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Models of mixtures
In general, two-reactant flames can be classified as diffusion or premixed. In a premixed flame the reactants are constituents of a homogeneous mixture that burns when raised to a sufficiently high temperature. In a diffusion flame the reactants are of separate origin; burning occurs only at a diffusion-blurred interface.
Both kinds of flames can be produced by a Bunsen burner. If the air hole is only partly open, so that a fuel-rich mixture of gas and air passes up the burner tube, a thin conical sheet of flame (typically blue-green in color) stands at the mouth; this is a premixed flame. Any excess gas escaping downstream mixes by diffusion with the surrounding atmosphere and burns as a diffusion flame (typically blue-violet). If the air hole is closed, only the diffusion flame is seen; if all the oxygen is removed from the atmosphere (but not from the air entering the hole), only the premixed flame is observed. (Yellow coloring due to carbon-particle luminosity may also be seen.)
A premixed flame can also be obtained by igniting a combustible mixture in a long uniform tube; under the right conditions, a flame propagates down the tube as a (more or less) steady process, called a deflagration wave. The Bunsen burner brings such a wave to rest by applying a counterflow and stabilizes it with appropriate velocity and thermal gradients.