Diffusionsflammen

Diffusionsflammen, or diffusion flames, are flames in which the fuel and the oxidizer are supplied separately and mix primarily by molecular diffusion before reacting. They are a major class of non-premixed flames, in contrast to premixed flames where fuel and oxidizer are combined before ignition.

In a typical laminar diffusion flame, fuel issues from a central jet or porous burner while air advances from the surrounding region. The reacting zone forms where diffusing fuel and oxidizer meet, releasing heat and creating a thin reaction zone. The flame’s location and shape are determined by the balance between diffusion fluxes, chemical kinetics, and the flow field, with buoyancy and geometry influencing lift-off height and stability. Adiabatic flame temperatures for hydrocarbon–air diffusion flames are high, and soot formation can occur in fuel-rich areas, giving a luminous yellow inner zone in many hydrocarbon flames.

Theory and modeling often use the concept of mixture fraction, a scalar that describes the local blending of fuel and oxidizer. The local equivalence ratio and reaction rates depend on this mixing and on transport properties. Unlike premixed flames, there is no single, universal laminar burning velocity for diffusion flames; the global burning rate is set by diffusion processes and the geometry of the system.

Examples and applications include Bunsen burner flames, candle flames, and various industrial burners and gas-turbine combustors. Diffusion flames are important in safety analyses and emissions studies, as soot formation and incomplete combustion are common concerns, especially under rich conditions.