tutorial 15. using the non-premixed combustion modelbarbertj/cfd training/fluent 12/t… · ·...

Tutorial 15. Using the Non-Premixed Combustion Model

Introduction

A 300KW BERL combustor simulation is modeled using a Probability Density Function(PDF) mixture fraction model. The reaction can be modeled using either the speciestransport model or the non-premixed combustion model. In this tutorial you will set upand solve a natural gas combustion problem using the non-premixed combustion modelfor the reaction chemistry.

This tutorial demonstrates how to do the following:

• Define inputs for modeling non-premixed combustion chemistry.

• Prepare the PDF table in ANSYS FLUENT.

• Solve a natural gas combustion simulation problem.

• Use the P-1 radiation model for combustion applications.

• Use the k-ε 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 β-function for the PDF. For details on thenon-premixed combustion modeling approach, see Chapter 16 in the separate User’sGuide.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1, andthat you are familiar with the ANSYS FLUENT navigation pane and menu structure.Some steps in the setup and solution procedure will not be shown explicitly.

Release 12.0 c© ANSYS, Inc. March 12, 2009 15-1

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 15.1, and Figure 15.2 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 15.1: Problem Description

15-2 Release 12.0 c© ANSYS, Inc. March 12, 2009


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 15.2: Close-Up of the Burner

Setup and Solution

Preparation

1. Download non_premix_combustion.zip from the User Services Center to yourworking folder (as described in Tutorial 1).

2. Unzip non_premix_combustion.zip.

The files, berl.msh and berl.prof can be found in the non premix combustion

folder, which will be created after unzipping the file.

The mesh file, berl.msh is a quadrilateral mesh describing the system geometryshown in Figures 15.1 and 15.2.

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

4. Enable Double-Precision.

For more information about FLUENT Launcher, see Section 1.1.2 in the separateUser’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.


http://www.fluentusers.com


Step 1: Mesh

1. Read the mesh file berl.msh.

File −→ Read −→Mesh...

The ANSYS FLUENT console will report that the mesh contains 9784 quadrilateralcells. A warning will be generated informing you to consider making changes to thezone type, or to change the problem definition to axisymmetric. You will changethe problem to axisymmetric swirl in Step 2.

Step 2: General Settings

General

1. Check the mesh.

General −→ Check

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

2. Scale the mesh.

General −→ Scale...

(a) Select mm from the View Length Unit In drop-down list.

All dimensions will now be shown in millimeters.

(b) Select mm from the Mesh Was Created In drop-down list in the Scaling groupbox.

(c) Click Scale to scale the mesh.



(d) Close the Scale Mesh dialog box.

3. Check the mesh.

General −→ Check

Note: It is a good idea to 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 quality of the mesh has not been compromised.

4. Examine the mesh (Figure 15.3).

MeshFLUENT 12.0 (2d, dp, pbns, lam)

Figure 15.3: 2D BERL Combustor Mesh Display

Due to the mesh 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 mesh 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.

Graphics and Animations −→ Views...

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

(b) Click Apply and close the Views dialog box.

The full geometry will be displayed, as shown in Figure 15.4.

Figure 15.4: 2D BERL Combustor Mesh Display Including the Symmetry Plane



6. Change the spatial definition to axisymmetric swirl.

General

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

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

(b) Select Axisymmetric Swirl in the 2D Space list.

Step 3: Models

Models

1. Enable the Energy Equation.

Models −→ Energy −→ Edit...

(a) Enable Energy Equation.

(b) Click OK to close the Energy dialog box.

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



2. Select the standard k-epsilon turbulence model.

Models −→ Viscous −→ Edit...

(a) Select k-epsilon (2 eqn) in 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 dialog box.

3. Select the P1 radiation model.

Models −→ Radiation −→ Edit...



(a) Select P1 in the Model list.

(b) Click OK to close the Radiation Model dialog box.

The ANSYS FLUENT console will list the properties that are required for themodel you have enabled. An Information dialog box will open, reminding youto confirm the 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.

For details on the different radiation models available in ANSYS FLUENT, see Chap-ter 13 in the separate User’s Guide.

4. Select the Non-Premixed Combustion model.

Models −→ Species −→ Edit...



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

The dialog box will expand to show the related inputs. You will use this dialogbox to create the PDF table.

When you use the non-premixed combustion model, you need to create a PDFtable. This table contains information on the thermo-chemistry and its in-teraction with turbulence. ANSYS FLUENT interpolates the PDF during thesolution of the non-premixed combustion model.

(b) Enable Inlet Diffusion in the PDF Options group box.

