Gaseous fuel jets

Radiation from carbon particles (luminous zone) 

Luminous zone with intensely luminous zone 

FIGURE 6.1 Structure of an acetylene-air diffusion flame at various pressures in mmHg (after Gaydon and Wolfhard [1]). 

Other types of gaseous diffusion flames are those in which the flow of the fuel and oxidizer oppose each other and are referred to as counterflow diffusion flames. The types most frequently used are shown in Fig. 6.4. Although 

FIGURE 6.2 Appearance of gaseous fuel jet flames in a co-flow cylindrical configuration. 

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Flame Types and Their Characteristics. There are two main types of flames diffusion and premixed. In diffusion flames, the fuel and oxidant are separately introduced and the rate of the overall process is determined by the mixing rate. Examples of diffusion flames include the flames associated with candles, matches, gaseous fuel jets, oil sprays, and large fires, whether accidental or otherwise. In premixed flames, fuel and oxidant are mixed thoroughly prior to combustion. A fundamental understanding of both flame types and their stmcture involves the determination of the dimensions of the various zones in the flame and the temperature, velocity, and species concentrations throughout the system. 

With the development in the previous section, it is now possible to approach the classical problem of determining the shape and height of a burning gaseous fuel jet in a coaxial stream as first described by Burke and Schumann and presented in detail in Lewis and von Elbe [8],... 

As in the case of burning gaseous fuel jets, it is assumed that the fuel and oxidant approach each other in stoichiometric proportions. The stoichiometric relations are written as... 

Figure 6.11 Close-up of gaseous fuel jet showing instability...

As in the case of burning gaseous fuel jets, the diffusion equations are combined readily by assuming Dp — (X/Cp) i.e., Le = 1. The same procedure can be followed in combining the boundary conditions for the three droplet burning equations to determine the appropriate b variables to simplify the solution for the mass consumption rate. 

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