页面提取自-ANSYS FLUENT 14.0 Tutorial Guide-2

Chapter 16: Modeling Species Transport and Gaseous Combustion

This tutorial is divided into the following sections:16.1. Introduction16.2. Prerequisites

16.3. Problem Description16.4. Background

16.5. Setup and Solution16.6. Summary

16.7. Further Improvements

16.1. Introduction

This tutorial examines the mixing of chemical species and the combustion of a gaseous fuel.A cylindrical combustor burning methane (ANSYS FLUENT.

) in air is studied using the eddy-dissipation model in

This tutorial demonstrates how to do the following:?????

Enable physical models, select material properties, and define boundary conditions for a turbulent flowwith chemical species mixing and reaction.

Initiate and solve the combustion simulation using the pressure-based solver.Examine the reacting flow results using graphics.Predict thermal and prompt NOx production.

Use custom field functions to compute NO parts per million.

16.2. Prerequisites

This tutorial is written with the assumption that you have completed one or more of the introductorytutorials found in this manual:???

Introduction to Using ANSYS FLUENT in ANSYS Workbench: Fluid Flow and Heat Transfer in a MixingElbow (p.1)

Parametric Analysis in ANSYS Workbench Using ANSYS FLUENT (p.77)

Introduction to Using ANSYS FLUENT: Fluid Flow and Heat Transfer in a Mixing Elbow (p.131)

and that you are familiar with the ANSYS FLUENT navigation pane and menu structure. Some steps inthe setup and solution procedure will not be shown explicitly.

To learn more about chemical reaction modeling, see \Chemistry\ in the User's Guide and \ in the Theory Guide.Otherwise, no previous experience with chemical reaction or combustion modeling is assumed.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

16.3. Problem Description

The cylindrical combustor considered in this tutorial is shown in Figure 16.1 (p.668).The flame consideredis a turbulent diffusion flame. A small nozzle in the center of the combustor introduces methane at 80

. Ambient air enters the combustor coaxially at 0.5 .The overall equivalence ratio is approximately0.76 (approximately 28 excess air).The high-speed methane jet initially expands with little interferencefrom the outer wall, and entrains and mixes with the low-speed air.The Reynolds number based on

the methane jet diameter is approximately

×

.

Figure 16.1 Combustion of Methane Gas in a Turbulent Diffusion Flame Furnace

16.4. Background

In this tutorial, you will use the generalized eddy-dissipation model to analyze the methane-air combus-tion system.The combustion will be modeled using a global one-step reaction mechanism, assumingcomplete conversion of the fuel to and .The reaction equation is

+

→

+

(16–1)

This reaction will be defined in terms of stoichiometric coefficients, formation enthalpies, and parametersthat control the reaction rate.The reaction rate will be determined assuming that turbulent mixing isthe rate-limiting process, with the turbulence-chemistry interaction modeled using the eddy-dissipationmodel.

16.5. Setup and Solution

The following sections describe the setup and solution steps for this tutorial:16.5.1. Preparation16.5.2. Step 1: Mesh

16.5.3. Step 2: General Settings16.5.4. Step 3: Models16.5.5. Step 4: Materials

16.5.6. Step 5: Boundary Conditions16.5.7. Step 6: Initial Reaction Solution16.5.8. Step 8: Postprocessing16.5.9. Step 9: NOx Prediction

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Setup and Solution

16.5.1. Preparation

1.

Extract the file species_transport.zip from the ANSYS_Fluid_Dynamics_Tutori-al_Inputs.zip archive which is available from the Customer Portal.

Note

For detailed instructions on how to obtain the ANSYS_Fluid_Dynamics_Tutori-al_Inputs.zip file, refer to Preparation (p.3) in Introduction to Using ANSYS FLU-ENT in ANSYS Workbench: Fluid Flow and Heat Transfer in a Mixing Elbow (p.1).

2.

Unzip species_transport.zip to your working folder.

The file gascomb.msh can be found in the species_transport folder created after unzippingthe file.3.

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

For more information about FLUENT Launcher, see Starting ANSYS FLUENT Using FLUENT Launcher inthe User's Guide.4.

