tut14

Tutorial 14. Using the Non-Premixed Combustion Model

Introduction

A 300KW BERL combustor simulation is modeled using the PDF mixture fraction model.The reaction can be modeled using either the species transport model or the non-premixedcombustion model. In this tutorial you will set up and solve a natural gas combustionproblem using the non-premixed combustion model for the reaction chemistry.

This tutorial demonstrates how to do the following:

Define inputs for modeling non-premixed combustion chemistry. Prepare a Probability Density Function (PDF) table in FLUENT. Solve a natural gas combustion simulation problem. Use the P-1 radiation model for combustion applications. Use the k- turbulence model.

The non-premixed combustion model uses a modeling approach that solves transportequations for one or two conserved scalars and the mixture fractions. Multiple chemicalspecies, including radicals and intermediate species, may be included in the problemdefinition. Their concentrations will be derived from the predicted mixture fractiondistribution.

Property data for the species are accessed through a chemical database and turbulence-chemistry interaction is modeled using a -function for the PDF. See Chapter 15 of theUsers Guide for details on the non-premixed combustion modeling approach.

Prerequisites

This tutorial assumes that you are familiar with the menu structure in FLUENT and thatyou have completed Tutorial 1. Some steps in the setup and solution procedure will notbe shown explicitly.

c Fluent Inc. September 21, 2006 14-1

Using the Non-Premixed Combustion Model

Problem Description

The flow considered is an unstaged natural gas flame in a 300 kW swirl-stabilized burner.The furnace is vertically-fired and of octagonal cross-section with a conical furnace hoodand a cylindrical exhaust duct. The furnace walls are capable of being refractory-linedor water-cooled. The burner features 24 radial fuel ports and a bluff centerbody. Air isintroduced through an annular inlet and movable swirl blocks are used to impart swirl.The combustor dimensions are described in Figure 14.1, and Figure 14.2 shows a close-up of the burner assuming 2D axisymmetry. The boundary condition profiles, velocityinlet boundary conditions of the gas, and temperature boundary conditions are based onexperimental data [1].

Figure 14.1: Problem Description

14-2 c Fluent Inc. September 21, 2006


Do 1.15 Do1.33 Do

1.66 Do

20o

0.66 Donatural gas

swirling combustion air

Do = 87 mm

24 holes 1.8 mm

195 mm

Figure 14.2: Close-Up of the Burner

Setup and Solution

Preparation

1. Download non_premix_combustion.zip from the Fluent Inc. User Services Centeror copy it from the FLUENT documentation CD to your working folder (as describedin Tutorial 1).

2. Unzip non_premix_combustion.zip.

berl.msh and berl.prof can be found in the non premix combustion folder, whichwill be created after unzipping the file.

The mesh file, berl.msh is a quadrilateral mesh describing the system geometryshown in Figures 14.1 and 14.2.

3. Start the 2D (2d) version of FLUENT.



Step 1: Grid

1. Read the mesh file berl.msh.

File Read Case...The FLUENT console will report that the mesh contains 9784 quadrilateral cells. Awarning will be generated informing you to consider making changes to the zonetype, or to change the problem definition to axisymmetric. You will change theproblem to axisymmetric swirl in Step 2.

2. Check the grid.

Grid CheckFLUENT will perform various checks on the mesh and will reports the progress inthe console. Make sure that the minimum volume reported is a positive number.

3. Scale the grid.

Grid Scale...

(a) Select mm (millimeters) from the Grid Was Created In drop-down list in theUnit Conversion group box.

(b) Click Change Length Units.

All dimensions will now be shown in millimeters.

(c) Click Scale to scale the grid.

(d) Close the Scale Grid panel.



4. Display the grid (Figure 14.3).

Display Grid...

(a) Retain the default settings.

(b) Click Display and close the Grid Display panel.

GridFLUENT 6.3 (2d, pbns, lam)

Figure 14.3: 2D BERL combustor Mesh Display

Due to the grid resolution and the size of the domain, you may find it more usefulto display just the outline, or to zoom in on various portions of the grid display.



Extra: You can use the mouse zoom button (middle button, by default) to zoomin to the display and the mouse probe button (right button, by default) to findout the boundary zone labels. The zone labels will be displayed in the console.

