diffusionflame

Diffusion flame, also referred to as a diffusionflame, is a flame in which the fuel and oxidizer are supplied separately and must diffuse toward the reaction zone before burning. The flame is sustained by the diffusive transport of reactants rather than premixed combustion. Common examples include hydrocarbon flames burning in air such as a candle flame, gas burners with separate fuel and air streams, and many industrial diffusion flames.

In laminar diffusion flames, the reaction zone forms at the interface between the fuel-rich and oxidizer-rich streams and takes on a characteristic surface shape, often conical in jet flames. The local mixture fraction, which describes the relative amounts of fuel and oxidizer, governs the temperature and reaction rate. The Lewis number, the ratio of thermal diffusivity to mass diffusivity, influences flame thickness and stability; diffusion flames are typically diffusion-limited by the slower diffusing species.

Modeling diffusion flames commonly involves diffusion-transport equations coupled to chemical kinetics. The Burke-Schumann model describes a steady, diffusion-controlled flame with infinitely fast reaction and mixing of two streams, providing a simple analytical framework. Experimental studies frequently use counterflow diffusion flames to investigate structure, extinction limits, and soot formation under controlled conditions.

Practical aspects include tendencies to form soot and emit high levels of NOx under fuel-rich conditions. Diffusion flames are robust to flow perturbations and have been utilized in certain industrial burners, welding torches, and safety-oriented flame studies. They contrast with premixed flames, where fuel and oxidizer are combined before ignition.