Enable Double-Precision.

Note

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

16.5.2. Step 1: Mesh

1.

Read the mesh file gascomb.msh.File ? Read ? Mesh...

After reading the mesh file, ANSYS FLUENT will report that 1615 quadrilateral fluid cells have beenread, along with a number of boundary faces with different zone identifiers.

16.5.3. Step 2: General Settings

General1.

Check the mesh.General ? Check

ANSYS FLUENT will perform various checks on the mesh and will report the progress in the console.Ensure that the reported minimum volume reported is a positive number.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

Note

ANSYS FLUENT will issue a warning concerning the high aspect ratios of some cells

and possible impacts on calculation of Cell Wall Distance.The warning message includesrecommendations for verifying and correcting the Cell Wall Distance calculation. In thisparticular case the cell aspect ratio does not cause problems so no further action isrequired. As an optional activity, you can confirm this yourself after the solution isgenerated by plotting Cell Wall Distance as noted in the warning message.

2.

Scale the mesh.General ? Scale...

Since this mesh was created in units of millimeters, you will need to scale the mesh into meters.

a.b.c.d.

Select mm from the Mesh Was Created In drop-down list in the Scaling group box.Click Scale.

Ensure that m is selected from the View Length Unit In drop-down list.Ensure that Xmax and Ymax are set to 1.8 m and 0.225 m respectively.

The default SI units will be used in this tutorial, hence there is no need to change any units in thisproblem.

e.3.

Close the Scale Mesh dialog box.

Check the mesh.General ? Check

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Setup and Solution

Note

You should check the mesh after you manipulate it (i.e., scale, convert to polyhedra,merge, separate, fuse, add zones, or smooth and swap.) This will ensure that the qualityof the mesh has not been compromised.

4.

Examine the mesh with the default settings.

Figure 16.2 The Quadrilateral Mesh for the Combustor Model

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Chapter 16: Modeling Species Transport and Gaseous Combustion

Extra

You can use the right mouse button to probe for mesh information in the graphicswindow. If you click the right mouse button on any node in the mesh, information willbe displayed in the ANSYS FLUENT console about the associated zone, including thename of the zone.This feature is especially useful when you have several zones of thesame type and you want to distinguish between them quickly.

5.

Select Axisymmetric in the 2D Space list.General

16.5.4. Step 3: Models

Models1.

Enable heat transfer by enabling the energy equation.Models ?

Energy ? Edit...

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Setup and Solution

2.

Select the standard - turbulence model.

Models ? Viscous ? Edit...

a.Select k-epsilon in the Model list.

The Viscous Model dialog box will expand to provide further options for the k-epsilon model.

b.c.3.

Retain the default settings for the k-epsilon model.Click OK to close the Viscous Model dialog box.

Enable chemical species transport and reaction.Models ?

Species ? Edit...

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Chapter 16: Modeling Species Transport and Gaseous Combustion

a.Select Species Transport in the Model list.

The Species Model dialog box will expand to provide further options for the Species Transportmodel.

b.c.

Enable Volumetric in the Reactions group box.

Select methane-air from the Mixture Material drop-down list.Scroll down the list to find methane-air.

Note

The Mixture Material list contains the set of chemical mixtures that exist in theANSYS FLUENT database.You can select one of the predefined mixtures to accessa complete description of the reacting system.The chemical species in the systemand their physical and thermodynamic properties are defined by your selectionof the mixture material.You can alter the mixture material selection or modify themixture material properties using the Create/Edit Materials dialog box (see Step4: Materials).

d.

Select Eddy-Dissipation in the Turbulence-Chemistry Interaction group box.

The eddy-dissipation model computes the rate of reaction under the assumption that chemicalkinetics are fast compared to the rate at which reactants are mixed by turbulent fluctuations(eddies).e.

Click OK to close the Species Model dialog box.

An Information dialog box will open, reminding you to confirm the property values before continuing.Click OK to continue.674

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Setup and Solution

Prior to listing the properties that are required for the models you have enabled, ANSYS FLUENT willdisplay a warning about the symmetry zone in the console.You may have to scroll up to see thiswarning.

