combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combust ...

, a diffusion flame is a

flame A flame (from Latin '' flamma'') is the visible, gaseous part of a fire. It is caused by a highly exothermic chemical reaction taking place in a thin zone. When flames are hot enough to have ionized gaseous components of sufficient density the ...

in which the

oxidizer An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or " accepts"/"receives" an electron from a (called the , , or ). In other words, an oxid ...

and

fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy bu ...

are separated before burning. Contrary to its name, a diffusion flame involves both

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical p ...

and convection processes. The name diffusion flame was first suggested by S.P. Burke and T.E.W. Schumann in 1928, to differentiate from

premixed flame A premixed flame is a flame formed under certain conditions during the combustion of a premixed charge (also called pre-mixture) of fuel and oxidiser. Since the fuel and oxidiser—the key chemical reactants of combustion—are available througho ...

where fuel and oxidizer are premixed prior to burning. The diffusion flame is also referred to as nonpremixed flame. The burning rate is however still limited by the rate of diffusion. Diffusion flames tend to burn slower and to produce more soot than

s because there may not be sufficient oxidizer for the reaction to go to completion, although there are some exceptions to the rule. The soot typically produced in a diffusion flame becomes incandescent from the heat of the flame and lends the flame its readily identifiable orange-yellow color. Diffusion flames tend to have a less-localized flame front than premixed flames. The contexts for diffusion may vary somewhat. For instance, a

candle A candle is an ignitable wick embedded in wax, or another flammable solid substance such as tallow, that provides light, and in some cases, a fragrance. A candle can also provide heat or a method of keeping time. A person who makes candl ...

uses the heat of the flame itself to

vaporize Vaporization (or vaporisation) of an element or compound is a phase transition from the liquid phase to vapor. There are two types of vaporization: evaporation and boiling. Evaporation is a surface phenomenon, whereas boiling is a bulk phenomeno ...

its wax fuel and the oxidizer (

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

) diffuses into the flame from the surrounding air, while a gaslight flame (or the safety flame of a

Bunsen burner A Bunsen burner, named after Robert Bunsen, is a kind of ambient air gas burner used as laboratory equipment; it produces a single open gas flame, and is used for heating, sterilization, and combustion. The gas can be natural gas (which is ma ...

) uses fuel already in the form of a vapor. Diffusion flames are often studied in counter flow (also called opposed jet) burners. Their interest is due to possible application in the flamelet model for turbulent combustion. Furthermore they provide a convenient way to examine strained flames and flames with holes. These are also known under the name of "edge flames", characterized by a local extinction on their axis because of the high strain rates in the vicinity of the stagnation point. Diffusion flames have an entirely different appearance in a

microgravity The term micro-g environment (also μg, often referred to by the term microgravity) is more or less synonymous with the terms '' weightlessness'' and ''zero-g'', but emphasising that g-forces are never exactly zero—just very small (on the ...

environment. There is no convection to carry the hot combustion products away from the fuel source, which results in a spherical flame front, such as in the candle seen here. This is a rare example of a diffusion flame which does not produce much soot and does not therefore have a typical yellow flame. File:Candleburning.jpg, The common flame of a

is a classic example of a diffusion flame. The yellow color of the flame is due to the large number of incandescent soot particles in the incomplete combustion reaction of the flame. File:Australia Cairns Flame.jpg, A nearly-

turbulent In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

diffusion flame File:Candlespace.jpg, A candle in a

environment. This is a rare example of a diffusion flame which does not produce much soot and does not therefore have a typical yellow flame

References

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Diffusion flames in microgravity

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedi ...

Diffusion and edge flames in opposed-jet burners
Fire Combustion engineering

See also

References

External links