The Inlet Diffusion option enables the mixture fraction to diffuse out of thedomain through inlets and outlets.

(c) Define chemistry models.

i. Retain the default selection of Equilibrium and Non-Adiabatic.

In most non-premixed combustion simulations, the Equilibrium chemistrymodel is recommended. The Steady Flamelets option can model local chem-ical non-equilibrium due to turbulent strain.

ii. Retain the default value for Operating Pressure.

iii. Enter 0.064 for Fuel Stream Rich Flammability Limit.

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

The Fuel Stream Rich Flammability Limit allows you to perform a “par-tial equilibrium” calculation, suspending equilibrium calculations when themixture fraction exceeds the specified rich limit. This increases the effi-ciency of the PDF calculation, allowing you to bypass the complex equi-librium calculations in the fuel-rich region. This is also more physicallyrealistic than the assumption of full equilibrium.



(d) Click the Boundary tab to add and define the boundary species.

i. Add c2h6, c3h8, c4h10, and co2.

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

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

All the four species will appear in the table.

ii. Select Mole Fraction in the Species Unit list.

iii. Retain the default values for n2 and o2 for Oxid.

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

iv. Specify the fuel composition by entering the following values for Fuel:

The fuel composition is entered in mole fractions of the species, c2h6,

c3h8, c4h10, and co2.

Species Mole Fractionch4 0.965

n2 0.013

c2h6 0.017

c3h8 0.001

c4h10 0.001

co2 0.003



Hint: Scroll down to see all the species.

Note: All boundary species with a mass or mole fraction of zero will beignored.

v. Enter 315 K for Fuel and Oxid in the Temperature group box.

(e) Click the Control tab and retain default species to be excluded from the equi-librium calculation.

(f) Click the Table tab to specify the table parameters and calculate the PDFtable.

i. Retain the default values for all the parameters in the Table Parametersgroup box.

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

ii. Click Calculate PDF Table to compute the non-adiabatic PDF table.

iii. Click the Display PDF Table... button to open the PDF Table dialog box.



A. Retain the default parameters and click Display (Figure 15.5).

B. Close the PDF Table dialog box.

Figure 15.5: 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, theslice selected is that corresponding to the adiabatic enthalpy values. You canalso select other slices of constant enthalpy for display.

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



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

File −→ Write −→PDF...

i. Retain berl.pdf for PDF File name.

ii. 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 SelectFile dialog box.

(h) Click OK to close the Species Model dialog box.

Step 4: Materials

Materials

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

Materials −→ pdf-mixture −→ Create/Edit...

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 to 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 mesh dependent.

For more details, see Section 5.3.8 in the separate Theory Guide.

(b) Click Change/Create and close the Create/Edit Materials dialog box.

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.

ANSYS FLUENT uses the gas law to compute the mixture density and a mass-weighted mixing law to compute the mixture Cp. When the non-premixed combustionmodel is used, do not alter the properties of the individual species. This will createan inconsistency with the PDF look-up table.

Step 5: Boundary Conditions

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

Boundary Conditions −→ poutlet-3 −→ Edit...

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

(b) Enter 5% for Backflow Turbulent Intensity.

(c) Enter 600 mm for Backflow Hydraulic Diameter.

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

(e) Click OK to close the Pressure Outlet dialog box.

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.



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

Boundary Conditions −→ air-inlet-4 −→ Edit...

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

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

(c) Select vel-prof w from the Swirl-Velocity 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.

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

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

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 retainthe zero default settings.



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

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

Boundary Conditions −→ fuel-inlet-5 −→ Edit...

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

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

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

(d) Enter 5% for Turbulent Intensity.

(e) Enter 1.8 mm for Hydraulic Diameter.

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

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

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

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



5. Set the boundary conditions for wall-6.

Boundary Conditions −→ wall-6 −→ Edit...

(a) Click the Thermal tab.

i. Select Temperature in the Thermal Conditions list.

ii. Enter 1370 K for Temperature.

iii. Enter 0.5 for Internal Emissivity.

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

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

Zone Name Temperature Internal Emissivitywall-7 312 0.6

wall-8 1305 0.5

wall-9 temp-prof t (from the drop-down list) 0.6

wall-10 1100 0.5

wall-11 1273 0.6

wall-12 1173 0.6

wall-13 1173 0.6



7. Plot the profile of temperature for the wall furnace (wall-9).

Plots −→ Profile Data −→ Set Up...

(a) Select temp-prof from the Profile selection list.

(b) Retain the selection of t and x from the Y Axis Function and X Axis Functionselection lists respectively.

(c) Click Plot (Figure 15.6).

