Diffusion-controlled combustion

Abstract

We devote this brief review to some relevant aspects of diffusion-controlled combustion. After a survey of the conservation equations involved, we shall describe the Burke-Schumann limit, which is applicable when the reaction time at the flame is very short compared with the mixing time. Using as a protopypical example the How downstream from a fuel injector in a combustor chamber, we next introduce some phenomena related to finite-rate kinetics. We shall see how the high temperature sensitivity typical of combustion reactions is responsible for the presence near the injector of chemically frozen regions of low temperature where the reactants mix without chemical reaction, these regions being separated by thin premixed flames, with rich and lean branches, from regions of near equilibrium flow, where the reactants coexist only in a thin trailing diffusion flame. The role of these triple llames in the ignition, anchoring, and lift-off processes of diffusion flames will be briefly discussed.