01-2d-berl-edm.pdf

Tutorial: 2D Simulation of a 300 KW BERL Combustor

Using the Magnussen Model

Introduction

The purpose of this tutorial is to provide guidelines and recommendations for setting upand solving a natural gas combustion problem.

This tutorial demonstrates how to do the following:

• Use the k-epsilon turbulence model and P-1 radiation model.

• Use the Eddy Dissipation Finite-rate Reaction model.

• Set up and solve a natural gas combustion problem.

• Postprocess the resulting data.

Prerequisites

This tutorial is written with the assumption that you have completed Tutorial 1 from theANSYS FLUENT 14.5 Tutorial Guide, and that you are familiar with the ANSYS FLUENTnavigation pane and menu structure. Some steps in the setup and solution procedure willnot be shown explicitly.

If you have not used k-epsilon turbulence, P-1 radiation, and Eddy Dissipation Finite-rateReaction models before, it would be helpful to first refer to the ANSYS FLUENT 14.5 User’sGuide.

Problem Description

This problem was modeled after the experiments carried out at the Burner EngineeringResearch Laboratory (BERL) as part of a large project (Scaling 400 study) for combustorsranging in size from 30 KW to 12 MW. The schematic of the problem is shown in Figure 1.

The flow under study is an unstaged natural gas flame in a 300 KW swirl-stabilized burner.The furnace is vertically fired. It has an octagonal cross-section with a conical furnacehood and a cylindrical exhaust duct. The furnace walls can be refractory-lined or water-cooled. The burner features 24 radial fuel ports and a bluff centerbody. Air is introducedthrough an annular inlet and movable swirl blocks are used to impart swirl. Figure 2 showsa closeup of the burner assuming 2D axisymmetry. Appropriate area adjustments weremade to account for the 2D representation of a 3D problem. It has been ensured that the

c© ANSYS, Inc. December 4, 2012 1

2D Simulation of a 300 KW BERL Combustor

cross-sectional areas of the model and real furnaces are the same. The input conditions forthis case, i.e. wall temperature, inlet boundary conditions, and profile have been derivedfrom this experimental data.

Figure 1: Schematic of the Problem

Figure 2: Closeup of the Burner

Preparation

1. Copy the files, (berl.msh.gz and berl.prof) to your working folder.

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

3. Enable Double Precision from the list of Options.

2 c© ANSYS, Inc. December 4, 2012


Setup and Solution

Step 1: Mesh

1. Read the mesh file (berl.msh.gz).

File −→ Read −→Mesh...

Figure 3: Mesh

Step 2: General Settings

General

1. Check the mesh.

General −→ Check

ANSYS FLUENT performs various checks on the mesh and reports the progress inthe console. Pay attention to the minimum volume reported and make sure this is apositive number.

2. Scale the mesh to mm.

General −→ Scale...



(a) Select mm from the Mesh Was Created In drop-down list.

(b) Ensure m is selected from the View Length Unit In drop-down list.

(c) Click Scale.

(d) Close the Scale Mesh dialog box

3. Retain the selection of Pressure-Based in the Type list.

4. Select Axisymmetric Swirl in the 2D Space list.

5. Enable Gravity.

6. Enter -9.81 for X under Gravitational Acceleration.

Step 3: Models

Models

1. Enable the Energy Equation.

Models −→ Energy −→ Edit...

2. Select the Standard k-epsilon (2 eqn) turbulence model.

Models −→ Viscous −→ Edit...

3. Select P1 from the radiation model list.

Models −→ Radiation −→ Edit...

Note: The P1 radiation model is used since it is quicker to run. However, the DOradiation model can be used for more accurate results.

An Information dialog box will appear informing that the material properties are changed.Click OK.



4. Select Species Transport as the species model.

Models −→ Species −→ Edit...

(a) Enable Volumetric in Reactions group box.

(b) Disable Diffusion Energy Source.

(c) Select Finite-Rate/Eddy-Dissipation in Turbulence-Chemistry Interaction group box.

