18.2.1 Setup and Solution Procedure

1.   Read your mesh file into ANSYS FLUENT and set up any other models you plan to use in conjunction with the partially premixed combustion model (turbulence, radiation, etc.).

2.   Enable the partially premixed combustion model.

(a)   Turn on the Partially Premixed Combustion model in the Species Model dialog box.

Models Species Edit...

(b)   If necessary, modify the Zimont Model Constants if you selected the Zimont Turbulent Flame Closure flame speed model in the Species Model dialog box. These are the same as the constants for the premixed combustion model and, in most cases, you will not need to change them from their default values. See Section  17.3.1 for details. Similarly, you can modify the Extended Coherent Flamelet Model Constants if you selected the Extended Coherent Flame Model. See Section  17.4.1 for details.

3.   Generate a PDF look-up table. You can follow the procedure for non-premixed combustion described in Section  16.1.

If ANSYS FLUENT warns you, during the partially premixed properties calculation, that any parameters are out of the range for the laminar flame speed function, you will need to modify the piecewise-linear points manually before saving the PDF file. See Section  18.2.2 for details. Also, the calculation of the thermal diffusivity uses the thermal conductivity in the Create/Edit Materials dialog box. More accurate thermal diffusivity polynomials can be obtained by editing the thermal conductivity in the Create/Edit Materials dialog box and then clicking Recalculate Properties in the Premixed tab.

4.   Define the physical properties for the unburnt material in the domain.

Materials

ANSYS FLUENT will automatically select the prepdf-polynomial function for Laminar Flame Speed, indicating that the piecewise-linear polynomial function from the PDF look-up table will be used to compute the laminar flame speed. You may also choose to use a user-defined function instead of a piecewise-linear polynomial function. See Section  17.3.2 for information about setting the other properties for the unburnt material.

5.   Set the values for the mean progress variable ( $\overline{c}$) and the mean mixture fraction ( $\overline{f}$) and its variance ( $\overline{f^{'2}}$) at flow inlets and exits. (For problems that include a secondary stream, you will define boundary conditions for the mean secondary partial fraction and its variance as well.)

Boundary Conditions

See Section  16.8 for guidelines on setting mixture fraction and variance conditions, as well as thermal and velocity conditions at inlets.

There are two ways to specify a premixed inlet boundary condition: (a)   If you defined the fuel composition in the Boundary tab to be the premixed inlet species, then you should set $\overline{f}=1$ and $\overline{c}=0$ in the boundary condition dialog boxes. (b)   If you set the fuel composition to pure fuel in the Boundary tab, you will need to set the correct equivalence ratio ( $0<\overline{f}<1$) and $\overline{c}=0$ at your premixed inlet boundary condition.

For example, if the premixed inlet of methane and air is at an equivalence ratio of 0.3, you can

(a)   specify the mass fraction of the fuel composition of $Y_{\rm CH_4}$ = 0.017, $Y_{\rm O_2}$ = 0.236, and $Y_{\rm N_2}$ = 0.747 in the Boundary tab and $\overline{f}$ = 1 and $\overline{c}$ = 0 in the boundary condition dialog box.

(b)   specify the mass fraction of the fuel composition of $Y_{\rm CH_4}$ = 1.0 in the Boundary tab and $\overline{f}$ = 0.017 and $\overline{c}$ = 0 in the boundary condition dialog box.

Method (a) is preferred since it will have more points in the flame zone than method (b).

6.   Initialize the value of the progress variable.

Solution Initialization Patch...

See Section  17.3.4 for details.

7.   Solve the problem and perform postprocessing.

See Section  16.10.4 for guidelines about setting solution parameters. (These guidelines are for non-premixed combustion calculations, but they are relevant for partially premixed as well.)