The structure of diffusion flames

Abstract

These notes are devoted to the analysis of reacting flows in systems where the reactants are initially unpremixed. We shall deal here with the combustión reactions, encountered very often in applications, which take place in the gas phase, between a fuel and an oxidizer, (typically the oxygen of the air). The combustión reactions are exothermic, so that we shall see large temperature differences in the flow field, and the reaction rates are very sensitively dependent on temperature. Then in regions of the flow field where the temperature is low the reactants will mix without chemical reactions, while in regions of high temperature the flow will be in chemical equilibrium. When the reaction can be modelled by an overall irreversible reaction between the fuel and the oxygen, the chemical equilibrium condition implies that one of the reactants has locally zero concentration, because it has been previously completely consumed by the chemical reaction. The chemical reaction has taken place in thin reaction layers in the form of rich or lean premixed flames, when they sepárate regions of frozen flow from regions of equilibrium flow, or diffusion flames, when they sepárate a región without fuel from a región without oxidizer.