Warning: It appears that symmetry zone 5 should actually be an axis (it has faces with zero area projections).

Unless you change the zone type from symmetry to axis, you may not be able to continue the solution without encountering floating point errors.

In the axisymmetric model, the boundary conditions should be such that the centerline is an axis typeinstead of a symmetry type.You will change the symmetry zone to an axis boundary in Step 5:Boundary Conditions.

16.5.5. Step 4: Materials

Materials

In this step, you will examine the default settings for the mixture material.This tutorial uses mixture propertiescopied from the FLUENT Database. In general, you can modify these or create your own mixture propertiesfor your specific problem as necessary.1.

Confirm the properties for the mixture materials.Materials ?

Mixture ? Create/Edit...

The Create/Edit Materials dialog box will display the mixture material (methane-air) that was selectedin the Species Model dialog box.The properties for this mixture material have been copied from theFLUENT Database... and will be modified in the following steps.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

a.

Click the Edit... button to the right of the Mixture Species drop-down list to open the Speciesdialog box.

You can add or remove species from the mixture material as necessary using the Species dialogbox.i.

Retain the default selections from the Selected Species selection list.

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Setup and Solution

The species that make up the methane-air mixture are predefined and require no modification.ii.b.

Click OK to close the Species dialog box.

Click the Edit... button to the right of the Reaction drop-down list to open the Reactions dialogbox.

The eddy-dissipation reaction model ignores chemical kinetics (i.e., the Arrhenius rate) and usesonly the parameters in the Mixing Rate group box in the Reactions dialog box.The ArrheniusRate group box will therefore be inactive.The values for Rate Exponent and Arrhenius Rateparameters are included in the database and are employed when the alternate finite-rate/eddy-dissipation model is used.i.ii.c.d.e.

Retain the default values in the Mixing Rate group box.Click OK to close the Reactions dialog box.

Retain the selection of incompressible-ideal-gas from the Density drop-down list.Retain the selection of mixing-law from the Cp (Specific Heat) drop-down list.Retain the default values for Thermal Conductivity,Viscosity, and Mass Diffusivity.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

f.g.

Click Change/Create to accept the material property settings.Close the Create/Edit Materials dialog box.

The calculation will be performed assuming that all properties except density and specific heat areconstant.The use of constant transport properties (viscosity, thermal conductivity, and mass diffusivitycoefficients) is acceptable because the flow is fully turbulent.The molecular transport properties willplay a minor role compared to turbulent transport.

16.5.6. Step 5: Boundary Conditions

Boundary Conditions

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Setup and Solution

1.Convert the symmetry zone to the axis type.Boundary Conditions ?

symmetry-5

The symmetry zone must be converted to an axis to prevent numerical difficulties where the radiusreduces to zero.a.

Select axis from the Type drop-down list.

A Question dialog box will open, asking if it is OK to change the type of symmetry-5 from sym-metry to axis. Click Yes to continue.

The Axis dialog box will open and display the default name for the newly created axis zone. ClickOK to continue.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

2.Set the boundary conditions for the air inlet (velocity-inlet-8).Boundary Conditions ?

velocity-inlet-8 ? Edit...

To determine the zone for the air inlet, display the mesh without the fluid zone to see the boundaries.Use the right mouse button to probe the air inlet. ANSYS FLUENT will report the zone name (velocity-inlet-8) in the console.

a.Enter air-inlet for Zone Name.

This name is more descriptive for the zone than velocity-inlet-8.

b.c.d.e.f.

Enter 0.5

for Velocity Magnitude.

Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in theTurbulence group box.Retain the default value of 10Enter 0.44

for Turbulent Intensity.

for Temperature.

for Hydraulic Diameter.

Click the Thermal tab and retain the default value of 300

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Setup and Solution

g.

Click the Species tab and enter 0.23 for o2 in the Species Mass Fractions group box.

h.3.

Click OK to close the Velocity Inlet dialog box.

Set the boundary conditions for the fuel inlet (velocity-inlet-6).Boundary Conditions ?

velocity-inlet-6 ? Edit...

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Chapter 16: Modeling Species Transport and Gaseous Combustion

a.Enter fuel-inlet for Zone Name.

