16.7.4 Postprocessing the Look-Up Table Data

It is important for you to view your temperature and species tables to ensure that they are adequately but not excessively resolved. Inadequate resolution will lead to inaccuracies, and excessive resolution will lead to unnecessarily slow calculation times.

After a PDF table has been generated or read into ANSYS FLUENT, you can display 2D plots and 3D surfaces showing the variation of species mole fraction, density, or temperature with the mean mixture fraction, mixture fraction variance, or enthalpy using the PDF Table dialog box (e.g., Figure  16.7.2). The PDF Table dialog box can be accessed in one of two ways: you can click on the Display PDF Table... button in the Table tab of the Species Model dialog box (as shown in Figure  16.7.1) or you can use the path

Display $\rightarrow$ PDF Tables/Curves...

Figure 16.7.2: The PDF Table Dialog Box (Non-Adiabatic Case With Flamelets)

To display the look-up tables graphically, use the following procedure:

1.   In the PDF Table dialog box, in the Plot Variable drop-down list you can select temperature, density, or species fraction as the variable to be plotted.

2.   (multiple flamelets only) Specify the value of the Scalar Dissipation. In the case of non-adiabatic flamelets, there is the additional parameter of mean enthalpy. In addition to varying the mean enthalpy and mean mixture fraction, you can vary the display of the PDF table by changing the value of the scalar dissipation, which gives the table a fourth "dimension''.

3.   Specify the Plot Type as either 3D Surface or 2D Curve on 3D Surface. In the equilibrium model, a 2D curve is a slice of a 3D surface, and thus some options selected for a 3D surface may impact the display of a 2D curve.

• For a 3D surface:

  (a)   Enable or disable Draw Numbers Box under Options. When this option is turned on, the display will include a wireframe box with the numerical limits in each coordinate direction.

  (b)   (non-adiabatic cases only) Under Surface Parameters, specify the discrete independent variable to be held constant in the look-up table ( Constant Value of).

  • For a single-mixture-fraction case, select Scaled Heat Loss/Gain ( $\overline{H}$), Mean Mixture Fraction ( $\overline{f}$), or Scaled Variance ( $\overline{f'^2}$). For any mean mixture fraction $\overline{f}$, the variance varies between a minimum of 0 and a maximum of $\overline{f}(1-\overline{f})$. In order to view the mixture fraction variance, it is normalized by Equation  16.7-1 so that for any mean mixture fraction the scaled variance ranges from 0 to 0.25.

    
    

    
    

  • For a two-mixture-fraction case, the Scaled Heat Loss/Gain is the only available option.

  (c)   (non-adiabatic cases only) Specify whether the 3D array of data points available in the look-up table will be sliced by Index or Value under Slice by.

  • If you selected Index, specify the discretization Index of the variable that is being held constant. The range of integer values that you are allowed to choose from is displayed under Min and Max, and is equivalent to the number of points specified for that variable in the Table tab of the Species Model dialog box (see Section  16.7). If you specified to hold the enthalpy ( Scaled Heat Loss/Gain) constant, the enthalpy slice index corresponding to the adiabatic case will be displayed in the Adiabatic field.

  • If you selected Value, specify the numerical Value of the variable that is being held constant. The range of values that you can specify is displayed under Min and Max.

• For a 2D curve on a 3D surface:

  (a)   Specify whether you want to write the plot data to a file by toggling Write To File under Options.

  (b)   Under Curve Parameters, specify the X-Axis Function against which the plot variable will be displayed.

  • For an adiabatic single-mixture-fraction case, select Mean Mixture Fraction ( $\overline{f}$), or Scaled Variance ( $\overline{f'^2}$).

  • For a non-adiabatic single-mixture-fraction case, the options will depend on what was selected under Constant Value of under Surface Parameters, but will include two of the following: Scaled Heat Loss/Gain ( $\overline{H}$), Mean Mixture Fraction, and Scaled Variance.

  • For a two-mixture-fraction case, select Fuel Mixture Fraction ( $f_{\rm fuel}$) or Secondary Partial Fraction ( $p_{\rm sec}$).

  (c)   Specify the type of discretization (i.e., how the look-up table data will be sliced) for the variable that is being held constant (under Constant Value of Mean Mixture Fraction, Constant Value of Scaled Variance, etc.). Note that for non-adiabatic cases, each 3D surface slice contains a full set of 2D slices.

  • If you selected Index under Slice by, specify the discretization Index of the variable that is being held constant. The range of integer values that you are allowed to choose from is displayed under Min and Max, and is equivalent to the number of points specified for that variable in the Table tab of the Species Model dialog box (see Section  16.7).

  • If you selected Value under Slice by, specify the numerical Value of the variable that is being held constant. The range of values that you can specify is displayed under Min and Max.

4.   Write or display the look-up table results. If you have turned on the Write To File option for a 2D plot, click Write and specify a name for the file in the Select File dialog box (see Section  2.1.6). Otherwise, click Plot or Display as appropriate to display a 2D plot or 3D surface in the graphics window.

Figures  16.7.3 and 16.7.4 shows examples of 2D plots derived for a very simple hydrocarbon system.

Figure 16.7.3: Mean Species Fraction Derived From an Equilibrium Chemistry Calculation

Figure 16.7.4: Mean Temperature Derived From an Equilibrium Chemistry Calculation

Figure  16.7.5 shows an example of a 3D surface derived for the same system.

