A
flame is the product of a highly
exothermic reaction (for example,
combustion, a self-sustaining
oxidation reaction). In other words, it 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, 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
free radicals, and 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.
http://genchem.chem.wisc.edu/demonstrations/Inorganic/pages/Group67/chlorine_and_hydrogen.htm 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 partially
ionized gases, 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.
Flame color
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
blowlamp 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 a ever-hotter flame with a temperature of over 4525°C (8180°F) when it burns in oxygen.
[{{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 = }}
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.
The
National Aeronautics and Space Administration (NASA) of the
United States has recently discovered that
gravity also indirectly plays a role in flame formation and composition.
[ Spiral flames in microgravity , National Aeronautics and Space Administration, 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
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.
[ CFM-1 experiment results , National Aeronautics and Space Administration, April 2005.] 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.
[LSP-1 experiment results , National Aeronautics and Space Administration, April 2005.][SOFBAL-2 experiment results , National Aeronautics and Space Administration, April 2005.] These discoveries have potential applications in
applied science and
industry, especially concerning
fuel efficiency.
Definition: Thamelin flow: the shape and speed of a projected flame (eg a gas flame inside a tubular radiant heater).