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A flame () is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction made in a thin zone. When flames are hot enough to have gaseous components of sufficient density, they are then considered plasma.
One may investigate different parts of a candle flame with the aid of a cold metal spoon:Archived at Ghostarchive and the
target="_blank" rel="nofollow"> Wayback Machine: 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 (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 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. Another of many possible chemical combinations is hydrazine and nitrogen tetroxide which is hypergolic and commonly used in rocket engines. 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 radicals. For instance, a well-known chemical kinetics scheme, GRI-Mech, 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.
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 . 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. A blue-colored flame only emerges when the amount of soot decreases and the Swan band from excited molecular radicals become dominant, though the blue can often be seen near the base of candles where airborne soot is less concentrated.
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 (or flame emission spectroscopy) to determine presence of some metal ions. In pyrotechnics, the pyrotechnic colorants are used to produce brightly colored fireworks.
Cyanogen, with the formula (CN)2, produces the second-hottest-known natural flame with a temperature of over when it burns in oxygen.
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:
Common flame temperatures
This is a rough guide to flame temperatures for various common substances (in air at 1 atm. pressure):
Butane ~300 °C (~600 °F) (a cool flame in low gravity) 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 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) 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 , and at up to in ozone. 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.
Cool flames
At temperatures as low as , 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 , or between "cool" and full ignition. Sometimes the variation can lead to an explosion.
In microgravity
In the year 2000, experiments by NASA confirmed that gravity plays an indirect role in flame formation and composition. Spiral flames in microgravity , NASA, 2000. 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 Weightlessness 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. Candle Flame in Microgravity . NASA 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.C. H. Kim et al. Laminar Soot Processes Experiment Shedding Light on Flame Radiation . NASA, HTML 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,Dold, J. W. (1989). Flame propagation in a nonuniform mixture: analysis of a slowly varying triple flame. Combustion and Flame, 76(1), 71-88. Joel Daou and Amable Liñán.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.
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 electrons).
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