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17.3.1 Setup and Solution Procedure

1.   Read your mesh file into 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 panel.

Define $\rightarrow$ Models $\rightarrow$ Species...

(b)   If necessary, modify the Model Constants in the Species Model panel. These are the same at the constants for the premixed combustion model and, in most cases, you will not need to change them from their default values. See Section  16.3.3 for details.

3.   Generate a PDF look-up table. You can follow the procedure for non-premixed combustion described in Section  15.6.

figure   

If 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  17.3.2 for details. Also, the calculation of the thermal diffusivity uses the thermal conductivity in the Materials panel. More accurate thermal diffusivity polynomials can be obtained by editing the thermal conductivity in the Materials panel and then clicking Recalculate Properties in the Premix tab.

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

Define $\rightarrow$ Materials...

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  16.3.4 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.)

Define $\rightarrow$ Boundary Conditions...

See Section  15.13 for guidelines on setting mixture fraction and variance conditions, as well as thermal and velocity conditions at inlets.

figure   

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 conditions panel.

(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 conditions panel.

(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 conditions panel.

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.

Solve $\rightarrow$ Initialize $\rightarrow$ Patch...

See Section  16.3.6 for details.

7.   Solve the problem and perform postprocessing.

See Section  15.16.4 for guidelines about setting solution parameters. (These guidelines are for non-premixed combustion calculations, but they are relevant for partially premixed as well.)


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