This name is more descriptive for the zone than velocity-inlet-6.

b.c.d.e.f.g.h.4.

Enter 80

for the Velocity Magnitude.

Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in theTurbulence group box.Retain the default value of 10Enter 0.01

for Turbulent Intensity.

for Temperature.

for Hydraulic Diameter.

Click the Thermal tab and retain the default value of 300Click OK to close the Velocity Inlet dialog box.

Click the Species tab and enter 1 for ch4 in the Species Mass Fractions group box.

Set the boundary conditions for the exit boundary (pressure-outlet-9).Boundary Conditions ?

pressure-outlet-9 ? Edit...

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Setup and Solution

a.b.c.d.e.f.g.

Retain the default value of 0

for Gauge Pressure.

Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the

Turbulence group box.Retain the default value of 10Enter 0.45

for Backflow Turbulent Intensity.

for Backflow Total Temperature.

for Backflow Hydraulic Diameter.

Click the Thermal tab and retain the default value of 300Click OK to close the Pressure Outlet dialog box.

Click the Species tab and enter 0.23 for o2 in the Species Mass Fractions group box.

The Backflow values in the Pressure Outlet dialog box are utilized only when backflow occurs at thepressure outlet. Always assign reasonable values because backflow may occur during intermediate it-erations and could affect the solution stability.5.

Set the boundary conditions for the outer wall (wall-7).Boundary Conditions ?

wall-7 ? Edit...

Use the mouse-probe method described for the air inlet to determine the zone corresponding to theouter wall.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

a.Enter outer-wall for Zone Name.

This name is more descriptive for the zone than wall-7.

b.Click the Thermal tab.i.ii.

Select Temperature in the Thermal Conditions list.Retain the default value of 300

for Temperature.

c.6.

Click OK to close the Wall dialog box.

Set the boundary conditions for the fuel inlet nozzle (wall-2).Boundary Conditions ?

wall-2 ? Edit...

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Setup and Solution

a.Enter nozzle for Zone Name.

This name is more descriptive for the zone than wall-2.

b.Click the Thermal tab.i.ii.

Retain the default selection of Heat Flux in the Thermal Conditions list.Retain the default value of 0

for Heat Flux, so that the wall is adiabatic.

c.Click OK to close the Wall dialog box.

16.5.7. Step 6: Initial Reaction Solution

You will first calculate a solution for the basic reacting flow neglecting pollutant formation. In a later step,you will perform an additional analysis to simulate NOx.1.

Select the Coupled Pseudo Transient solution method.Solution Methods

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Chapter 16: Modeling Species Transport and Gaseous Combustion

a.b.c.

Select Coupled from the Scheme drop-down list in the Pressure-Velocity Coupling group box.Retain the default selections in the Spatial Discretization group box.Enable Pseudo Transient.

The Pseudo Transient option enables the pseudo transient algorithm in the coupled pressure-based solver.This algorithm effectively adds an unsteady term to the solution equations in orderto improve stability and convergence behavior. Use of this option is recommended for generalfluid flow problems.

2.Modify the solution controls.Solution Controls

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Setup and Solution

a.Enter 0.25 under Density in the Pseudo Transient Explicit Relaxation Factors group box.The default explicit relaxation parameters in ANSYS FLUENT are appropriate for a wide range ofgeneral fluid flow problems. However, in some cases it may be necessary to reduce the relaxationfactors to stabilize the solution. Some experimentation is typically necessary to establish the op-timal values. For this tutorial, it is sufficient to reduce the density explicit relaxation factor to 0.25for stability.

b.

Click Advanced... to open the Advanced Solution Controls dialog box and select the Experttab.

The Expert tab in the Advanced Solution Controls dialog box allows you to individually specifythe solution method and Pseudo Transient Time Scale Factors for each equation, except for the

flow equations.When using the Pseudo Transient method for general reacting flow cases, increasingthe species and energy time scales is recommended.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

i.ii.

3.

Enter 10 for the Time Scale Factor for ch4,o2,co2,h2o, and Energy.Click OK to close the Advanced Solution Controls dialog box.

