Either by nature or design, in most combustion systems fuel and oxidizer are initially spatially separated. If the subsequent mixing between them is not sufficiently fast before chemical reaction is initiated, then the mixing and reaction will take place only in thin reaction zones that separate them. Examples are a wood panel on fire, an oil spray burning in a furnace, a candle flame, and the sparks (i.e., burning metal particles) generated when a metal surface is abraded.

The structure of a nonpremixed flame therefore consists of three zones, with a reaction zone separating a fuel-rich zone and an oxidizer-rich zone. Figure 6.1.1a shows a typical configuration for the model problem to be studied in the next section. As the fuel and oxidizer are transported toward each other, through diffusion as well as whatever convective motion the system may have, they become heated and eventually meet and mix within the reaction zone. Reaction between them subsequently takes place rapidly. The combustion products together with the heat of combustion are then transported away from the reaction zone in both directions. Since reaction occurs at a finite rate and the reaction zone has a finite thickness, complete reaction cannot be accomplished. Small amounts of fuel and oxidizer invariably leak through the reaction zone, as shown in Figure 6.1.1b.