Triple flames in mixing layers with nonunity Lewis numbers

Abstract

The present paper is devoted to the study of the effects of nonunity Lewis numbers on triple-flame propagation in nonuniform mixtures. For definiteness, the case of a strained reactive mixing layer is considered. The fuel and oxidizer that are fed to the mixing layer are allowed to have different initial temperatures. Specifically, we examine how the triple flames encountered in this context are influenced by (a) the transverse gradients in the temperature and composition of the fresh reactive mixture and (b) by differential-diffusion effects. The analysis is carried for a single irreversible reaction with a large activation energy and using the thermo-diffusive model. Analytical expressions describing the flame shape, the local burning speed, and the propagation velocity of the triple flame are obtained. In particular, it is found that the Lewis numbers affect the propagation of the triple flame in a way similar to that obtained in the studies of stretched premixed flames. For example, the flame curvature determined by the transverse gradients in the frozen mixing layer leads to flame-front velocities that grow with decreasing values of the Lewis numbers.