Premixed Combustion Tutorial FLUENT in Conical Chamber using Zimont Model

Tutorial: utorial: Premixed Premixed Combustion Combustion in a Conical Conical Chamber Chamber Using the Zimont Model

Introduction The purpose of this tutorial is to provide guidelines and recommendations for setting up and solving a premixed gaseous mixture using the premixed combustion (Zimont) model. Both, adiabatic and non-adiabatic premixed combustion models will be used. This tutorial demonstrates how to do the following: •

Use premixed combustion (Zimont) model.

•

Use the adiabatic and non-adiabatic premixed combustion models.

•

Set up and solve the case with appropriate solver settings.

•

Postprocess the resulting data.

Prerequisites This tutorial is written with the assumption that you have completed Tutorial 1 from the ANSYS FLUENT 13.0 FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS the ANSYS FLUENT navigation navigation pane and menu structure. structure. Some steps in the setup and solution procedure procedure will not be shown explicitly. In this tutorial, you will use the premixed combustion model. This tutorial does not cover the mechan mechanics ics of using using this model. model. Instea Instead, d, it focuses focuses on the applicat application ion of this this model model to solve solve the proble problem. m. For more informa information tion refer refer to Chapte Chapterr 18. Modeling Modeling Premix Premixed ed Combustion in the ANSYS the ANSYS FLUENT 13.0 FLUENT 13.0 User’s Guide.

Problem Description The conical combustor considered is shown in Figure 1. A small small nozz nozzle le at the the cente centerr of the combustor introduces the lean methane/air mixture (equivalence ratio = 0.6) at 60 m/s and 650 K. The high speed flow reverses direction in the combustor and exits through the co-axial outlet.

c ANSYS,  ANSYS, Inc. January January 5, 2011

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Premixed Combustion in a Conical Chamber Using the Zimont Model

Figure 1: Problem Schematic

The chemical reaction for an equivalence ratio of 0.6 is: CH 4 + 3.33(O2 + 3.76N 2 ) = CO 2 + 2H 2 O + 1.33O2 + 12.53N 2 Some related parameters are as follows: Table 1: Premixed Mixture Properties Parameter Value Mass of air (for equivalence ratio 0.6) 2 × (32 + 3.76 × 28)/0.6 = 457.6 Mass of 1 mole of fuel 16 Mass fraction of fuel 0.0338 Heat of combustion (j/kg) 3.84e+07 Adiabatic Temperature (K) 1950 Critical Strain Rate (1/s) 5000 Laminar Flame Speed (m/s) 0.35

Preparation 1. Copy the files, conreac.msh to your working folder. 2. Use FLUENT Launcher to start the (2D) version of ANSYS FLUENT. For more information about FLUENT Launcher see Section 1.1.2, Starting ANSYS FLUENT Using FLUENT Launcher in the ANSYS FLUENT 13.0 User’s Guide.

Note: The Display Options are enabled by default. Therefore, after you read in the mesh, it will be displayed in the embedded graphics window.

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c ANSYS, Inc. January 5, 2011 


Setup and Solution Step 1: Mesh 1. Read the mesh file (conreac.msh ). File

−→

Read −→Mesh...

Step 2: General Settings 1. Define the solver settings. General

(a) Select Axisymmetric in the 2D Space list. 2. Check the mesh. General −→ Check ANSYS FLUENT will perform various checks on the mesh and will report the progress in the console. Make sure the minimum volume reported is a positive number.

3. Examine the mesh.


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Figure 2: Mesh Display

Step 3: Models Models 1. Define the viscous model. Models −→ Viscous −→ Edit... (a) Select the Standard k-epsilon (2 eqn) turbulence model. (b) Click OK to close the Viscous Model dialog box. 2. Define the species model. Models −→ Species −→ Edit...

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(a) Select Premixed Combustion in the Model list. (b) Retain the default selection of Adiabatic in the Premixed Combustion Model Options list. (c) Enter 0.637 for Turbulent Flame Speed Constant. (d) Click OK to close the Species Model dialog box. An Information dialog box will appear, reminding you to confirm the property values that have been extracted from the database.

