FLUENT can import one or more flamelet files, convolute these flamelets with the assumed-shape PDFs (see Equation 15.3-3), and construct look-up tables. The flamelet files can be generated in FLUENT, or with separate, stand-alone computer codes.
Two types of flamelet files can be imported into FLUENT: binary files generated by the OPPDIF code [ 224], and standard format files described in Section 15.12.3 and in Peters and Rogg [ 283].
When flamelets are generated in physical space (such as with OPPDIF), the species and temperature vary in one spatial dimension. The species and temperature must then be mapped from physical space to mixture fraction space. If the diffusion coefficients of all species are equal, a unique definition of the mixture fraction exists. However, with differential diffusion, the mixture fraction can be defined in a number of ways.
FLUENT provides four methods of computing the mixture fraction profile along the laminar flamelet:
, , and are the mass fractions of carbon, hydrogen, and oxygen atoms, and , , and are the molecular weights. and are the values of at the oxidizer and fuel inlets.
where is the elemental mass fraction of nitrogen along the flamelet, is the mass fraction of nitrogen at the oxidizer inlet, and is the mass fraction of nitrogen at the fuel inlet.
The flamelet profiles in the multiple-flamelet data set should vary only in the strain rate imposed; the species and the boundary conditions should be the same. In addition, it is recommended that an extinguished flamelet is excluded from the multiple-flamelet data set. The formats for multiple flamelets are as follows:
For either type of file, FLUENT will determine the number of flamelet profiles and sort them in ascending strain-rate order. For flamelets generated in physical space, you can select one of the four methods available for the calculation of mixture fraction. The scalar dissipation will be calculated from the strain rate using Equation 15.3-2.