle flame<br>The interior of the luminous zone can be much hotter, beyond {{cvt|1500|C|F}}.<ref>{{Cite journal |last1=Zheng |first1=Shu |last2=Ni |first2=Li |last3=Liu |first3=Huawei |last4=Zhou |first4=Huaichun |date=2019-04-01 |title=Measurement of the distribution of temperature and emissivity of a candle flame using hyperspectral imaging technique |url=https://www.sciencedirect.com/science/article/pii/S0030402619301949 |journal=Optik |language=en |volume=183 |pages=222–231 |doi=10.1016/j.ijleo.2019.02.077 |bibcode=|s2cid=126553613 |issn=0030-4026|url-access=subscription }}</ref>]]
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]], the applied heat causes the fuel molecules in the [[Paraffin wax|candle wax]] to vaporize.{{notetag|If this process happens in an inert atmosphere without [[oxidizer]], it is called {{em|[[pyrolysis]]}}.}} 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 causes the vaporized fuel molecules to [[Chemical decomposition|decompose]], forming various incomplete combustion products and [[radical (chemistry)|free radicals]], and these products then react with each other and with the [[Oxidizing agent|oxidizer]] involved in the reaction of the following flame (fire).

One may investigate different parts of a candle flame with the aid of a cold metal spoon:<ref>Archived at [https://ghostarchive.org/varchive/youtube/20211211/tMDKeBaLWDw Ghostarchive]{{cbignore}} and the [https://web.archive.org/web/20150804032938/https://www.youtube.com/watch?v=tMDKeBaLWDw Wayback Machine]{{cbignore}}: {{cite web|title=What Is Fire?| website=[[YouTube]] | date=3 August 2015 |url=https://www.youtube.com/watch?v=tMDKeBaLWDw|language=en|access-date=2019-11-27}}{{cbignore}}</ref> the higher parts of the flame produce water vapor deposition, the result of combustion, the yellow parts in the middle produce [[soot]], and the area near the candle wick produces unburned wax. Goldsmiths use higher parts of a flame with a metallic blow-pipe for melting gold and silver. Sufficient energy in the flame will excite the electrons in some of the transient reaction intermediates such as the [[methylidyne radical]] (CH) and [[diatomic carbon]] (C<sub>2</sub>), 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 [[Black body]]).

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>{{cite web|url=http://genchem.chem.wisc.edu/demonstrations/Inorganic/pages/Group67/chlorine_and_hydrogen.htm|archive-url=https://web.archive.org/web/20080820080559/http://genchem.chem.wisc.edu/demonstrations/Inorganic/pages/Group67/chlorine_and_hydrogen.htm|archive-date=2008-08-20|title=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 engines. [[Fluoropolymer]]s can be used to supply [[fluorine]] as an oxidizer of metallic fuels, e.g. in the [[magnesium/teflon/viton]] composition.

The [[chemical kinetics]] occurring in the flame are very complex and typically involve 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>{{cite web |author1=Gregory P. Smith |author2=David M. Golden |author3=Michael Frenklach |author4=Nigel W. Moriarty |author5=Boris Eiteneer |author6=Mikhail Goldenberg |author7=C. Thomas Bowman |author8=Ronald K. Hanson |author9=Soonho Song |author10=William C. Gardiner Jr. |author11=Vitali V. Lissianski |author12=Zhiwei Qin |title=GRI-Mech 3.0 |url=http://www.me.berkeley.edu/gri_mech/ |url-status=dead |archive-url=https://web.archive.org/web/20071029194024/http://www.me.berkeley.edu/gri_mech/ |archive-date=29 October 2007|access-date=8 November 2007 }}</ref> uses 53 species and 325 elementary reactions to describe combustion of [[biogas]].

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; the flame occurs where they meet. 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.

==Color==

{{See also|Flame test}}

[[File:Spectrum of blue flame - intensity corrected.png|thumb|Spectrum of the blue (premixed, i.e., complete combustion) flame from a [[butane]] torch showing molecular [[Radical (chemistry)|radical]] band emission and [[Swan bands]]. 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.]]

Flame color depends on several factors, the most important typically being [[black-body 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-mixing, which determines the [[Burn rate (chemistry)|rate of combustion]] and thus the temperature and reaction paths, thereby producing different color hues.
[[File: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.]]

In a laboratory under normal gravity conditions and with a closed air inlet, a Bunsen burner burns with yellow flame (also called a safety flame) with a peak temperature of about {{convert|2000|K|F|-2|abbr=}}. The yellow arises from [[incandescence]] of very fine soot particles that are produced in the flame. Also, [[carbon monoxide]] is produced, and the flame tends to take oxygen from the surfaces it touches. When the air inlet is opened, less soot and carbon monoxide are produced. When enough air is supplied, no soot or carbon monoxide is produced and the flame becomes blue. (Most of this blue had previously been obscured by the bright yellow emissions.) 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 [[radical (chemistry)|radicals]] in the flame, which emit most of their light well below ≈565&nbsp;nanometers in the blue and green regions of the visible spectrum.

