The thickness of laminar flames in pre-mixed reactants: optical considerations

The concept of flame thickness and the necessary criteria for its definition and measurement are briefly discussed. Owing to ray deflexions occasioned by the flames’ refractive index fields, the apparent thicknesses of luminous zones are always in part optical illusion. Expres­sions in terms of burning velocity, flame geometry and physical properties of the reactants are derived for the apparent thickness of an idealized luminous zone of no real thickness and numerical values are deduced for typical flames used in combustion research. Their similarity to measured values leads to the conclusion that, at normal pressures, such measurements do not, in general, furnish a true measure of luminous flame thickness. The second part deals with the examination of flame by extraneous light. Simple optical methods of measuring flame thickness according to various definitions are presented, their theory is discussed and their use is illustrated by sample results.