Figure 16.7.5: 3D Plot of Look-Up Table for Temperature Generated for a Simple Hydrocarbon System

Files for Flamelet Modeling

In this section, information is provided about the standard flamelet files used for flamelet generation and import.

Standard Flamelet Files

The data structure for the standard flamelet file format is based on keywords that precede each data section. If any of the keywords in your flamelet data file do not match the supported keywords, you will have to manually edit the file and change the keywords to one of the supported types. (The ANSYS FLUENT flamelet filter is case-insensitive, so you need not worry about capitalization within the keywords.)

The following keywords are supported by the ANSYS FLUENT filter:

• Header section: HEADER

• Main body section: BODY

• Number of species: NUMOFSPECIES

• Number of grid points: GRIDPOINTS

• Pressure: PRESSURE

• Strain rate: STRAINRATE

• Scalar dissipation: CHI

• Temperature: TEMPERATURE and TEMP

• Mass fraction: MASSFRACTION-

• Mixture fraction: Z

Sample File

A sample flamelet file in the standard format is provided below. Note that not all species are listed in this file.

HEADER
STRAINRATE 100.
NUMOFSPECIES 12
GRIDPOINTS 39
PRESSURE 1.
BODY
Z
  0.0000E+00  4.3000E-07  2.1780E-06  1.2651E-05  7.8456E-05
  2.1876E-04  5.9030E-04  9.4701E-04  1.4700E-03  1.8061E-03
  2.1967E-03  2.6424E-03  3.1435E-03  4.3038E-03  5.6637E-03
  8.9401E-03  1.2800E-02  1.7114E-02  2.1698E-02  2.6304E-02
  2.8522E-02  3.0647E-02  3.2680E-02  3.4655E-02  4.2784E-02
  5.2655E-02  6.5420E-02  8.2531E-02  1.0637E-01  1.4122E-01
  1.9518E-01  2.8473E-01  4.4175E-01  6.6643E-01  8.6222E-01
  9.5897E-01  9.9025E-01  9.9819E-01  1.0000E+00
TEMPERATURE
  3.0000E+02  3.0013E+02  3.0085E+02  3.0475E+02  3.2382E+02
  3.5644E+02  4.3055E+02  4.9469E+02  5.8260E+02  6.3634E+02
  6.9655E+02  7.6268E+02  8.3393E+02  9.8775E+02  1.1493E+03
  1.4702E+03  1.7516E+03  1.9767E+03  2.1403E+03  2.2444E+03
  2.2766E+03  2.2962E+03  2.3044E+03  2.3027E+03  2.2164E+03
  2.0671E+03  1.8792E+03  1.6655E+03  1.4355E+03  1.1986E+03
  9.6530E+02  7.5025E+02  5.7496E+02  4.4805E+02  3.6847E+02
  3.2730E+02  3.0939E+02  3.0248E+02  3.0000E+02
MASSFRACTION-H2
  3.2354E-07  7.4290E-07  1.6979E-06  3.8179E-06  8.3038E-06
  1.2219E-05  1.7873E-05  2.1556E-05  2.5872E-05  2.8290E-05
  3.0888E-05  3.3684E-05  3.6720E-05  4.3768E-05  5.4359E-05
  1.0484E-04  2.6807E-04  6.1906E-04  1.2615E-03  2.3555E-03
  3.1422E-03  4.1281E-03  5.3302E-03  6.7434E-03  1.4244E-02
  2.4296E-02  3.7472E-02  5.5159E-02  7.9788E-02  1.1573E-01
  1.7135E-01  2.6359E-01  4.2527E-01  6.5658E-01  8.5814E-01
  9.5775E-01  9.8996E-01  9.9814E-01  1.0000E+00
MASSFRACTION-CH4
     .          .            .           .           .
     .          .            .           .           .
     .          .            .           .           .
MASSFRACTION-O
  6.8919E-10  2.8720E-09  1.1905E-08  4.8669E-08  2.0370E-07
  5.5281E-07  1.7418E-06  3.6996E-06  8.3107E-06  1.3525E-05
  2.2484E-05  3.8312E-05  6.6385E-05  1.8269E-04  4.4320E-04
  1.4284E-03  2.7564E-03  3.9063E-03  4.3237E-03  3.7141E-03
  3.0916E-03  2.3917E-03  1.7345E-03  1.2016E-03  2.4323E-04
  5.2235E-05  1.1469E-05  2.3011E-06  3.7414E-07  4.2445E-08
  2.7470E-09  8.7551E-11  2.9341E-12  7.0471E-13  0.0000E+00
  7.2143E-14  0.0000E+00  0.0000E+00  0.0000E+00

Missing Species

ANSYS FLUENT will check whether all species in the flamelet data file exist in the thermodynamic properties databases thermo.db. If any of the species in the flamelet file do not exist, ANSYS FLUENT will issue an error message and halt the flamelet import. If this occurs, you can either add the missing species to the database, or remove the species from the flamelet file.

You should not remove a species from the flamelet data file unless its species concentration is very small ( $10^{-3}$ or less) throughout the flamelet profile. If you remove a low-concentration species, you will not have the species concentrations available for viewing in the ANSYS FLUENT calculation, but the accuracy of the ANSYS FLUENT calculation will otherwise be unaffected.

If you choose to remove any species, be sure to also update the number of species (keyword NUMOFSPECIES) in the flamelet data file, to reflect the loss of any species you have removed from the file.

If a species with relatively large concentration is missing from the ANSYS FLUENT thermodynamic databases, you will have to add it. Removing a high-concentration species from the flamelet file is not recommended.