3. Click OK in the Information dialog box to continue.


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Step 4: Materials Create a new fluid material called premixed-mixture. Materials −→ Fluid −→ Create/Edit...

1. Enter premixed-mixture for Name 2. Enter the values as shown in the Table 2. Table 2: Premixed Mixture Material Parameter Density (kg/m3 ) Adiabatic Unburnt Density (kg/m3 ) Adiabatic Unburnt Temperature (K) Adiabatic Burnt Temperature (K) Laminar Flame Speed (m/s) Critical Rate of Strain (1/s)

Value premixed-combustion 1.2 650 1950 0.35 5000

For the adiabatic model, the temperature is calculated from the temperature of the unburnt mixture and the adiabatic temperature of the burnt products. For more in-

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formation, refer to the Section 9.6.1.1, Adiabatic Temperature Calculation in ANSYS FLUENT 13.0 Theroy Guide.

3. Click Change/Create and close the Create/Edit Materials dialog box. When you click Change/Create , a Question dialog box will appear, asking you if air should be overwritten. Click No to retain air and add the new material, premixedmixture, to the list. The Create/Edit Materials dialog box will be updated to show the new material name in the FLUENT Fluid Materials list.

Step 5: Cell Zone Conditions Set the boundary conditions for fluid-6. Cell Zone Conditions −→ fluid-6 −→ Edit...

1. Select premixed-mixture from the Material Name drop-down list. 2. Click OK to close the Fluid dialog box. Step 6: Boundary Conditions 1. Set the boundary conditions for velocity-inlet-5. Boundary Conditions


−→

velocity-inlet-5 −→ Edit...

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(a) Enter 60 m/s for Velocity Magnitude. (b) Select Intensity and Length Scale from the Specification Method drop-down list. (c) Enter 0.003 m for Turbulence Length Scale. (d) Click OK to close the Velocity Inlet dialog box. 2. Set the boundary conditions for pressure-outlet-4. Boundary Conditions

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−→

pressure-outlet-4

−→

Edit...



(a) Select Intensity and Length Scale from the Specification Method drop-down list. (b) Enter 0.003 m for Backflow Turbulence Length Scale. (c) Click the Species tab and enter 1 for Backflow Progress Variable. (d) Click OK to close the Pressure Outlet dialog box. 3. Retain the default adiabatic boundary conditions for wall-1. Step 7: Solution 1. Solve for flow and turbulence equations. Solution Controls −→ Equations...

(a) De-select Premixed Combustion from the Equations list (b) Ensure that Flow and Turbulence are selected. (c) Click OK to close the Equations dialog box. 2. Initialize the solution. Solution Initialization (a) Select all-zones from the Compute from drop-down list. (b) Click Initialize. 3. Save the case file (zimont.cas.gz ). File

−→

Write −→Case...

4. Start the calculation by requesting 250 iterations. Run Calculation −→ Calculate The solution will converge in approximately 145 iterations.

5. Save the case and data files (zimont.cas/dat.gz ). File

−→

Write −→Case & Data...


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6. Solve using all the equations. Solution Controls −→ Equations... (a) Select Premixed Combustion from the Equations drop-down list. (b) Click OK to close the Equations dialog box. 7. Patch progress variable. Solution Initialization −→ Patch

(a) Select Progress Variable from the list of Variable. (b) Enter 1 for Value. (c) Select fluid-6 from Zones to Patch. (d) Click Patch and close the Patch dialog box. 8. Request an additional 200 iterations or perform iterations until the solution converges (Figure 3). Run Calculation −→ Calculate Note: In the first iteration you will notice that the solution is converged. However it is not converged at this stage. Click Calculate again. Solution will converge in approximately 80 additional iterations.

9. Save the case and data files, (zimont-ad.cas/dat.gz . File

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−→

Write −→Case & Data...