The colder part of a diffusion (incomplete combustion) flame will be red, transitioning to orange, yellow, and white as the temperature increases as evidenced by changes in the black-body 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 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.<ref name=temp/> A blue-colored flame only emerges when the amount of soot decreases and the [[Swan band|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.<ref>{{cite book|url=https://books.google.com/books?id=hadB8msSl1EC&pg=PA172|page=172|title=Combustion Phenomena: Selected Mechanisms of Flame Formation, Propagation and Extinction|author1=Jozef Jarosinski|author2=Bernard Veyssiere|publisher=CRC Press|year=2009|isbn=978-0-8493-8408-0}}</ref>

Specific colors can be imparted to the flame by introduction of excitable species with bright [[emission spectrum]] lines. In analytical chemistry, this effect is used in [[flame test]]s (or [[Emission spectrum#Experimental technique in flame emission spectroscopy|flame emission spectroscopy]]) to determine presence of some metal ions. In [[pyrotechnics]], the [[pyrotechnic colorant]]s are used to produce brightly colored fireworks.

==Temperature==
[[File:Flametest--Na.swn.jpg|thumb|upright|A [[flame test]] for [[sodium]]. The yellow color in this gas flame does not arise from the [[black body|black-body]] 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.]]

When looking at a flame's temperature there are many factors which can change or apply. An important one is that a flame's color does not necessarily determine a temperature comparison because black-body radiation is not the only thing that produces or determines the color seen; therefore it is only an estimation of temperature. Other factors that determine its temperature are:
{{Flowlist|
* [[Adiabatic|Adiabatic flame]]; i.e., no loss of heat to the atmosphere (may differ in certain parts)
* [[Atmospheric pressure]]
* Percentage oxygen content of the [[atmosphere]]
* The kind of fuel used (i.e., depends on how quickly the process occurs; how violent the combustion is)
* Any [[oxidation]] of the fuel
* Temperature of atmosphere links to adiabatic flame temperature (i.e., heat will transfer to a cooler atmosphere more quickly)
* How [[stoichiometric]] the combustion process is (a 1:1 stoichiometricity) assuming no dissociation will have the highest flame temperature; excess air/oxygen will lower it as will lack of air/oxygen
* The distance from the source of the flame (i.e., the further from the source of the flame the lower temperature)}}
* In fires (particularly house fires), the cooler flames are often red and produce the most smoke. Here the red color compared to typical yellow color of the flames suggests that the temperature is lower. This is because there is a lack of oxygen in the room and therefore there is incomplete combustion and the flame temperature is low, often just {{convert|600|to|850|C|F}}. This means that a lot of [[carbon monoxide]] is formed (which is a flammable gas) which is when there is greatest risk of [[backdraft]]. When this occurs, combustible gases at or above the [[flash point]] of spontaneous combustion are exposed to oxygen, carbon monoxide and superheated hydrocarbons combust, and temporary temperatures of up to {{convert|2000|C|F}} occur.{{citation needed|date=April 2012}}

===Common flame temperatures===
{{Original research section|date=December 2019}}
This is a rough guide to flame temperatures for various common substances (in {{convert|20|C|F}} air at 1&nbsp;atm. pressure):
<!-- {{cvt}} templates would be useful for these data tables. -->
{| class="wikitable"
|-
! Material burned
! Flame temperature
|-
|[[Butane]]
|~300&nbsp;°C (~600&nbsp;°F) (a [[cool flame]] in low gravity)<ref name="Pearlman-2000" />
|-
| [[Charcoal]] fire
| 750–1,200&nbsp;°C (1,382–2,192&nbsp;°F)
|-
| [[Methane]] (natural gas)
| 900–1,500&nbsp;°C (1,652–2,732&nbsp;°F)
|-
| [[Bunsen burner]] flame
| 900–1,600&nbsp;°C (1,652–2,912&nbsp;°F) [depending on the air valve, open or close.]
|-
| Candle flame
| ≈1,100&nbsp;°C (≈2,012&nbsp;°F) [majority]; hot spots may be 1,300–1,400&nbsp;°C (2,372–2,552&nbsp;°F)
|-
| [[Propane]] [[blowtorch]]
| 1,200–1,700&nbsp;°C (2,192–3,092&nbsp;°F)
|-
| [[Backdraft]] flame peak
| 1,700–1,950&nbsp;°C (3,092–3,542&nbsp;°F)
|-
| [[Magnesium]]
| 1,900–2,300&nbsp;°C (3,452–4,172&nbsp;°F)
|-
| [[Oxyhydrogen#Oxyhydrogen torch|Hydrogen torch]]
| Up to ≈2,000&nbsp;°C (≈3,632&nbsp;°F)
|-
| [[MAPP gas]]
| 2,020&nbsp;°C (3,668&nbsp;°F)
|-
| [[Acetylene]] blowlamp/[[blowtorch]]
| Up to ≈2,300&nbsp;°C (≈4,172&nbsp;°F)
|-
| [[Oxyacetylene]]
| Up to 3,300&nbsp;°C (5,972&nbsp;°F)
|}

