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Ebook Combustion (4th edition): Part 2

(BQ) Part 2 book "Combustion" has contents: Diffusion flames, ignition, environmental combustion considerations, combustion of nonvolatile fuels. | Chapter 6 Diffusion Flames A. INTRODUCTION 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 mixingrate-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. Since diffusion rates vary with pressure and the rate of overall combustion reactions varies approximately with the pressure squared, at very low pressures the flame formed will exhibit premixed combustion characteristics even though the fuel and oxidizer may be separate concentric gaseous streams. Figure 6.1 details how the flame structure varies with pressure for such a configuration where the fuel is a simple higher-order hydrocarbon [1]. Normally, the concentric fuel–oxidizer configuration is typical of diffusion flame processes. B. GASEOUS FUEL JETS Diffusion flames have far greater practical application than premixed flames. Gaseous diffusion flames, unlike premixed flames, have no fundamental characteristic property, such as flame velocity, which can be measured readily; even initial mixture .