Figure 3: Scaled Residuals

Step 8: Postprocessing 1. Display the velocity vectors in the domain (Figure 4). Graphics and Animations −→ Vectors −→ Setup (a) Sensure that Velocity is selected from the Vectors of drop-down list. (b) Enter 10 for Scale and click Display. (c) Close the Vectors dialog box.

Figure 4: Velocity Vectors


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2. Display contours of stream function (Figure 5). Graphics and Animations −→ Contours −→ Set Up... (a) Select Velocity... and Stream Function from the Contours of drop-down lists and click Display.

Figure 5: Contours of Stream Function

3. Display filled contours of mean progress variable (Figure 6). (a) Enable Filled. (b) Select Premixed Combustion... and Progress Variable from the Contours of dropdown lists and click Display.

Figure 6: Contours of Mean Progress Variable

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4. Display filled contours of static temperature (Figure 7). (a) Select Premixed Combustion... and Static Temperature from the Contours of dropdown lists and click Display.

Figure 7: Contours of Static Temperature

Step 9: Setup for Non-Adiabatic Premixed Combustion 1. Change the premixed combustion species model to Non-Adiabatic. Models −→ Species −→ Edit... (a) Select Non-Adiabatic from Premixed Combustion Model Options group box. (b) Close the Species Model dialog box. An information dialog box will open reminding you to confirm the property valuesthat have been extracted from the database. Click OK.

2. Modify the following properties for the premixed-mixture material: Materials −→ premixed-mixture −→ Create/Edit...

Parameter Value Heat of Combustion 3.85e+07 Unburnt Fuel Mass Fraction 0.0338

For the non-adiabatic model, ANSYS FLUENT will solve an energy transport equation to account for heat losses or gains within the system. The temperature is calculated


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from the heat of combustion and the fuel mass fraction. For more information, refer to the Section 9.6.1.2, Non-Adiabatic Temperature Calculation in ANSYS FLUENT 13.0 Theroy Guide.

3. Enter 650 K for Temperature in the Thermal tab for velocity-inlet-5. Boundary Conditions

−→

velocity-inlet-5 −→ Edit...

Step 10: Solution (Non-Adiabatic Premixed Combustion) 1. Solve for flow and turbulence equations. (a) Select PRESTO! from the Pressure drop-down list in the Spatial Discretization group box. Solution Methods (b) Select Flow and Turbulence from the Equations selection list. Solution Controls −→ Equations... 2. Initialize the flow field and compute from all-zones. Solution Initialization 3. Start the calculation by requesting 250 iterations. Run Calculation −→ Calculate Solution will converge in approximately 135 iterations.

4. Solve using all the equations. Solution Controls −→ Equations... (a) Select all the equations from the Equations selection list. (b) Click OK to close the Equations dialog box. 5. Patch the progress variable to 1. Solution Initialization −→ Patch... 6. Request another 150 iterations or perform iterations until the solution is converged (Figure 8). Run Calculation −→ Calculate The solution will converge in approximately 120 additional iterations.

7. Save the case and data files, (zimont-nonad.cas/dat.gz) .

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Figure 8: Scaled Residuals

Step 11: Postprocessing (Non-Adiabatic Premixed Combustion) 1. Display the velocity vectors in the domain with a scale factor of 10 (Figure 9).

Figure 9: Velocity Vectors


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2. Display filled of stream function (Figure 10) with Filled from Options group box disabled.

Figure 10: Contours of Stream Function

3. Display filled contours of mean progress variable (Figure 11).

Figure 11: Contours of Mean Progress Variable

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4. Display filled contours of static temperature (Figure 12). Display contours of static temperature by selecting Temperature... and Static Temperature from the Contours of drop-down lists.

Figure 12: Contours of Static Temperature

Results Postprocessing results can be used to study the application of the premixed combustion model in ANSYS FLUENT.

Summary Application of premixed combustion model (Zimont model) in a premixed gaseous mixture case has been demonstrated in this tutorial.


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Premixed Combustion Tutorial FLUENT in Conical Chamber using Zimont Model

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