Figure 15.6: Profile Plot of Temperature for wall-9



8. Plot the profiles of velocity for the swirling air inlet (air-inlet-4).

(a) Plot the profile of axial-velocity for the swirling air inlet.


i. Select vel-prof from the Profile selection list.

ii. Retain the selection of u from the Y Axis Function selection list.

iii. Select y from the X Axis Function selection list.

iv. Click Plot (Figure 15.7).

Figure 15.7: Profile Plot of Axial-Velocity for the Swirling Air Inlet (air-inlet-4)



(b) Plot the profile of swirl-velocity for swirling air inlet.


i. Retain the selection of vel-prof from the Profile selection list.

ii. Select w from the Y Axis Function selection list.

iii. Retain the selection of y from the X Axis Function selection list.

iv. Click Plot (Figure 15.8) and close the Plot Profile Data dialog box.

Figure 15.8: Profile Plot of Swirl-Velocity for the Swirling Air Inlet (air-inlet-4)



Step 6: Operating Conditions

Boundary Conditions

1. Retain the default operating conditions.

Boundary Conditions −→ Operating Conditions...

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



Step 7: Solution

1. Set the solution parameters.

Solution Methods

(a) Select PRESTO! from the Pressure drop-down list in the Spatial Discretizationgroup box.

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



2. Set the solution controls.

Solution Controls

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

Under-Relaxation Factor ValuePressure 0.5

Density 0.8

Momentum 0.3

Turbulent Kinetic Energy 0.7

Turbulent Dissipation Rate 0.7

P1 1

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



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

Monitors −→ Residuals −→ Edit...

(a) Ensure that the Plot is enabled in the Options group box.

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



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

Solution Initialization

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

(b) Enter 0 m/s for Axial Velocity and Swirl Velocity.

(c) Enter 1300 K for Temperature.

(d) Click Initialize.

5. Save the case file (berl-1.cas.gz).

File −→ Write −→Case...



6. Start the calculation by requesting 1500 iterations.

Run Calculation

The solution will converge in approximately 1100 iterations.

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

File −→ Write −→Data...

8. Switch to second-order upwind for improved accuracy.

Solution Methods

(a) Ensure that PRESTO! is selected from the Pressure drop-down list in the SpatialDiscretization group box.

(b) Select Second Order Upwind for all the parameters except Mixture FractionVariance.

9. Save the case file (berl-2.cas.gz).

File −→ Write −→Case...

10. Request an additional 800 iterations.

Run Calculation

The solution will converge in approximately 720 iterations.

11. Save the converged second-order flow data (berl-2.dat.gz).

File −→ Write −→Data...



Step 8: Postprocessing

1. Display the predicted temperature field (Figure 15.9).

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.

The peak temperature in the system is 1987 K.



Contours of Static Temperature (k)FLUENT 12.0 (axi, swirl, dp, pbns, pdf19, ske)

1.99e+01.90e+01.82e+01.74e+01.65e+01.57e+01.48e+01.40e+01.32e+01.23e+01.15e+01.06e+09.81e+08.97e+08.13e+07.29e+06.45e+05.62e+04.78e+03.94e+03.10e+0

Y

XZ

Figure 15.9: Temperature Contours

2. Display contours of velocity (Figure 15.10).


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

(b) Click Display.

Figure 15.10: Velocity Contours



3. Display the contours of mass fraction of o2 (Figure 15.11).


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

(b) Click Display and close the Contours dialog box.

Figure 15.11: Contours of Mass Fraction of o2



Step 9: Energy Balances Reporting

ANSYS FLUENT can report the overall energy balance and details of the heat and masstransfer.

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

Reports −→ Fluxes −→ Set Up...

(a) Retain the default Selection of Mass Flow Rate in the Options group box.

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

Reports −→ Fluxes −→ Set Up...

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

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

(c) Click Compute and close the Flux Reports dialog box.

The value will be displayed in the console. Positive flux reports indicate heataddition to the domain. Negative values indicate heat leaving the domain.Again, the net heat imbalance should be a small fraction (say, 0.5% or less) ofthe total energy flux through the system. The reported value may change fordifferent runs.



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

Reports −→ Surface Integrals −→ Set Up...

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

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

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

(d) Click Compute.

A value of 1297.46 K will be displayed in the console.

(e) Close the Surface Integrals dialog box.



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 mesh. Mesh adaption can also ensure that your solution isindependent of the mesh. These steps are demonstrated in Tutorial 1.


tutorial 15. using the non-premixed combustion modelbarbertj/cfd training/fluent 12/t… · ·...

Documents