Radical-rich situation flame propagation

In this case, radical-radical reactions are important because of the large radical concentrations in a fully developed flame front. 

It can be shown (Warnatz, 1981) that the hydrocarbon fuel is attacked by H, O, and OH in the first steps. The alkyl radicals formed in this way decompose rapidly to smaller alkyl radicals and alkenes (Figs. 2 and 3). Only for the smallest alkyl radicals (CH3 and C2H5) does the relatively slow thermal decomposition compete with recombination and with oxidation reactions with O atoms and O2 (Figs. 2 and 3). This part of the mechanism is ratecontrolling in alkane and alkene flames and is responsible for the similarity of all alkane and alkene flame properties. 

According to this picture, hydrocarbon combustion, at least in lean and moderately rich mixtures, is mainly governed by elementary reactions which are not specific for the fuel considered, e.g., H + O2 OH + O and CO + OH CO2 + H. This is demonstrated by the sensitivity analysis shown in Fig. 4 for the responses of the flame velocity of freely propagating stoichiometric CH4-air and C2H6-air flames at atmospheric pressure. 

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