Introduction - Mr-CFDdl.mr-cfd.com/tutorials/ansys-fluent/10-catalytic-comb-ch4-surface-reactions.pdfIntroduction Catalyzed combustion is a ameless process occurring at low temperature

Tutorial: Modeling Catalytic Combustion of Methane Using

Wall Surface Reactions

Introduction

Catalyzed combustion is a flameless process occurring at low temperature and therebyemitting less nitrogen oxide. It also offers constraints for flammability limits and reactordesign. These advantages of catalyzed combustion determine its potential applications.

This tutorial provides guideline and recommendation for the setup of catalyzed combustionof methane on platinum surface.

This tutorial demonstrates how to do the following:

• Set up and solve catalyzed combustion problem.

• Use CHEMKIN import for setting up reactions and materials.

• Solve the case using appropriate solver settings.

• Postprocess the resulting data.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 from ANSYSFluent Tutorial Guide, and that you are familiar with the ANSYS Fluent navigation paneand menu structure. Some steps in the setup and solution procedure will not be shownexplicitly.

Problem Description

In this tutorial a catalyzed combustion of methane, hydrogen, and air mixture is modeledon a heated platinum wall of cylindrical channel. The cylindrical channel is modeled with2D axisymmetric solver. Two CHEMKIN mechanism files are used to solve this problem:one has only the definition of gaseous species and the other has definitions of surface speciesas well as surface reactions. Volumetric reactions are not considered in this problem. Thecircular channel has three sections: inlet, catalytic, and outlet. The catalyzed reactionstake place on the wall surface of the catalytic section.

c© ANSYS, Inc. January 4, 2013 1

Modeling Catalytic Combustion of Methane Using Wall Surface Reactions

Setup and Solution

Preparation

1. Copy the files (test.msh, gas chem.che, and surf chem.che) to your working folder.

2. Use FLUENT Launcher to start 2D version of ANSYS Fluent.

3. Enable Double-Precision in the Options list.

Step 1: Mesh

1. Read the mesh file (test.msh).

File −→ Read −→Mesh...

As the mesh file is read, ANSYS Fluent will report the progress in the console.

Step 2: General Settings

1. Define the solver settings.

General −→ Axisymmetric

(a) Select Axisymmetric in the 2D Space list.

2. Check the mesh (see Figure 1).

General −→ Check

2 c© ANSYS, Inc. January 4, 2013


Figure 1: Graphics Display of the Mesh with Enlarged View

ANSYS Fluent will perform various checks on the mesh and will report the progress inthe console. Make sure the minimum volume reported is a positive number.

Step 2: Models

1. Import chemical mechanism files.

File −→ Import −→CHEMKIN Mechanism...

(a) Enable Import Surface CHEMKIN Mechanism.



(b) Import the files (gas chem.che, surf chem.che).

As the mechanism files are read, messages will appear in the console reportingthe progress of the reading.You are provided with the thermodynamic database files along with the solutionfiles. By default ANSYS Fluent should select these files from the library filesprovided with the package. If not, then select the appropriate files.

(c) Close the CHEMKIN Mechanism Import dialog box.

2. Define the species model.

Models −→ Species −→ Edit...

(a) Select Species Transport in the Model group box to open Species Model dialogbox.

i. Enable Volumetric in the Reactions group box.

ii. Enable Wall Surface in the Reactions group box.

iii. Enable Heat of Surface Reactions in the Wall Surface Reaction Options groupbox.

iv. Ensure that Inlet Diffusion is disabled in the Options group box.

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



The information dialog box will appear informing that available material properties ormethods have changed. Confirm the property values. Click OK to close the Informationdialog box.

Step 3: Materials

In this step, you will set initial site coverage for PT (Platinum).

Materials −→ Create/Edit...

1. Ensure that chemkin-import is selected from the FLUENT Mixture Materials drop-downlist.

2. Click Edit button for the Mechanism to open the Reaction Mechanisms dialog box.

(a) Ensure that Wall Surface is selected for Reaction Type.

(b) Click Define... for pt surface.



i. Enter 1 for Initial Site Coverage for pt<s>.

ii. Enter 0 for all other species in the Site Species list.

iii. Click Apply and close the Site Parameters dialog box.

(c) Click OK to close the Reaction Mechanisms dialog box.

3. Select kinetic-theory from the Mass Diffusivity drop-down list.

4. Click Change/Create and close the Create/Edit Materials dialog box.



Step 4: Boundary Conditions

Boundary Conditions

1. Define boundary conditions for inlet.

Boundary Conditions −→ inlet −→ Edit...



(a) Enter 0.8 m/s for Velocity Magnitude.

(b) Ensure that Magnitude, Normal to Boundary is selected from the Velocity Specifi-cation Method.

(c) Click the Thermal tab and retain 300 K for Temperature.

(d) Click the Species tab and set the following species mass fractions:

Species Species Mass Fractionsh2 0.0045o2 0.23ch4 0.05

(e) Click OK to close the Velocity Inlet dialog box.

