8.4.4 Flamelet Import

ANSYS FLUENT can import one or more flamelet files, convolute these flamelets with the assumed-shape PDFs (see Equation  8.4-3), and construct look-up tables. The flamelet files can be generated in ANSYS FLUENT, or with separate stand-alone computer codes.

Three types of flamelet files can be imported into ANSYS FLUENT: binary files generated by the OPPDIF code [ 210] , ASCII files generated by the CFX-RIF code [ 15, 16, 272], and standard format files described in this section in the separate User's Guide and in Peters and Rogg [ 270].

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.

ANSYS FLUENT provides four methods of computing the mixture fraction profile along the laminar flamelet:

• Average of C and H: Following the work of Drake and Blint [ 77], the mixture fraction is calculated as the mean value of $f_{\rm C}$ and $f_{\rm H}$, where $f_{\rm C}$ and $f{\rm _H}$ are the mixture fraction values based on the carbon and hydrogen elements.

• Hydrocarbon formula: Following the work of Bilger et al. [ 26], the mixture fraction is calculated as

  
  

  
  

  where

  
  

  
  

  $Y_{\rm C}$, $Y_{\rm H}$, and $Y_{\rm O}$ are the mass fractions of carbon, hydrogen, and oxygen atoms, and $M_{w,{\rm C}}$, $M_{w,{\rm H}}$, and $M_{w,{\rm O}}$ are the molecular weights. $b_{\rm ox}$ and $b_{\rm fuel}$ are the values of $b$ at the oxidizer and fuel inlets.

• Nitrogen method: The mixture fraction is computed in terms of the mass fraction of the nitrogen species:

  
  

  
  

  where $Y_{\rm N}$ is the elemental mass fraction of nitrogen along the flamelet, $Y_{\rm N, ox}$ is the mass fraction of nitrogen at the oxidizer inlet, and $Y_{\rm N, fuel}$ is the mass fraction of nitrogen at the fuel inlet.

• Read from a file (standard format files only): This option is for flamelets solved in mixture fraction space. If you choose this method, ANSYS FLUENT will search for the mixture fraction keyword Z, as specified in Peter and Roggs's work [ 270], and retrieve the data. If ANSYS FLUENT does not find mixture fraction data in the flamelet file, it will instead use the hydrocarbon formula method described above.

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:

• OPPDIF format : The multiple-flamelet OPPDIF files should be produced using the CNTN keyword in the OPPDIF script. Alternatively, you can use ANSYS FLUENT to merge a number of single-flamelet OPPDIF files into a multiple-flamelet file.

• Standard format : If you have a set of standard format flamelet files, you can import them all at the same time, and ANSYS FLUENT will merge them internally into a multiple-flamelet file. When you import the set of flamelet files, ANSYS FLUENT will search for and count the occurrences of the HEADER keyword to determine the number of flamelets in the file.

• CFX-RIF format : A CFX-RIF flamelet file contains multiple flamelets at various strains and the file should not be modified manually. Only one CFX-RIF flamelet file should be imported.

For either type of file, ANSYS 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  8.4-2.