Combustion 2

STAR-CCM+ User Guide Adiabatic PPDF Flamelets Tutorial 4493

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• Open the Displayers node and select the Geometry 1 node.

• In the Properties window, change the Opacity expert property to 0.2.

The baffle and the five injection holes in it are now visible through theexternal surface of the combustor.

You can now proceed to Setting up the Models

Setting up the Models

Models define the primary variables of the simulation, including pressure,temperature and velocity, and what mathematical formulation will be usedto generate the solution. In this example, the flow involves a turbulent,compressible, multi-component gas whose components are reactingchemically. The Segregated Flow model will be used together with thestandard K-Epsilon turbulence model and the PPDF reaction model.

To select the models:

• Open the Continua node, right-click on the Physics 1 node and select


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• In the Properties window, change the Convection property to 1st-order.

• Repeat this process for the Segregated Flow node.

• Make sure that the Standard K-Epsilon node has its Convection propertyset at 2nd-order.

• Save the simulation by clicking on the (Save) button.

Setting Initial Conditions

The initial velocity needs to be changed as follows:

• Select the Physics 1 > Initial Conditions > Velocity > Constant node

In the Properties window, change the Value property to 0,0,1

Constructing the PPDF Flamelet Table

STAR-CCM+ is not currently capable of producing internally-generatedPPDF Flamelet tables. However, it does contain facilities for importingpre-generated PPDF tables or constructing them from flamelet library dataproduced by the DARS package. In this tutorial, the latter method will beused.

Before we can construct the table, the directory containing the flameletlibrary files must be copied into your working directory:

• Navigate to the doc/tutorials/flamelet/ directory of yourSTAR-CCM+ installation.

• Copy the directory named ppdf into the directory containing your


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Note that the species stored in the flamelet libraries will now be listed underthe Multi-Component Gas > Gas Mixture > Gas Components node.

No further specification of species or reactions is required since allnecessary information is contained within the table.


The combustor’s initial condition is a stationary flow field consistingentirely of air. By default, the initial mixture fraction, mixture fractionvariance and velocity are all zero so no changes are required to these.However, the initial turbulence parameters must be changed.

• Select the Continua > Physics 1 > Initial Conditions > Turbulence Specification


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• Ctrl+click to select the Cyclic1 and Cyclic2 nodes.

• Right-click on the selected nodes then choose Create Interface > Periodic.

Two new periodic boundary nodes will appear in the Boundaries node anda new node named Periodic 1 will appear in the Interfaces node.

Setting Boundary Conditions and Values

All wall boundaries, including the baffle, are adiabatic no-slip walls. As thisis the default boundary type, no changes are required here. The default


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• Select the Fuel_Inlet > Physics Values > Mixture Fraction > Constant node.

• In the Properties window, enter a Value of 1.0.

• Select the Fuel_Inlet > Physics Values > Velocity > Constant node.

• In the Properties window enter -28,-60,100 m/s for the Value property.

Specification of the boundary conditions is now complete.

• Save the simulation.

Setting Solver Parameters and Stopping Criteria

To ensure rapid convergence, the under-relaxation factor for the pressureequation will be reduced. To do this:

• Select the Solvers > Segregated Flow > Pressure node.

• In the Properties window, change the Under-Relaxation Factor to 0.15.

The default stopping criterion is for the run to stop after 1000 iterations. Thismay be reduced for the current case.


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appears, select Temperature.

• Rotate the geometry scene so that the view is roughly perpendicular tothe beige plane section and the inlet boundaries are on the left.

• Select the Scenes > Geometry Scene 1 > Displayers > Section Scalar 1 node.

• In the Properties window, change the Contour Style property to SmoothFilled.


Running the Simulation

• To run the simulation, click the (Run) button on the toolbar.

If this is not displayed, use the Solution > Run menu item. You may alsoactivate the Solution toolbar by selecting Tools > Toolbars > Solution and thenclicking the toolbar button.