Ensure the plotting of residuals during the calculation.Monitors ?

Residuals ? Edit...

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Setup and Solution

a.b.4.

Ensure that Plot is enabled in the Options group box.Click OK to close the Residual Monitors dialog box.

Initialize the field variables.Solution Initialization

a.5.

Click Initialize to initialize the variables.

Save the case file (gascomb1.cas.gz).File ? Write ? Case...a.b.c.

Enter gascomb1.cas.gz for Case File.

Ensure that Write Binary Files is enabled to produce a smaller, unformatted binary file.Click OK to close the Select File dialog box.

6.Run the calculation by requesting 200 iterations.Run Calculation

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Chapter 16: Modeling Species Transport and Gaseous Combustion

a.Select Aggressive from the Length Scale Method drop-down list.

When using the Automatic Time Step Method ANSYS FLUENT computes the Pseudo Transient timestep based on characteristic length and velocity scales of the problem.The Conservative LengthScale Method uses the smaller of two computed length scales emphasizing solution stability.TheAggressive Length Scale Method uses the larger of the two which may provide faster convergencein some cases.

b.Enter 5 for the Timescale Factor.

The Timescale Factor allows you to further manipulate the computed Time Step calculated byANSYS FLUENT. Larger time steps can lead to faster convergence. However, if the time step is toolarge it can lead to solution instability.

c.d.

Enter 200 for Number of Iterations.Click Calculate.

The solution will converge after approximately 160 iterations.7.

Save the case and data files (gascomb1.cas.gz and gascomb1.dat.gz).File ? Write ? Case & Data...

Note

If you choose a file name that already exists in the current folder, ANSYS FLUENT willask you to confirm that the previous file is to be overwritten.

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Setup and Solution

16.5.8. Step 8: Postprocessing

Review the solution by examining graphical displays of the results and performing surface integrations atthe combustor exit.1.

Report the total sensible heat flux.Reports ?

Fluxes ? Set Up...

a.b.c.

Select Total Sensible Heat Transfer Rate in the Options list.Select all the boundaries from the Boundaries selection list.Click Compute and close the Flux Reports dialog box.

Note

The energy balance is good because the net result is small compared to the heatof reaction.

2.

Display filled contours of temperature (Figure 16.3 (p.692)).Graphics and Animations ? a.b.c.

Contours ? Set Up...

Ensure that Filled is enabled in the Options group box.

Ensure that Temperature... and Static Temperature are selected in the Contours of drop-downlists.

Click Display.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

Figure 16.3 Contours of Temperature

The peak temperature is approximately 2310 .

3.Display velocity vectors (Figure 16.4 (p.694)).Graphics and Animations ?

Vectors ? Set Up...

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Setup and Solution

a.b.

Enter 0.01 for Scale.

Click the Vector Options... button to open the Vector Options dialog box.

i.Enable Fixed Length.

The fixed length option is useful when the vector magnitude varies dramatically.With fixedlength vectors, the velocity magnitude is described only by color instead of by both vectorlength and color.

ii.c.

Click Apply and close the Vector Options dialog box.

Click Display and close the Vectors dialog box.

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Chapter 16: Modeling Species Transport and Gaseous Combustion

Figure 16.4 Velocity Vectors

4.Display filled contours of stream function (Figure 16.5 (p.695)).Graphics and Animations ? a.b.

Click Display.

Contours ? Set Up...

Select Velocity... and Stream Function from the Contours of drop-down lists.

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Setup and Solution

Figure 16.5 Contours of Stream Function

The entrainment of air into the high-velocity methane jet is clearly visible in the streamline display.5.

Display filled contours of mass fraction for

(Figure 16.6 (p.696)).

Graphics and Animations ? a.b.

Click Display.

Contours ? Set Up...

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

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Chapter 16: Modeling Species Transport and Gaseous Combustion

Figure 16.6 Contours of CH4 Mass Fraction

6.

In a similar manner, display the contours of mass fraction for the remaining species ,, and (Figure 16.7 (p.697),Figure 16.8 (p.698), and Figure 16.9 (p.699)) Close the Contours dialog box whenall of the species have been displayed.

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