The influence of diffusion on flame propagation

In stationary pre-mixed flames of hydrocarbons at pressures between 1 atm. and a few mm. the thickness of the reaction zone varies approximately inversely with the pressure, and the flame speed is independent of pressure. Marked exothermic reaction begins around 700 to 800° C, and the time of passage through the pre-heating zone is very much less than the induction period for ignition at this temperature; a purely thermal mechanism of flame propagation is inadequate. The thickness of the reaction zone is related to the distance which free hydrogen atoms may diffuse against the gas stream. Calculations are made on the relation between rates of diffusion, flame thickness and flame speed, and are compared with experimental values. It appears that diffusion of free atoms or radicals can account for flame propagation satisfactorily; the correct relation that the flame speed is nearly independent of pressure is obtained.