Flame
A flame (from
Mechanism
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
Other oxidizers besides oxygen can be used to produce a flame. Hydrogen burning in chlorine produces a flame and in the process emits gaseous
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 radicals. For instance, a well-known chemical kinetics scheme, GRI-Mech,[6] 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
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 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 air inlet, a Bunsen burner burns with yellow flame (also called a safety flame) with a peak temperature of about 2,000 K (3,100 °F). 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 radicals in the flame, which emit most of their light well below ≈565 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.[7] 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.[8]
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 tests (or flame emission spectroscopy) to determine presence of some metal ions. In pyrotechnics, the pyrotechnic colorants are used to produce brightly colored fireworks.
Temperature
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:
- 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 oxidationof the fuel
- Temperature of atmosphere links to adiabatic flame temperature (i.e., heat will transfer to a cooler atmosphere more quickly)
- How stoichiometricthe 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 600 to 850 °C (1,112 to 1,562 °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 2,000 °C (3,630 °F) occur.[citation needed]
Common flame temperatures
This section possibly contains original research. (December 2019) |
This is a rough guide to flame temperatures for various common substances (in 20 °C (68 °F) air at 1 atm. pressure):
Material burned | Flame temperature |
---|---|
Butane | ~300 °C (~600 °F) (a cool flame in low gravity)[9] |
Charcoal fire | 750–1,200 °C (1,382–2,192 °F) |
Methane (natural gas) | 900–1,500 °C (1,652–2,732 °F) |
Bunsen burner flame | 900–1,600 °C (1,652–2,912 °F) [depending on the air valve, open or close.] |
Candle flame | ≈1,100 °C (≈2,012 °F) [majority]; hot spots may be 1,300–1,400 °C (2,372–2,552 °F) |
Propane blowtorch | 1,200–1,700 °C (2,192–3,092 °F) |
Backdraft flame peak | 1,700–1,950 °C (3,092–3,542 °F) |
Magnesium | 1,900–2,300 °C (3,452–4,172 °F) |
Hydrogen torch | Up to ≈2,000 °C (≈3,632 °F) |
MAPP gas | 2,020 °C (3,668 °F) |
Acetylene blowlamp/blowtorch | Up to ≈2,300 °C (≈4,172 °F) |
Oxyacetylene
|
Up to 3,300 °C (5,972 °F) |
Material burned | Max. flame temperature (in air, diffusion flame)[7] |
---|---|
Animal fat | 800–900 °C (1,472–1,652 °F) |
Kerosene | 990 °C (1,814 °F) |
Gasoline | 1,026 °C (1,878.8 °F) |
Wood | 1,027 °C (1,880.6 °F) |
Methanol | 1,200 °C (2,192 °F) |
Charcoal (forced draft) | 1,390 °C (2,534 °F) |
Highest temperature
Dicyanoacetylene, a compound of carbon and nitrogen with chemical formula C4N2 burns in oxygen with a bright blue-white flame at a temperature of 5,260 K (4,990 °C; 9,010 °F), and at up to 6,000 K (5,730 °C; 10,340 °F) in ozone.[10] 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)2, produces the second-hottest-known natural flame with a temperature of over 4,525 °C (8,177 °F) when it burns in oxygen.[11][12]
Cool flames
At temperatures as low as 120 °C (248 °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 100 °C (212 °F), or between "cool" and full ignition. Sometimes the variation can lead to an explosion.[9][13]
In microgravity
In the year 2000, experiments by NASA confirmed that gravity plays an indirect role in flame formation and composition.
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
See also
- Flame detector
- International Flame Research Foundation
- Olympic flame
- Oxidizing and reducing flames
- The Combustion Institute
References
- ISBN 0-521-87052-6.
- ^ "Do flames contain plasma?". Science Questions with Surprising Answers. Retrieved 26 June 2022.
- S2CID 126553613.
- ^ Archived at Ghostarchive and the Wayback Machine: "What Is Fire?". YouTube. Retrieved 27 November 2019.
- ^ "Reaction of Chlorine with Hydrogen". Archived from the original on 20 August 2008.
- ^ Gregory P. Smith; 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; Zhiwei Qin. "GRI-Mech 3.0". Archived from the original on 29 October 2007. Retrieved 8 November 2007.
- ^ ISBN 978-0-12-372510-3.
- ISBN 978-0-8493-8408-0.
- ^ a b Pearlman, Howard; Chapek, Richard M. (24 April 2000). "Cool Flames and Autoignition in Microgravity". NASA. Archived from the original on 1 May 2010. Retrieved 13 May 2010.
- .
- .
- .
- ISBN 978-1-59370-004-1.
- ^ Spiral flames in microgravity Archived 19 March 2010 at the Wayback Machine, National Aeronautics and Space Administration, 2000.
- ^ Candle Flame in Microgravity Archived 26 October 2011 at the Wayback Machine. NASA
- ^ C. H. Kim et al. Laminar Soot Processes Experiment Shedding Light on Flame Radiation Archived 11 January 2014 at the Wayback Machine. NASA, HTML Archived 20 July 2012 at the Wayback Machine
- doi:10.1086/171746.
External links
- A candle flame strongly influenced and moved about by an electric field due to the flame having ions. (archived 30 September 2011)
- Ultra-Low Emissions Low-Swirl Burner Archived 13 June 2017 at the Wayback Machine
- 7 Shades of Fire (archived 31 August 2017)
- Licence, Peter. "Coloured Flames". The Periodic Table of Videos. University of Nottingham.