5. Mirror the display about the symmetry plane.

Display Views...

(a) Select axis-2 from the Mirror Planes list.

(b) Click Apply and close the Views panel.

The full geometry will be displayed, as shown in Figure 14.4.



GridFLUENT 6.3 (2d, pbns, lam)

Figure 14.4: 2D BERL Combustor Mesh Display Including the Symmetry Plane



Step 2: Models

1. Change the spatial definition to axisymmetric swirl.

Define Models Solver...

(a) Retain the default selection of Pressure Based in the Solver list.

The non-premixed combustion model is available only with the pressure-basedsolver.

(b) Select Axisymmetric Swirl in the Space list.

(c) Click OK to close the Solver panel.

2. Enable the Energy Equation.

Define Models Energy...Since heat transfer occurs in the system considered here, you will have to solve theenergy equation.



3. Select the standard k-epsilon turbulence model.

Define Models Viscous...

(a) Select k-epsilon (2 eqn) from the Model list.

For axisymmetric swirling flow, the RNG k-epsilon model can also be used.

(b) Retain all other default settings.

(c) Click OK to close the Viscous Model panel.



4. Select the P1 radiation model.

Define Models Radiation...

(a) Select P1 from the Model list.

(b) Click OK to close the Radiation Model panel.

The FLUENT console will list the properties that are required for the model youhave enabled. An Information dialog box will open, reminding you to confirmthe property values.

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

The DO radiation model produces a more accurate solution than the P1 radiationmodel but it can be CPU intensive. The P1 model will produce a quick, acceptablesolution for this problem.

See Chapter 13 of the Users Guide for details on the different radiation modelsavailable in FLUENT.



5. Select the Non-Premixed Combustion model.

Define Models Species Transport & Reaction...

(a) Select Non-Premixed Combustion from the Model list.

The panel will expand to show the related inputs. You will use this panel tocreate the PDF table.

When you use the non-premixed combustion model, you need to create a PDF table.This table contains information on the thermo-chemistry and its interaction withturbulence. FLUENT interpolates the PDF during the solution of the non-premixedcombustion model.



Step 3: Non Adiabatic PDF Table

1. Enable the Create Table option, in the PDF Options group box of the Species Modelpanel.

This will update the panel to display the inputs for creating the PDF table. The InletDiffusion option enables the mixture fraction to diffuse out of the domain throughinlets and outlets.

2. Click the Chemistry tab to define chemistry models.

(a) Retain the default selection of Equilibrium and Non-Adiabatic.

In most non-premixed combustion simulations, the Equilibrium chemistry modelis recommended. The Steady Flamelets option can model local chemical non-equilibrium due to turbulent strain.

(b) Retain the default value for Operating Pressure.

(c) Enter 0.064 for Fuel Stream in the Rich Flammability Limit box.

For combustion cases, a value larger than 10% 50% of the stoichiometricmixture fraction can be used for the rich flammability limit of the fuel stream.In this case, the stoichiometric fraction is 0.058, therefore a value that is 10%greater is 0.064.


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The Fuel Rich Flammability Limit allows you to perform a partial equilibriumcalculation, suspending equilibrium calculations when the mixture fraction ex-ceeds the specified rich limit. This increases the efficiency of the PDF cal-culation, allowing you to bypass the complex equilibrium calculations in thefuel-rich region. This is also more physically realistic than the assumption offull equilibrium.

3. Click the Boundary tab to add and define the boundary species.

(a) Add c2h6, c3h8, c4h10, and co2.

i. Enter c2h6 in the Boundary Species text-entry field and click Add.

ii. Similarly, add c3h8, c4h10, and co2.

All four species will appear in the table.

(b) Select Mole Fraction from the Species Units list.

(c) Retain default values for n2 and o2 under Oxid.

The oxidizer (air) consists of 21% O2 and 79% N2 by volume.

(d) Specify the fuel composition by entering the following values under Fuel:

The fuel composition is entered in mole fractions of the species, c2h6, c3h8,c4h10, and co2.




Species Mole Fractionch4 0.965n2 0.013c2h6 0.017c3h8 0.001c4h10 0.001co2 0.003

Hint: Scroll down to see all the species.

Note: All boundary species with a mass or mole fractions of zero will be ig-nored.