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

An Information dialog box will appear informing that the material properties arechanged. Click OK.

Step 4: Materials

Materials

1. Copy the following fluid materials from the database.

Materials −→ Fluid −→ Create/Edit...

(a) carbon-dioxide (co2)

(b) methane (ch4)

2. Rename methane to fuel, delete its chemical formula, and click Change/Create tooverwrite methane.

Click Yes in the question dialog box.

3. Modify the properties for mixture-template.

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



(a) Click on Edit... next to Mixture Species and reorder the species as follows:

i. h2o

ii. o2

iii. fuel

iv. co2

v. n2

A transport equation is not solved for the last species in the list, instead itsconcentration is determined by difference. To reduce the round-off error, thespecies of the greatest quantity should be placed last in the list. In most cases,this is n2.

(b) Click on Edit... next to Reaction and define the following reaction.

Reactants Stoich.Coefficient

Rate Exponent Products Stoich.Coefficient

Rate Exponent

fuel 1 1 co2 1.022 0o2 2.033 1 h2o 2.022 0

(c) Ensure mixing-law is selected from the Cp drop-down list.

(d) Select polynomial from the Thermal Conductivity drop-down list. Define two poly-nomial coefficients with 0.0076736 and 5.8837e-05 as the first and second co-efficients.

(e) Select polynomial from the Viscosity drop-down list. Define two polynomial coef-ficients with 7.6181e-06 and 3.2623e-08 as the first and second coefficients.

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

(g) Enter 1e-09 for Scattering Coefficient.

(h) Click Change/ Create.

4. Enter 16.313 for Molecular Weight and -1.0629e+08 Standard State Enthalpy for fuelunder Material Type mixture.

5. Use the following TUI command to change the specific heat of the species included inthe mixture.

(set-ifrf-cp-polynomials ’mixture-template)

6. Select polynomial from the Cp drop-down list for fuel under Material Type mixture andset the values for the coefficients as shown below:

Species 1 2 3 4 5fuel 2005 -0.3407 2.362e-03 -1.178e-6 1.703e-10



Note: Global reaction mechanisms with one or two steps inevitably neglect the inter-mediate species. In high-temperature flames, neglecting these dissociated speciesmay cause the temperature to be over-predicted. A more realistic temperaturefield can be obtained by increasing the specific heat capacity for each species.Above command sets the Cp polynomial coefficient to that mentioned by Peterand Weber (1995). Fuel named as fuel in this tutorial, is not a standard species.Therefore, Cp polynomial coefficients for fuel need to be specified manually.

7. Click Change/Create and close the Create/Edit Materials dialog box.

Step 5: Boundary Conditions

Boundary Conditions

1. Read the profile file (berl.prof).

File −→ Read −→Profile...

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 velocity-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 radiativeheat transfer.



2. Set the boundary conditions for velocity-inlet-4 zone.

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

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

(b) Select vel-prof u and vel-prof w for Axial-Velocity and Swirl-Velocity respectively.

(c) Select Intensity and Length Scale from the Specification Method drop-down list.

(d) Enter 17 % and 0.0076 m for Turbulence Intensity and Turbulence Length Scalerespectively.

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

(f) Click the Species tab and enter 0.2315 for Species Mass Fractions for o2.

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



3. Set the boundary conditions for velocity-inlet-5.

Boundary Conditions −→ velocity-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 Length Scale from the Specification Method.

(d) Retain 5 % for Turbulence Intensity.

(e) Enter 0.0009 m for Turbulence Length Scale.

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

(g) Click the Species tab and enter 0.97 and 0.008 for Species Mass Fractions forfuel and co2 respectively.

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



4. Change the Type for outflow-3 zone to pressure-outlet.

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

(a) Select Select Intensity and Hydraulic Diameter from the Specification Method.

(b) Retain 5 % for Backflow Turbulent Intensity.

(c) Enter 0.6 m for Backflow Hydraulic Diameter.

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

(e) Click the Species tab and enter 0.2315 for Species Mass Fractions for o2.

