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Figure 14.7: Contours of Static Temperature on the Combustor Walls

g.Rotate the contour plot to examine the temperature field of the combusting flow on the canister walls from different angles.

7.Save the case and data files (combustor_edm.cas.gz and combustor_edm.dat.gz).

File → Write → Case & Data...

14.5. Steady Diffusion Flamelet Model Setup and Solution

In the first part of the tutorial, the combustion reaction was modeled using the Eddy Dissipation model. In this part of the tutorial, you will use the Steady Diffusion Flamelet model to simulate a turbulent non-premixed reacting flow. The Steady Diffusion Flamelet model can model local chemical non-equi- librium due to turbulent strain.

In the Steady Diffusion Flamelet model, reactions take place in a thin laminar locally one-dimensional zone, called 'flamelet'. The turbulent flame is represented by an ensemble of such flamelets. Detailed chemical kinetics is used to describe the combustion reaction. The chemistry is assumed to respond rapidly to the turbulent strain, and as the strain relaxes to zero, the chemistry tends to equilibrium. Despite the tendency toward equilibrium, a flamelet solution can often yield more accurate results than an Eddy Dissipation or oneor two-step Finite Rate solution. This is because all the chemistry details

are included, making it possible to capture some of the faster intermediate reactions. To model turbulent mixing, a probability density function (PDF) table is used as a lookup table at run time.

Note

To reduce the solution time for this tutorial, the mesh used is very coarse. This is not a suitable mesh to obtain accurate results, but it is sufficient for demonstration purposes.

To watch a video that demonstrates some steps shown below, go to

• ANSYS Fluent: Describing Non-premixed Combustion using the Steady Flamelet Model

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14.5.1. Models

Specify settings for non-premixed combustion.

Setting Up Physics → Models → Species...

1.In the Model group box, select Non-Premixed Combustion.

2.In the State Reaction group box, select Steady Diffusion Flamelet.

3.Retain the selection of Create Flamelet in the Options group box.

If you are generating a flamelet file yourself, you need to read in the chemical kinetics mechanism and thermodynamic data, which must be in CHEMKIN format.

4.Click Import CHEMKIN Mechanism...

5.In the CHEMKIN Mechanism Import dialog box, in the Kinetics Input File text entry field, enter the following:

path\KINetics\data\grimech30_50spec_mech.inp

where path is the ANSYS Fluent installation directory (for example, C:\Program Files\ANSYS Inc\v193\fluent\fluent19.3.0).

6.Click Import.

Once the reacting data file has been imported, the tab for specifying the fuel and oxidizer compositions, flamelet and PDF table become accessible.

7.In the Boundary tab, specify the fuel (methane) and oxidizer (air) stream compositions in mass fractions.

a.In the Specify Species in group box, make sure that Mass Fraction is selected.

b.Configure the following settings:

Group	Species	Mass Fraction
Fuel	ch4	1.0
Oxid	o2	0.233 (default)
	n2	0.767 (default)

Tip

Scroll down to see all the species.

Note

All boundary species with a mass or mole fraction of zero will be ignored.

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c.In the Temperature group box, retain the default values of 300 K for Fuel and Oxid.

8.In the Control tab, retain the default settings.

9.In the Flamelet tab, retain the default settings and click Calculate Flamelets.

Once the diffusion flamelets are generated, a Question dialog box opens, asking whether you want to save flamelets to a file. Click No.

10.In the Table tab, retain the default settings for the table parameters and click Calculate PDF Table to compute a non-adiabatic probability density function (PDF) table.

11.Click Display PDF Table...

12.In the PDF Table dialog box, retain the selection of Mean Temperature from the Plot Variable drop-down list and all the other default parameters and click Display.

In the graphical display of the 3D look-up table, the Z axis represents the mean temperature of the reacting fluid, and the X and Y axes represent the mean mixture fraction and the scaled variance, respectively.

The maximum and minimum values for mean temperature and the corresponding mean mixture fraction and scale variance are also reported in the console.

The 3D look-up tables are reviewed on a slice-by-slice basis. By default, the slice selected corresponds to the adiabatic enthalpy values. You can also select other slices of constant enthalpy for display.

13.Save the PDF output file (combustor_flamelet.pdf.gz).

File → Write → PDF...

a.Enter combustor_flamelet.pdf.gz for PDF File name.

b.Click OK to write the file.

By default, the file will be saved as formatted (ASCII, or text). To save a binary (unformatted) file, enable the Write Binary Files option in the Select File dialog box.

14.Click Close to close the PDF Table dialog box.

