nonpremixed

Nonpremixed, in combustion science, refers to a type of flame in which the reactants are not mixed before ignition. The fuel and oxidizer meet and react by diffusion, with combustion occurring primarily at the interface where the two streams meet. This contrasts with premixed flames, where the fuel and oxidizer are combined prior to ignition and a thin, propagating flame front forms.

In laminar nonpremixed flames, such as a Bunsen burner flame or a candle flame, the structure consists of a broad diffusion region surrounding the jet of fuel or oxidizer. The flame temperature rises toward the adiabatic flame temperature, typically in the range of about 1800 to 2300 K for hydrocarbon fuels, depending on the exact mixture and pressure. Because mixing controls the local reactant concentrations, the local stoichiometry varies spatially, and soot formation can occur in fuel-rich pockets while oxidation zones form where oxygen availability is higher.

In practical devices, nonpremixed combustion occurs when fuel is injected into air or vice versa, including gas turbines, diesel engines, and many industrial furnaces. Turbulent mixing increases flame surface area and enhances burning rates, influencing heat release, stability, and emissions. Emissions are sensitive to the degree of mixing: poor mixing can lead to unburned hydrocarbons and soot, whereas very lean pockets can reduce some pollutants but may raise NOx under certain conditions.

Nonpremixed flames are a common subject of theoretical and computational study, with models often using mixture fraction concepts, scalar dissipation rates, and detailed chemical kinetics to capture the interplay between transport, mixing, and reaction. They are valued for robustness to flow disturbances but may require careful control of mixing to manage emissions.