Luminous flame

A luminous flame is a burning flame which is brightly visible. Much of its output is in the form of visible light, as well as heat or light in the non-visible wavelengths.

An early study of flame luminosity was conducted by Michael Faraday and became part of his series of Royal Institution Christmas Lectures, The Chemical History of a Candle.

In the simplest case, the yellow flame is luminous due to small soot particles in the flame which are heated to incandescence. Producing a deliberately luminous flame requires either a shortage of combustion air (as in a Bunsen burner) or a local excess of fuel (as for a kerosene torch). Because of this dependency upon relatively inefficient combustion, luminosity is associated with diffusion flames and is lessened with premixed flames.

The flame is yellow because of its temperature. To produce enough soot to be luminous, the flame is operated at a lower temperature than its efficient heating flame (see Bunsen burner). The colour of simple incandescence is due to black-body radiation. By Planck's law, as the temperature decreases, the peak of the black-body radiation curve moves to longer wavelengths, i.e. from the blue to the yellow.

Other factors, particularly the fuel chemistry and its propensity for forming soot, have an influence on luminosity.

The Victorian chemist, Edward Frankland, studied luminosity in flames and discovered that their luminosity also increased with pressure.

One of the most familiar instances of a luminous flame is produced by a Bunsen burner. This burner has a controllable air supply and a constant gas jet: when the air supply is reduced, a highly luminous, and thus visible, orange 'safety flame' is produced. For heating work, the air inlet is opened and the burner produces a much hotter blue flame.

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