15.Click OK to close the Species Model dialog box.

14.5.2. Boundary Conditions

Specify the boundary condition for the fuel inlet.

Setup → Boundary Conditions → fuelinlet Edit...

1.In the Velocity Inlet dialog box, under the Species tab, enter 1 for Mean Mixture Fraction.

The value of 1 indicates that only pure methane will be entering the fuelinlet boundary.

2.Click OK.

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14.5.3. Solution

1.Edit the output filename for mass-weighted average of co2 at the outlet.

Solution → Monitors → Report Files → co2-out-rfile Edit...

a.Enter co2-out-fl-rfile.out for File Name.

b.Click OK to close the Edit Report File dialog box.

2.Save the case file (combustor_flamelet.cas.gz).

File → Write → Case...

3.Reinitialize the solution.

Solution → Initialization → Initialize

4.In the Run Calculation task page, retain the settings of 5 for Timescale Factor and 500 for Number of Iterations and click Calculate.

Solution → Run Calculation → Advanced...

5.Save the case and data files (combustor_flamelet.cas.gz and combustor_flamelet.dat.gz).

File → Write → Case & Data...

14.5.4. Postprocessing for the Steady Diffusion Flamelet Solution

1.Check the mass flux balance and the total sensible heat flux as described in Postprocessing for the EddyDissipation Solution (p. 489).

2.Display filled contours of mean mixture fraction on the surface plane_xz (Figure 14.8: Contours of Mean Mixture Fraction (p. 499)).

Results → Graphics → Contours → New...

a.Enter mean-mixture-fraction for Contour Name.

b.From the Contours of drop-down lists, select Pdf... and Mean Mixture Fraction.

c.From the Surfaces selection list, deselect all surfaces and select plane_xz.

d.Enable Filled in the Options group box.

e.Clear the Auto Range and Clip to Range options.

f.Enter 0.15 for Max.

g.In the Coloring group box, select Smooth.

h.Click Save/Display.

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Figure 14.8: Contours of Mean Mixture Fraction

3.Display filled contours of CO2 mass fraction in the combustion chamber (Figure 14.9: Contours of CO2 Mass Fraction (p. 499)).

Results → Graphics → Contours → co2-mass-fraction

Display

Figure 14.9: Contours of CO2 Mass Fraction

The steady diffusion flamelet simulation yields a significantly different CO2 mass fraction distribution as compared to the eddy dissipation model calculation. The lower CO2 concentration at the base of the flamelet flame is caused by low local temperature in the area, which results in slower combustion. In the eddy dissipation model, chemical kinetics is ignored, and the reaction is controlled by turbulent mixing of the materials. In this case, the CO2 concentration is greater near the base of the flame because the rate of mixing is high in the area (see Figure 14.5: Contours of CO2 Mass Fraction (p. 493)).

4. Display the outlet CO2 concentration profiles for both solutions on a single plot.

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Results → Plots → File

a.In the File XY Plot dialog box, click the Load... button to open the Select File dialog box.

b.In the Select File dialog box that opens, click once on co2-out-fl-rfile.out and co2-out-rfile.out.

Each of these files will be listed with their folder path in the bottom list to indicate that they have been selected.

Tip

If you select a file by mistake, simply click the file in the bottom list and then click Remove.

c.Click OK to save the files and close the Select File dialog box.

d.In the Plot group box, enter co2-out for Title.

e.From the Curve Information selection list, select co2-out-rfile.out | Iteration | co2-out

f.Enter co2-EDM in the lower-right text-entry box under the Legend Names selection list.

g.Click the Change Legend Entry button.

The item in the Legend Entries list for co2-out-rfile.out | Iteration | co2-out will be changed to co2-EDM. This legend entry will be displayed in the upper-left corner of the XY plot generated in a later step.

h.In a similar manner, change the legend entry for the co2-out-fl-rfile.out | Iteration | co2-out curve to be co2-Flamelet.

i.Click the Axes... button to open the Axes dialog box.

i.From the Axis list, select Y.

ii.Enter 2 for Precision.

iii.Click Apply and close the Axes dialog box.

j.Click the Curves... button to open the Curves dialog box, where you will define a different curve symbol for the CO2 concentration data.

i.Retain 0 for the Curve #.

ii.Select ---- from the Pattern drop-down list.

iii.From the Symbol drop-down list, select the "blank" choice, which is the first item in the Symbol list.

iv.Click Apply.

v.Set Curve # to 1by clicking the up-arrow button.

vi.Modify the settings for Pattern and Symbol in a manner similar to that for the previous curve.

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