Restrictions on the Mixture Fraction Approach
The unique dependence of (species mass fractions, density, or temperature) on (Equation 15.2-11 or 15.2-13) requires that the reacting system meet the following conditions:
It is important to emphasize that these restrictions eliminate the use of the non-premixed approach for directly modeling premixed combustion. This is because the unburned premixed stream is far from chemical equilibrium. Note, however, that an extended mixture fraction formulation, the partially premixed model (see Chapter 17), can be applied to non-premixed, premixed, and partially premixed flames.
Figures 15.2.12 and 15.2.13 illustrate typical reacting system configurations that can be handled by the non-premixed model in FLUENT. Figure 15.2.14 shows a premixed configuration that cannot be modeled using the non-premixed model.
Using the Non-Premixed Model for Liquid Fuel or Coal Combustion
You can use the non-premixed model if your FLUENT simulation includes liquid droplets and/or coal particles. In this case, fuel enters the gas phase within the computational domain at a rate determined by the evaporation, devolatilization, and char combustion laws governing the dispersed phase. In the case of coal, the volatiles and the products of char can be defined as two different types of fuel (using two mixture fractions) or as a single composite off-gas (using one mixture fraction), as described in Section 15.9.5.
Using the Non-Premixed Model with Flue Gas Recycle
While most problems you solve using the non-premixed model will involve inlets that contain either pure oxidant or pure fuel ( or 1), you can include an inlet that has an intermediate value of mixture fraction ( ) provided that this inlet represents a completely reacted mixture. Such cases arise when there is flue gas recirculation, as depicted schematically in Figure 15.2.15. Since is a conserved quantity, the mixture fraction at the flue gas recycle inlet can be computed as
where is the exit mixture fraction (and the mixture fraction at the flue gas recycle inlet), is the mass flow rate of the oxidizer inlet, is the mass flow rate of the fuel inlet, is the mass flow rate of the recycle inlet.
If a secondary stream is included,