Laminar diffusion flame

Combustion is one of today's main means of energy conversion. It is used in many practical systems to produce thermal energy (domestic and industrial boilers or furnaces) or electricity (thermal power plants), as well as for transport (automobile and aircraft engines, rocket motors, etc.) or waste destruction (incinerators). Combustion can be characterized as a highly exothermic, irreversible reaction(s) between a fuel (or reductant) and an oxidizer (or oxidant) according to the global scheme :

This reaction induces a high heat release that takes place in a very thin zone (most common flames have typical thicknesses δ _L of the order of 0.1 to 1 mm) leading to very high thermal gradients (the ratio of absolute temperatures between burnt and fresh gases, T _b / T _u , is of the order of 5 to 7) and wide variations in density ρ.

A wide variety of gaseous, liquid and solid fuels can be used. Among the most common are wood, coal, hydrocarbons (methane CH ₄ , propane C ₃ H ₈ , gasoline, diesel, kerosene, fuel oil, ...), hydrogen (H ₂ )... The oxidizer is most often oxygen from the air, more exceptionally pure oxygen (rocket engines, certain industrial furnaces) which enables higher temperatures to be reached and avoids the storage of inert nitrogen but poses safety problems. More rarely, other oxidizers are used (pyrotechnic rocket engines).

In many practical systems, fuel and oxidizer are injected separately into the reaction zone, without any initial premixing. Combustion is then controlled not only by the chemical reaction, but also by the diffusive transport of the reactants towards each other, hence the name diffusion flame.

Although laminar diffusion flames seem to be used only in a few rather anecdotal applications (candles, lighter flames, etc.), we will show that understanding the structure of these flames is fundamental to the description and modeling of many industrial situations.