Non-premixed Flames (Diffusion Flames)

Abstract

In many combustion processes, the fuel and oxidizer are separated before entering the reaction zone where they mix and burn. The combustion reactions in such cases are called ``non-premixed flames,'' or traditionally, ``diffusion flames'' because the transport of fuel and oxidizer into the reaction zone occurs primarily by diffusion. Many combustors operate in the non-premixed burning mode, often for safety reasons. Since the fuel and oxidizer are not premixed, the risk of sudden combustion (explosion) is eliminated. Chemical reactions between fuel and oxidizer occur only at the molecular level, so ``mixing'' between fuel and oxidizer must take place before combustion. In non-premixed combustion the fuel and oxidizer are transported independently to the reaction zone, by convection and diffusion, where mixing of the fuel and oxidizer occurs prior to their reaction. Often the chemical reactions are fast, hence the burning rate is limited by the transport and mixing process rather than by the chemical kinetics. Consequently, greater flame stability can be maintained. This stable characteristic makes diffusion flames attractive for many applications, notably aircraft gas-turbine engines. Topics covered in this chapter include: (1) a detailed description of a candle flame, (2) the structure of non-premixed laminar jet flames, (3) theoretical and empirical expressions for laminar jet flame height, (4) Burke-Schumann jet diffusion flames, (5) turbulent jet flames including liftoff height and blowout limit, and (6) a short discussion of condensed fuel fires.