The Near-Stoichiometric Behavior of Combustible Mixtures Part I: Diffusion of the Reactants†

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Abstract
The burning rate of a fuel-oxidant mixture is invariably found by experiment to have a maximum slightly on the fuel-rich side of stoichiometry. The usual explanation is that the maximum adiabatic flame temperature is shifted there by dissociation of the products. However, several aspects of this explanation are unsatisfactory: the burning rate depends on other factors than the flame temperature, especially near stoichiometry; verification is always numerical and hence for a limited range of parameters; and cool flames, where dissociation is negligible, are excluded. The present paper is an analysis of a two-reactant model, with not necessarily equal Lewis numbers K and L for oxidant and fuel (respectively),in the absence of dissociation. In particular, it is found that the phenomenon occurs whenever K is large enough and L smallenough, the precise conditions nearly always being met in practice. In other words, diffusion alone provides a satisfactory explanation. Part II will incorporate dissociation to show that, while it does explain the shift in maximum temperature, it has a subsidiary effect on the maximum burning rate.

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