{| class="wikitable"
|-
! Material burned
! Max. flame temperature (in air, diffusion flame)<ref name=temp>{{cite book |url=https://books.google.com/books?id=Q7Pb2wXV2woC&pg=PA4 |pages=2–4 |title=The analysis of burned human remains |author1=Christopher W. Schmidt |author2=Steve A. Symes |publisher=Academic Press |year=2008 |isbn=978-0-12-372510-3}}</ref>
|-
| Animal fat
| 800–900&nbsp;°C (1,472–1,652&nbsp;°F)
|-
| [[Kerosene]]
| 990&nbsp;°C (1,814&nbsp;°F)
|-
| [[Gasoline]]
| 1,026&nbsp;°C (1,878.8&nbsp;°F)
|-
| Wood
| 1,027&nbsp;°C (1,880.6&nbsp;°F)
|-
| [[Methanol]]
| 1,200&nbsp;°C (2,192&nbsp;°F)
|-
| [[Charcoal]] (forced draft)
| 1,390&nbsp;°C (2,534&nbsp;°F)
|}

===Highest temperature===
[[Dicyanoacetylene]], a compound of carbon and nitrogen with chemical formula C<sub>4</sub>N<sub>2</sub> burns in oxygen with a bright blue-white flame at a temperature of {{convert|5260|K|lk=in|C F}}, and at up to {{convert|6000|K|C F}} in [[ozone]].<ref>{{cite journal
 | last = Kirshenbaum
 | first = A. D.
 |author2=A. V. Grosse
 |date=May 1956
 | title = The Combustion of Carbon Subnitride, NC<sub>4</sub>N, and a Chemical Method for the Production of Continuous Temperatures in the Range of 5000–6000K
 | journal = Journal of the American Chemical Society
 | volume = 78
 | issue = 9
 | page = 2020
 | doi = 10.1021/ja01590a075
 }}</ref> This high flame temperature is partially due to the absence of hydrogen in the fuel (dicyanoacetylene is not a hydrocarbon) thus there is no water among the combustion products.

[[Cyanogen]], with the formula (CN)<sub>2</sub>, produces the second-hottest-known natural flame with a temperature of over {{convert|4525|C|F}} when it burns in oxygen.<ref>{{ cite journal |author1=Thomas, N. |author2=Gaydon, A. G. |author3=Brewer, L. | title = Cyanogen Flames and the Dissociation Energy of N<sub>2</sub> | journal = The Journal of Chemical Physics | year = 1952 | volume = 20 | issue = 3  | pages = 369–374 | doi = 10.1063/1.1700426 | bibcode = 1952JChPh..20..369T }}</ref><ref>{{ cite journal |author1=J. B. Conway |author2=R. H. Wilson Jr. |author3=A. V. Grosse |title=The Temperature of the Cyanogen-Oxygen Flame | journal = Journal of the American Chemical Society | year = 1953 | volume = 75 | issue = 2  | pages = 499 | doi = 10.1021/ja01098a517 }}</ref>

===Cool flames===
{{main|Cool flame}}
At temperatures as low as {{convert|120|C|F}}, fuel-air mixtures can react chemically and produce very weak flames called cool flames. The phenomenon was discovered by [[Humphry Davy]] in 1817. The process depends on a fine balance of temperature and concentration of the reacting mixture, and if conditions are right it can initiate without any external ignition source. Cyclical variations in the balance of chemicals, particularly of intermediate products in the reaction, give oscillations in the flame, with a typical temperature variation of about {{convert|100|C|F}}, or between "cool" and full ignition. Sometimes the variation can lead to an explosion.<ref name="Pearlman-2000">{{cite web|url=http://www.grc.nasa.gov/WWW/RT/RT1999/6000/6711wu.html|title=Cool Flames and Autoignition in Microgravity|last=Pearlman|first=Howard|author2=Chapek, Richard M.|date=24 April 2000|publisher=[[NASA]]|access-date=13 May 2010|url-status=dead|archive-url=https://web.archive.org/web/20100501223626/http://www.grc.nasa.gov/WWW/RT/RT1999/6000/6711wu.html|archive-date=1 May 2010}}</ref><ref>{{cite book|last=Jones|first=John Clifford |title=Hydrocarbon process safety: a text for students and professionals|date=September 2003|publisher=PennWell|location=Tulsa, OK|isbn=978-1-59370-004-1|pages=32–33|chapter=Low temperature oxidation}}</ref>

==In microgravity==
{{Redirect|Fire in space|the ''Battlestar Galactica'' episode|Fire in Space}}
[[File:Candlespace.jpg|thumb|In [[Weightlessness|zero-G]], convection does not carry the hot combustion products away from the fuel source, resulting in a spherical flame front.]]

