Lewis number effects on the structure and extinction of diffusion flames due to strain

Abstract

In turbulent diffusion flames chemical reaction and mixing occur simultaneously in thin strained and distorted laminar mixing layers separating fuel from the oxidizer. If the reaction can be modelled in terms of an irreversible reaction F + n 02 —> Products the relative importance of chemical production and transportation terms in the conservaron equations is measured by the Damkohler number, or ratio of the character-istic mixing time tm and the characteristic chemical time tc. A typical mixing time is the inverse of the straining rate y, while tc~' is proportional to the frequency factor of the reaction times the Arrhenius exponential exp (-E/RT) involving the ratio of the activation energy of the reaction E to the thermal energy TR. Combustión reactions are exothermic and with large valúes of the ratio E/RT, and, as a result we find in the flow-fiéld regions of low temperature, where the Damkohler number is very small and, therefore, the chemical reaction is frozen. These regions coexist with regions of high temperature where because the reaction is so fast, one of the reactants is depleted, so that either the local concentration of the fuel or of the oxidizer must be zero.