(e) Enter 315 for Fuel and Oxid each in the Temperature group box.

(f) Click Apply.

4. Click the Control tab and retain default species to be excluded from the equilibriumcalculation.

5. Click the Table tab to specify the table parameters and calculate the PDF table.

(a) Retain the default values for all the paremeters in the Table Parameters groupbox.

(b) Click Apply.

The maximum number of species determines the number of most preponderantspecies to consider after the equilibrium calculation is performed.





(c) Click Calculate PDF Table to compute the non-adiabatic PDF table.

(d) Click the Display PDF Table... button to open the PDF Table panel.

i. Retain the default parameters and click Display (Figure 14.5).

ii. Close the PDF Table panel.

Mean Temperature(K)FLUENT 6.3 (axi, swirl, pbns, pdf20, ske)

ZYX

Figure 14.5: Non-Adiabatic Temperature Look-Up Table on the Adiabatic Enthalpy Slice

The 3D look-up tables are reviewed on a slice-by-slice basis. By default, the sliceselected is that corresponding to the adiabatic enthalpy values. You can select otherslices of constant enthalpy for display, as well.



The maximum and minimum values for mean temperature and the correspondingmean mixture fraction will also be reported in the console. The maximum meantemperature is reported as 2246 K at a mean mixture fraction of 0.058.

6. Save the PDF output file (berl.pdf).

File Write PDF...(a) Enter berl.pdf for the PDF File name.

(b) Click OK to write the file.

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

7. Click OK to close the Species Model panel.

Step 4: Materials

1. Specify the continuous phase (pdf-mixture) material.

Define Materials...



All thermodynamic data for the continuous phase, including density, specific heat,and formation enthalpies are extracted from the chemical database when the non-premixed combustion model is used. These properties are transferred as the pdf-mixture material, for which only transport properties, such as viscosity and thermalconductivity, need to be defined.

(a) Select wsggm-domain-based from the Absorption Coefficient drop-down list.

Hint: Scroll down to view the Absorption Coefficient option.

This specifies a composition-dependent absorption coefficient, using the weighted-sum-of-gray-gases model. WSGGM-domain-based is a variable coefficient thatuses a length scale, based on the geometry of the model. Note that WSGGM-cell-based uses a characteristic cell length and can be more grid dependent.

See Section 13.3.8 of the Users Guide for more details.

(b) Click Change/Create and close the Materials panel.

You can click the View... button next to Mixture Species to view the species includedin the pdf-mixture material. These are the species included during the system chem-istry setup. The Density and Cp laws cannot be altered: these properties are storedin the non-premixed combustion look-up tables.

FLUENT uses the gas law to compute the mixture density and a mass-weightedmixing law to compute the mixture cp. When the non-premixed combustion modelis used, do not alter the properties of the individual species. This will create aninconsistency with the PDF look-up table.

Step 5: Operating Conditions

1. Keep the default operating conditions.

Define Operating Conditions...

The Operating Pressure was already set in the PDF table generation in Step 3.



Step 6: Boundary Conditions

1. Read the boundary conditions profile file.

File Read Profile...(a) Select berl.prof from the Select File dialog box.

(b) Click OK.

The CFD solution for reacting flows can be sensitive to the boundary conditions, inparticular the incoming velocity field and the heat transfer through the walls. Here,you will use profiles to specify the velocity at air-inlet-4, and the wall temperaturefor wall-9. The latter approach of fixing the wall temperature to measurements iscommon in furnace simulations, to avoid modeling the wall convective and radia-tive heat transfer. The data used for the boundary conditions was obtained fromexperimental data [1].

2. Define the boundary conditions for the zones.

Define Boundary Conditions...



3. Set the boundary conditions for pressure outlet (poutlet-3).

(a) Click the Momentum tab.

(b) Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(c) Enter 5% for Backflow Turbulent Intensity.

(d) Enter 600 mm for Backflow Hydraulic Diameter.

(e) Click the Thermal tab and enter 1300 for the Backflow Total Temperature.

(f) Click OK to close the Pressure Outlet panel.

The exit gauge pressure of zero defines the system pressure at the exit to be theoperating pressure. The backflow conditions for scalars (temperature, mixture frac-tion, turbulence parameters) will be used only if flow is entrained into the domainthrough the exit. It is a good idea to use reasonable values in case flow reversaloccurs at the exit at some point during the solution process.