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Mass Fraction of H2.

This tutorial is now complete.

Summary

This tutorial introduced the following STAR-CCM+ features:

• Importing the mesh and saving the simulation

• Visualizing the geometry

• Defining an adiabatic Laminar Flamelets combustion model

• Constructing a PPDF Flamelet table

• Setting initial conditions

• Creating interfaces and defining boundary conditions

• Setting solver parameters and stopping criteria

• Creating scalar displays for examining the results

• Running the solver for a set number of iterations

• Analyzing results using STAR-CCM+’s visualization facilities

STAR-CCM+ User Guide Complex Chemistry Operator Splitting Tutorial 4516

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A premixed mixture of hydrogen and air enters the pipe through an inlet ata pressure of 1 bar and a temperature of 1000K.

Importing the Mesh and Naming the Simulation

Start up STAR-CCM+ in a manner that is appropriate to your workingenvironment and select the New Simulation option from the menu bar.

Continue by importing the mesh and naming the simulation. Atwo-dimensional mesh has been prepared for this analysis and saved in theSTAR .ccm file format.

• Select File > Import... from the menu bar. In the Open dialog, navigate tothe doc/tutorials/combustor subdirectory of your STAR-CCM+

H2 O2 H O OH HO2 H2O H2O2

N2

Molecularweights

2.01 32.0 1.005 16.0 17.005 35.005 18.01 24.01 28.0

For each species:

Density Ideal gas

Molecularviscosity 1.76 x 10–5 Pa.s

Specific heat Determined via thermodynamic polynomial functions

Thermalconductivity Determined via the Lewis number


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The selected models now appear within that node.


Importing the Complex Chemistry Definition

This step involves importing the complex chemistry definition fromexternal files.

• Select the Continua > Physics 1 > Models > DarsCFD Reaction Model node.


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installation directory and select the directory h2o2.

Click Open.

• Look at the Properties window. The name and path of the foldercontaining the complex chemistry definition should appear in theCase Path property of the DarsCFD Reaction Model object. Also, thecheckbox of the Is DarsCFD library loaded property should be ticked.

Setting Material Properties

Species are automatically imported with the complex chemistry definition.For each species, you need to verify that the specific heat method uses theThermodynamic Polynomial.

• Open the Continua > Physics 1 > Models > Multi-Component Gas > Gas


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The initial temperature of the domain will be set to 1000K. We do not expectto see very low or very high temperatures for this run, so the temperaturerange between the minimum and maximum allowable temperatures issufficient.

• Select the Continua > Physics 1 >Initial Conditions > Static Temperature > Constant node.

• In the Properties window, change the Value property to 1000 K.


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• Select the Constant node within the Velocity node.

• In the Properties window, set the Value property to 1,0.



The boundary symmetry-5 will be set to Symmetry Plane for this problem.When the mesh was originally created, this boundary was designated to bea wall, so a change of boundary type will be required. Also, boundaryconditions for the inlet and outlet need to be specified. For the outlet, thebackflow conditions should be provided, which are identical to the initialconditions specified above. For the inlet, a slightly faster velocity field willbe specified.


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• Set the Value property to 1000.

• Select the Mass Fraction > Constant node.

• In the Properties window, click on the customizer button for the Valueproperty to open the Constant - Value dialog, and enter the same valuesas for the outlet previously.




The default under-relaxation factors for the flow and turbulence equationsare suitable for this case but those for the species and energy equations needto be reduced to ensure solution convergence.


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• Change the Maximum Steps property to 300.

The solution will not run for more than 300 iterations, unless this stoppingcriterion is changed or disabled.


Plotting Simulation Data

We are going to create three plots to visualize the solution: temperature,pressure and mass fractions.

• To begin with the temperature plot, right-click the Plots node and selectNew Plot > XY Plot.


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to the next Mass Fraction scalar in the list.

• Repeat these steps until you have added Y types for all of the massfraction scalars. (There should be a total of 9.)


