Earlier chapters were concerned with flames in which the fuel and oxidizer are homogeneously mixed. Even if the fuel and oxidizer are separate entities in the initial stages of a combustion event and mixing occurs rapidly compared to the rate of combustion reactions, or if mixing occurs well ahead of the flame zone (as in a Bunsen burner), the burning process may be considered in terms of homogeneous premixed conditions. There are also systems in which the mixing rate is slow relative to the reaction rate of the fuel and oxidizer, in which case the mixing controls the burning rate. Most practical systems are mixing-rate-controlled and lead to diffusion flames in which fuel and oxidizer come together in a reaction zone through molecular and turbulent diffusion. The fuel may be in the form of a gaseous fuel jet or a condensed medium (either liquid or solid), and the oxidizer may be a flowing gas stream or the quiescent atmosphere. The distinctive characteristic of a diffusion flame is that the burning (or fuel consumption) rate is determined by the rate at which the fuel and oxidizer are brought together in proper proportions for reaction.