4. Set the boundary conditions for the velocity inlet (air-inlet-4).

(a) Select Components from the Velocity Specification Method drop-down list.

(b) Select vel-prof u from the Axial-Velocity(m/s) drop-down list.

(c) Select vel-prof w from the Swirl-Velocity(m/s) drop-down list.

(d) Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(e) Enter 17% for Turbulent Intensity.

(f) Enter 29 mm for Hydraulic Diameter.

(g) Click the Thermal tab and enter 312 for Temperature.

Turbulence parameters are defined based on intensity and length scale. Therelatively large turbulence intensity of 17% may be typical for combustion airflows.

For the non-premixed combustion calculation, you have to define the inlet MeanMixture Fraction and Mixture Fraction Variance in the Species tab. In this case,the gas phase air inlet has a zero mixture fraction. Therefore, you can acceptthe zero default settings.

(h) Click OK to close the Velocity Inlet panel.



5. Set the boundary conditions for velocity inlet (fuel-inlet-5).

(a) Click the Momentum tab.

(b) Select Components from the Velocity Specification Method drop-down list.

(c) Enter 157.25 m/s for the Radial-Velocity.

(d) Select Intensity and Hydraulic Diameter from the Specification Method drop-down list in the Turbulence group box.

(e) Enter 5% for Turbulent Intensity.

(f) Enter 1.8 mm for Hydraulic Diameter.

The hydraulic diameter has been set to twice the height of the 2D inlet stream.

(g) Click the Thermal tab and enter 308 for Temperature.

(h) Click the Species tab and enter 1 for Mean Mixture Fraction for the fuel inlet.

(i) Click OK to close the Velocity Inlet panel.



6. Set the boundary conditions for wall-6.

(a) Click the Thermal tab.

i. Select Temperature from the Thermal Conditions list.

ii. Enter 1370 K for Temperature.

iii. Enter 0.5 for Internal Emissivity.

(b) Click OK to close the Wall panel.

7. Similarly, set the boundary conditions for wall-7 through wall-13 using the followingvalues:

Zone Name Temperature Internal Emissivitywall-7 312 0.6wall-8 1305 0.5wall-9 temp-prof t (from the drop-down list) 0.6wall-10 1100 0.5wall-11 1273 0.6wall-12 1173 0.6wall-13 1173 0.6



8. Close the Boundary Conditions panel.

Step 7: Solution

1. Set the solution control parameters.

Solve Controls Solution...

(a) Set the following parameters in the Under-Relaxation Factors group box:

Under-Relaxation Factor ValuePressure 0.5Density 0.8Momentum 0.3Turbulent Kinetic Energy 0.7Turbulent Dissipation Rate 0.7P1 1

The default under-relaxation factors are considered to be too aggressive forreacting flow cases with high swirl velocity.

(b) Select PRESTO! from the Pressure drop-down list in the Discretization groupbox.

(c) Retain the default selection of First Order Upwind for other parameters.

(d) Click OK to close the Solution Controls panel.



2. Initialize the flow field using the conditions at air-inlet-4.

Solve Initialize Initialize...

(a) Select air-inlet-4 from the Compute From drop-down list.

(b) Enter 0 for the Axial Velocity and Swirl Velocity each.

(c) Enter 1300 for the Temperature.

(d) Click Init and close the Solution Initialization panel.

3. Enable the display of residuals during the solution process.

Solve Monitors Residual...(a) Enable Plot in the Options group box.

(b) Click OK to close the Residual Monitors panel.

4. Save the case file (berl-1.cas).

File Write Case...5. Start the calculation by requesting 1500 iterations.

Solve Iterate...



The solution will converge in approximately 1100 iterations.

6. Save the first-order converged solution (berl-1.dat).

File Write Data...7. Switch to second-order upwind for improved accuracy.

Solve Controls Solution...(a) Ensure that PRESTO! is selected in the Pressure drop-down list in the Dis-

cretization group box.

(b) Select Second Order Upwind from the drop-down lists next to all the parametersexcept Mixture Fraction Variance in the Discretization group box.

(c) Click OK to close the Solution Controls panel.

8. Save the case file (berl-2.cas).

File Write Case...9. Request an additional 800 iterations.