Reporting, Monitoring and Plotting

STAR-CCM+ can dynamically monitor virtually any quantity while thesolution develops. This requires setting up a report defining the quantity ofinterest and the parts of the region to be monitored. A monitor is thendefined based on that report. The former also helps to create an appropriateX-Y graph plot.

In this simple tutorial, the CPU time needed to perform complex chemistrycalculations is compared with the CPU time for the entire solvercalculations. From the plots it can be observed that complex chemistrycalculations are time-consuming and that they take a big portion of the totalsolver CPU time. However, complex chemistry calculations also offer themost detailed description of the underlying fluid chemistry, and for someproblems these calculations are needed to predict physical values of interestaccurately.

• Right-click the Reports node and select New Report > Total Solver CPU


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them and select Create Monitor and Plot from Report.

In the dialog that appears, choose Single Plot.

• Right-click the Plots > Reports Plot node, and rename it CPU Plot.

The analysis is now ready to be run.




If this is not displayed, use the Solution > Run menu item. You may alsoactivate the Solution toolbar by selecting Tools > Toolbars > Solution and thenclicking the toolbar button.

The Residuals display will be created automatically and will show thesolver’s progress. If necessary, click on the Residuals tab to bring theResiduals plot into view. An example of a residual plot is shown in aseparate part of the User Guide. This example will look different from yourresiduals, since the plot depends on the models selected.


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• Mass Fractions

Summary


• Importing the mesh and saving the simulation.

• Defining models for complex chemistry combustion with Dars-CFD.

• Importing the complex chemistry input for Dars-CFD.

• Defining material properties required for multi-component gases.

• Setting initial conditions.

• Defining boundary conditions.

• Setting solver parameters and stopping criteria.

• Creating XY-plots for examining the results.

• Setting up monitoring reports and plots for total CPU time.

• Running the solver for a set number of iterations.

• Analyzing the results using STAR-CCM+’s plotting facilities.

STAR-CCM+ User Guide Surface Chemistry and Chemkin File Import Tutorial 4546

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• Ensure that the options Don’t show this dialog during import andOpen geometry scene after import are not selected and then click OK.

STAR-CCM+ will provide feedback on the import process, which will takea few seconds, in the Output window.

Finally, save the new simulation to disk under the file namemethaneOnPt.sim.

Setting up the Models

Models define the primary variables of the simulation, including pressure,temperature, velocity, and what mathematical formulation will be used togenerate the solution. In this example, the flow involves an laminar,compressible, multi-component gas whose components are reactingchemically with the platinum surface. The Segregated Flow model will beused together with the Homogeneous Reactor with Surface ChemistryModel.

• To select these models, open the Continua node, right-click the Physics 1node and then select the item Select models...

In the Physics Model Selection dialog:

• Select the Stationary radio button in the Motion group box.


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The selected models now appear within that node.


Importing the Complex Chemistry Definition

This part of the simulation involves importing the complex chemistrydefinition from Chemkin-formatted files. Copy the following files fromdoc/tutorials/combustor to the directory where methaneOnPt.sim islocated:

• chem.inp

• surf.inp

• surf-therm.dat

• therm.dat

• tran.dat

The two chemkin files chem.inp and surf.inp define the surface/gaschemistry mechanism. Thermodynamic data comes from therm.dat forspecies in the gas phase, and from surf-therm.dat for species in thesurface phase. The file tran.dat has molecular data.


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• Look at the Properties window. The name and path of the foldercontaining the complex chemistry definition should appear in theCase Path property of the DarsCfd Reaction Model object. Also, thecheckbox of the Is DarsCFD library loaded property should be ticked.

Setting Material Properties

Gas and surface species are automatically imported with the complexchemistry definition. When the complex chemistry definition is given inChemkin format, gas and surface phase reactions are also imported. Foreach gas phase species, it is necessary to verify that the specific heat methoduses the Thermodynamic Polynomial.