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

5. Set the boundary conditions for wall zones.

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



(a) Click the Thermal tab and select Temperature from the Thermal Conditions list.

(b) Set the following conditions:

ZoneName

Temperature Internal Emissivity

wall-6 1370 K 0.5wall-7 312 K 0.6wall-8 1305 K 0.5wall-9 temp-prof t 0.6wall-10 1100 K 0.5wall-11 1273 K 0.6wall-12 1173 K 0.6wall-13 1173 K 0.6

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

Step 6: Solution

1. Set the solution parameters.

Solution Methods

(a) Select Coupled from the Scheme drop-down list.



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

This is often useful for buoyant flows where velocity vectors near walls may notalign with the wall due to assumption of uniform pressure in the boundary layer.Thus, PRESTO! can only be used with quadrilateral or hexahedral meshes.

(c) Enable Pseudo Transient.

2. Deselect P1 from the Equations selection list.

Solution Controls −→ Equations...

3. Enable Set All Species URFs Together.

Solution Controls

4. Change the time scale factor for species and energy.

Solution Controls −→ Advanced



(a) Click the Expert tab.

(b) Enter 0.1 for Time Scale Factor of Turbulent Kinetic Energy and Turbulent Dissi-pation Rate.

(c) Enter 10 for Time Scale Factor of Species and Energy.

(d) Click OK to close the Advanced Solution Controls dialog box.

Note: Higher time scale size is used for the energy and species equations toconverge the solution in less number of iterations.

5. Initialize the flow field.

Solution Initialization

(a) Click More Settings... and enable Maintain Constant Velocity Magnitude in theInitialization Options group box.

Note: This option will help to rid of the reverse flow from the pressure outlet.

(b) Click Initialize.

6. Save the initialized case and data files, berl-mag-init.cas.gz and berl-mag-init.dat.gz.

File −→ Write −→Case & Data

7. Change the time scale factor and start calculation.

Run Calculation

(a) Enter 0.1 for the Timescale Factor.

(b) Start the calculation by requesting 100 iterations (Figure 4).

Figure 4: Scaled Residuals



8. Save the case and data files, berl-mag-1.cas.gz and berl-mag-1.dat.gz.


9. Enter 100 for Time Scale Factor of Species and Energy.


10. Change the time scale factor and start calculation.

Run Calculation

(a) Enter 0.5 for the Timescale Factor.

(b) Start the calculation by requesting 100 iterations.



12. Enter 1 for Time Scale Factor of Turbulent Kinetic Energy and Dissipation Rate.


13. Start the calculation by requesting 100 iterations.

Run Calculation

14. Select P1 from the Equations selection list.

Solution Controls −→ Equations...

15. Request for an additional 700 iterations (Figure 5).

Figure 5: Scaled Residuals





17. Compute the gas phase mass fluxes through all the boundaries.

Reports −→ Fluxes −→ Set Up...

18. Compute the gas phase energy fluxes through all the boundaries.

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

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



(c) Close the Flux Reports dialog box.

19. Display contours of flow variables of interest.

Graphics and Animations −→ Contours −→ Set Up...

In particular, look at temperature, velocities, and species variables. (Figures 6—8).

Figure 6: Contours of Static Temperature

Figure 7: Contours of Velocity Magnitude



Figure 8: Contours of Mass Fraction of O2

Results

Use of the DO radiation model, which is more CPU intensive, and also a second ordersolution, can help to increase the accuracy of the predictions.

Summary

Inherent limitations in the available models result in inaccuracies while predicting interme-diate species. Overall, fairly meaningful results within engineering accuracy are obtained.

References

A. A. A. Peters and R. Weber ”Mathematical Modeling of a 2.25 MWt Swirling NaturalGas Flame. Part1: Eddy Break-up Concept for Turbulent Combustion, Probability DensityFunction Approach for Nitric Oxide Formation.” Combustion Science and Technology. 110.67-101. 1995


01-2d-berl-edm.pdf

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