In the year 2000, experiments by NASA confirmed that gravity plays an indirect role in flame formation and composition.<ref>[https://science.nasa.gov/headlines/y2000/ast12may_1.htm Spiral flames in microgravity] {{webarchive|url=https://web.archive.org/web/20100319113411/http://science.nasa.gov/headlines/y2000/ast12may_1.htm |date=19 March 2010 }}, [[NASA|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 [[Micro-g environment|microgravity]] or [[Weightlessness|zero gravity]] environment, such as in orbit, natural 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://quest.nasa.gov/space/teachers/microgravity/9flame.html Candle Flame in Microgravity] {{webarchive|url=https://web.archive.org/web/20111026130422/http://quest.nasa.gov/space/teachers/microgravity/9flame.html |date=26 October 2011 }}. NASA</ref> 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.<ref>C. H. Kim ''et al.'' [https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040053584_2004055201.pdf Laminar Soot Processes Experiment Shedding Light on Flame Radiation] {{webarchive|url=https://web.archive.org/web/20140111155132/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20040053584_2004055201.pdf |date=11 January 2014 }}. [[NASA]], [http://www.grc.nasa.gov/WWW/RT/RT1997/6000/6711urban.htm HTML] {{webarchive|url=https://web.archive.org/web/20120720203036/http://www.grc.nasa.gov/WWW/RT/RT1997/6000/6711urban.htm |date=20 July 2012 }}</ref> These discoveries have potential applications in [[applied science]] and private industry, especially concerning [[fuel efficiency]].

==Edge flame==
An '''edge flame''' or a '''triple flame''' refers to a stationary or moving flame edge in partially premixed reacting mixture. The canonical edge flame has a tribriachial structure that comprises two premixed flames, namely one fuel rich and one fuel lean, and a trailing diffusion flame. The theoretical development of triple flames was carried out by [[John W. Dold]],<ref>Dold, J. W. (1989). Flame propagation in a nonuniform mixture: analysis of a slowly varying triple flame. Combustion and Flame, 76(1), 71-88.</ref> Joel Daou and [[Amable Liñán]].<ref>Daou, J., & Linán, A. (1998). The role of unequal diffusivities in ignition and extinction fronts in strained mixing layers. Combustion Theory and Modelling, 2(4), 449.</ref>

==Thermonuclear flames==

Flames do not need to be driven only by chemical energy release. In stars, subsonic burning fronts driven by burning light nuclei (like carbon or helium) to heavy nuclei (up to iron group) propagate as flames. This is important in some models of [[Type Ia supernovae]]. In thermonuclear flames, thermal conduction dominates over species diffusion, so the flame speed and thickness is determined by the [[thermonuclear energy]] release and [[thermal conductivity]] (often in the form of [[Degenerate matter#Electron degeneracy|degenerate electrons]]).<ref>{{cite journal |last1=Timmes |first1=F. X. |last2=Woosley |first2=S. E. |title=The conductive propagation of nuclear flames. I - Degenerate C + O and O + Ne + Mg white dwarfs |journal=The Astrophysical Journal |date=1 September 1992 |volume=396 |pages=649–667 |doi=10.1086/171746 |bibcode=1992ApJ...396..649T|doi-access=free }}</ref>

==See also==
* [[Flame detector]]
* [[International Flame Research Foundation]]
* [[Olympic flame]]
* [[Oxidizing and reducing flames]]
* [[The Combustion Institute]]

==Notes==
{{Notefoot}}

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20110930075348/http://www.plasma-universe.com/Image:Electric-candle-flame.jpg A candle flame strongly influenced and moved about by an electric field due to the flame having ions.] (archived 30 September 2011)
* [http://techportal.eere.energy.gov/technology.do/techID=147 Ultra-Low Emissions Low-Swirl Burner] {{Webarchive|url=https://web.archive.org/web/20170613105229/https://techportal.eere.energy.gov/technology.do/techID=147 |date=13 June 2017 }}
* [https://web.archive.org/web/20170831225214/http://avgminds.com/what-different-colors-of-fire-exist/ 7 Shades of Fire] (archived 31 August 2017)
* {{cite web|last=Licence|first=Peter|title=Coloured Flames|url=http://www.periodicvideos.com/videos/feature_colour_flames.htm|work=[[The Periodic Table of Videos]]|publisher=[[University of Nottingham]]}}

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[[Category:Fire]]

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