Structure of Flames:

Flames are not uniform within composition, length or cross section. The structure of a premixed flame that supported as a laminar flow is displays in Figure.

Figure: Schematic structure of a laminar flow flame showing various zones

As display in the figure, the flame might be separated within the following regions or zones.

i) Preheating zones

ii) Primary reaction zone or inner zone

iii) Internal zone

iv) Secondary reaction zone

The first or the innermost region of the flame is the preheating zone while the combustion mixture is heated to the ignition temperature through thermal conduction from the basic reaction zone. The another zone is the primary reaction zone or inner zone. That zone is about 0.1 mm thick at atmospheric pressure and is visible through virtue of its blue green light ascribed to radicals .C and .CH. There is no thermodynamic equilibrium in that zone and the concentration of ions and free radicals is extremely high. That region is not used for flame photometry.