Solve Iterate...10. Save the converged second-order flow data (berl-2.dat).

File Write Data...



Step 8: Postprocessing

1. Display the predicted temperature field (Figure 14.6).

Display Contours...

(a) Enable Filled in the Options group box.

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

(c) Click Display.

The peak temperature in the system is about 1994 K.

2. Display contours of velocity (Figure 14.7).

(a) Select Velocity... and Velocity Magnitude from the Contours of drop-down listsin the Contours panel.

(b) Click Display.



Contours of Static Temperature (k)FLUENT 6.3 (axi, swirl, pbns, pdf20, ske)

1.99e+031.91e+031.83e+031.74e+031.66e+031.57e+031.49e+031.40e+031.32e+031.24e+031.15e+031.07e+039.84e+028.99e+028.15e+027.31e+026.47e+025.63e+024.78e+023.94e+023.10e+02

Z

Y

X

Figure 14.6: Temperature Contours

Figure 14.7: Velocity Contours



3. Display the Mass fraction of o2 (Figure 14.8).

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

(b) Click Display.

Figure 14.8: Contours of o2 Mass Fraction

4. Close the Contours panel.



Step 9: Energy Balances Reporting

FLUENT can report the overall energy balance and details of the heat and mass transfer.

1. Compute the gas phase mass fluxes through the domain boundaries.

Report Fluxes...

(a) Select Mass Flow Rate in the Options list.

(b) Select poutlet-3, air-inlet-4, and fuel-inlet-5 from the Boundaries list.

(c) Click Compute.

The net mass imbalance should be a small fraction (say, 0.5% or less) of the totalflux through the system. If a significant imbalance occurs, you should decrease yourresidual tolerances by at least an order of magnitude and continue iterating.

2. Compute the fluxes of heat through the domain boundaries.

(a) Select Total Heat Transfer Rate in the Options list.

(b) Select all the zones from the Boundaries list.

(c) Click Compute.

A value of -16.51W will be displayed in the console. Positive flux reportsindicate heat addition to the domain. Negative values indicate heat leaving thedomain. Again, the net heat imbalance should be a small fraction (say, 0.5%or less) of the total energy flux through the system. The reported value maychange for different runs.

3. Close the Flux Reports panel.



4. Compute the mass weighted average of the temperature at the pressure outlet.

Report Surface Integrals...

(a) Select Mass-Weighted Average from the Report Type drop-down list.

(b) Select Temperature and Static Temperature... in the Field Variable drop-downlists.

(c) Select poutlet-3 from the Surfaces list.

(d) Click Compute.

A value of 1297.97 K will be displayed in the console.

5. Close the Surface Integrals panel.

Summary

In this tutorial you learned how to use the non-premixed combustion model to representthe gas phase combustion chemistry. In this approach the fuel composition was definedand assumed to react according to the equilibrium system data. This equilibrium chem-istry model can be applied to other turbulent, diffusion-reaction systems. You can alsomodel gas combustion using the finite-rate chemistry model.

You also learned how to set up and solve a gas phase combustion problem using the P1radiation model, and applying the appropriate absorption coefficient.



References

1. A. Sayre, N. Lallement, and J. Dugu, and R. Weber Scaling Characteristics ofAerodynamics and Low-NOx Properties of Industrial Natural Gas Burners, TheSCALING 400 Study, Part IV: The 300 KW BERL Test Results, IFRF Doc NoF40/y/11, International Flame Research Foundation, The Netherlands.

Further Improvements

This tutorial guides you through the steps to reach first generate an initial solution,and then reach a more-accurate second-order solution. You may be able to increase theaccuracy of the solution even further by using an appropriate higher-order discretizationscheme and by adapting the grid. Grid adaption can also ensure that your solution isindependent of the grid. These steps are demonstrated in Tutorial 1.




14 Using the Non-Premixed Combustion ModelIntroductionPrerequisitesProblem DescriptionSetup and SolutionPreparationStep 1: GridStep 2: ModelsStep 3: Non Adiabatic PDF TableStep 4: MaterialsStep 5: Operating ConditionsStep 6: Boundary ConditionsStep 7: SolutionStep 8: PostprocessingStep 9: Energy Balances Reporting

SummaryReferencesFurther Improvements

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