• Open the Continua > Physics 1 > Models > Multi-Component Gas >Gas Mixture > Gas Components node. It contains the species for thisproblem.


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These subnodes can be specified with the drop-down list of the Methodproperty of the mixture properties nodes.



The initial temperature of the domain will be set to 600K. Temperatures forthis run are not expected to be very low or very high, so the temperaturerange between the minimum and maximum allowable temperatures issufficient.

• Select the Continua > Physics 1 >Initial Conditions > Static Temperature > Constant node.


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• Enter the data shown in the following screenshot and click OK.



The boundaries symmetry plane, symmetry plane-2 and symmetry plane-3 willbe set to Symmetry Plane for this problem. The boundary Default BoundaryRegion will be set to a reactive wall and boundary conditions for surfacespecies will be specified. Also, boundary conditions for the inlet and outletneed to be specified. For the outlet, the backflow conditions should beprovided, which are identical to the initial conditions specified above. Forthe inlet, a slightly faster velocity field will be specified.


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• Select the Physics Values > Static Temperature > Constant node.

• Set the Value property to 1000 K.




The cold flow simulation will be run for the first 50 iterations to achieve astable steady-state field.

• Select the Continua > Physics 1 > Models >


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The solution will not run for more than 1100 iterations, unless this stoppingcriterion is changed or disabled.


Plotting Simulation Data

Three plots will be created to visualize the solution: temperature, pressureand mass fractions. These plots will use a type of derived part called a lineprobe.

• To create a line-probe, right-click the Derived Parts node and selectNew Part > Probe > Line...

A new node named line-probe will appear in the Derived Parts node.

• With the line-probe node selected, make the following entries in theProperties window:

• Point 1: 0.0002,0.0,0.0


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plot.

• Select the Pt(S) Plot > X Type > Position node.

• In the Properties window, enter 0.0,0.0,1.0.

• Select the Pt(S) Plot > Y Types > Y Type 1 > Scalar node. SpecifySite Surface Fraction of PT(S).


Reporting, Monitoring and Plotting

STAR-CCM+ can dynamically monitor virtually any quantity while thesolution develops. This requires setting up a report defining the quantity ofinterest and the parts of the region to be monitored. A monitor is thendefined based on that report. The former also helps to create an appropriateX-Y graph plot.

In this simple tutorial, the CPU time needed to perform complex chemistrycalculations is compared with the CPU time for the entire solvercalculations. From the plots it can be observed that complex chemistrycalculations are time-consuming and that they take a big portion of the totalsolver CPU time. However, complex chemistry calculations also offer themost detailed description of the underlying fluid chemistry, and for someproblems these calculations are needed to predict physical values of interestaccurately.

• Right-click the Reports node and select New Report > Total Solver CPU


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them and select Create Monitor and Plot from Report.

In the dialog that appears, choose Single Plot.

• Right-click the Plots > Reports Plot node, and rename it CPU Plot.

The analysis is now ready to be run.




If this is not displayed, use the Solution > Run menu item. The Solutiontoolbar may also be activated by selecting Tools > Toolbars > Solution andthen clicking the toolbar button.

The Residuals display will be created automatically and will show thesolver’s progress. If necessary, click on the Residuals tab to bring theResiduals plot into view. An example of a residual plot is shown in aseparate part of the User Guide. This example will look different from theresiduals generated in this exercise, since the plot depends on the modelsselected.


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• Site Fraction of Pt(S)

Summary


• Importing the mesh and saving the simulation.

• Defining models for complex chemistry combustion with Dars-CFD.

• Importing the Chemkin-formatted complex chemistry input forDars-CFD.

• Defining material properties required for multi-component gases.

• Setting initial conditions.

• Defining boundary conditions.

• Setting solver parameters and stopping criteria.

• Creating XY-plots for examining the results.

• Setting up monitoring reports and plots for total CPU time.

• Running the solver for a set number of iterations.

• Analyzing the results using STAR-CCM+’s plotting facilities.

Documents

Combustion 2