Flammability Limits: A Re-examination

Abstract

Flammability limits of both premixed and diffusion flames for n-alkanes and n-alcohols found in the literature are assembled. Several sets of theoretical flame temperatures and combustion stoichiometrics corresponding to the limits are denned and tabulated. These data are used to discuss the implication of the commonly accepted view that extinction (i.e., limit of flammability) occurs at temperatures at which the flame reaction rates become too low to overcome the dissipation processes. It is shown that the customary assumption of thermodynamic equilibrium at the limits leads to two serious difficulties in interpretation: (i) the inference that in fuel-rich regions the flame reactions are favored by a decrease of oxygen concentration, which is unreasonable from the kinetic point of view; and (ii) the indication that, in the case of several fuels, diffusion flames have a wider range of stability than their premixed counterpart. Thus, the assumption of thermodynamic equilibrium at the limits is unrealistic. It is also shown that when the assumption of equilibrium is replaced by an alternative assumption of incomplete reaction in a way suggested by chemical-kinetic considerations, a simple qualitative explanation of the limit phenomenon, removing both difficulties, becomes apparent. It is concluded that experimental measurements of combustion-product concentrations and temperatures near the limits are needed for quantitative explanations of the observed phenomena.