2. Define boundary conditions for wall.

Boundary Conditions −→ wall −→ Edit...

(a) In the Thermal tab select Temperature from the Thermal Conditions list and enter1290 K for Temperature.

(b) Click the Species tab.

i. Enable Reaction.

ii. Ensure that mechanism-1 is selected from Reaction Mechanisms drop-downlist.

(c) Click OK to close the Wall dialog box.

The reaction will take place only on the heated portion of the outer wall.

3. Retain the default boundary conditions for all other boundaries.



Step 5: Solution

1. Set the solution parameters.

Solution Methods

(a) Select PRESTO! from the Pressure drop-down list.

2. Initialize the solution.

Solution Initialization

(a) Select Standard Initialization from the Initialization Methods group box.

(b) Enter 0.8 m/s for the Axial Velocity.

(c) Click Initialize.

Since the wall is heated up, you do not need any higher temperature patching toignite the solution.



3. Run the calculation for 200 iterations.

Run Calculation

(a) Enter 200 for Number of Iterations.

(b) Click Calculate.

The solution will converge in approximately 180 iterations (see Figure 2). However,this is not a stable solution.

Figure 2: Scaled Residuals After 180 Iterations



4. Enable the plotting of surface monitor for mass fraction of co2.

Monitors (Surface Monitors)−→ Create...

(a) Enable Plot and Write in the Options group box.

(b) Select Area-Weighted Average from the Report Type drop-down list.

(c) Select Species... and Mass fraction of co2 from the Field Variable drop-down lists.

(d) Select outlet from the Surfaces list.

(e) Click OK to close the Surface Monitor dialog box.

5. Disable the convergence criteria for the residuals.

Monitors −→ Residuals −→ Edit...



(a) Select none from the Convergence Criterion drop-down list.

(b) Click OK to close the Residual Monitors dialog box.

6. Run the calculation for another 1400 iterations.

Run Calculation

(a) Enter 1400 for Number of Iterations.

(b) Click Calculate.

The scaled residuals are as shown in Figure 3.

Figure 3: Scaled Residuals After 1600 Iterations



The surface monitors for mass fraction of co2 are as shown in Figure 4.

Figure 4: Surface Monitors for Mass Fraction of co2

7. Save the case and data files (surf-cat-comb.cas/dat.gz).

File −→ Write −→Case & Data

Step 6: Postprocessing

1. Change the location of the colormap in the graphics color window.

Graphics and Animations −→ Options...



(a) Ensure that Axes is deselected in the Layout group box.

(b) Select Bottom from the Colormap Alignment drop-down list.

(c) Click Apply and close the Display Options dialog box.

2. Display the temperature contours.

Graphics and Animations −→ Contours −→ Set Up...

(a) Enable Filled in the Options group box.

(b) Select Temperature... and Static Temperature from the Contours of drop-downlists.

(c) Click Display (see Figure 5).



Figure 5: Contours of Static Temperature with Enlarged View

3. Display contours of mass fraction of ch4.

(a) Select Species... and Mass fraction of ch4 from the Contours of drop-down lists.

(b) Click Display (see Figure 6).

Figure 6: Contours of Mass Fraction of ch4 with Enlarged View



4. Display contours of mass fraction of h2.

(a) Select Species... and Mass fraction of h2 from the Contours of drop-down lists.


Figure 7: Contours of Mass Fraction of h2 with Enlarged View

5. Display contours of mass fraction of o2.

(a) Select Species... and Mass fraction of o2 from the Contours of drop-down lists.


Figure 8: Contours of Mass Fraction of o2 with Enlarged View



6. Display contours of mass fraction of co.

(a) Select Species... and Mass fraction of co from the Contours of drop-down lists.


Figure 9: Contours of Mass Fraction of co with Enlarged View

7. Display contours of mass fraction of co2.

(a) Select Species... and Mass fraction of co2 from the Contours of drop-down lists.


Figure 10: Contours of Mass Fraction of co2 with Enlarged View



Appendix

If you run a complex case and come across the warning (in console) such as Surface StiffSolver did not converge for xx cells/faces, then you can try to run it by modifyingthe RP variables using the following TUI commands:

(rpsetvar ’species/cvd-stiff-loop-max 100)

(rpsetvar ’species/cvd-stiff-sweep 5)

The default values for the RP variables are 60 and 5, respectively.

Summary

This tutorial demonstrated how to simulate surface reaction using CHEMKIN file in ANSYSFluent. For wall surface reactions you always need two sets of CHEMKIN file, one to definegaseous species and reactions, and another to define site and bulk species along with wallsurface reaction.


Documents

Introduction - Mr-CFDdl.mr-cfd.com/tutorials/ansys-fluent/10-catalytic-comb-ch4-surface-reactions.pdfIntroduction Catalyzed combustion is a ameless process occurring at low temperature