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Tutorial: PDF Transport Simulation of Sandia Flame

Introduction

The purpose of this tutorial is to demonstrate the setup and solution of a flame using thePDF transport model. The PDF transport model is used to include realistic finite ratechemical effects such as non-equilibrium CO and OH, as well as local flame extinction.

This tutorial demonstrates how to do the following:

• Use the PDF transport model to setup and solve the piloted jet diffusion flame.

• Obtain the initial solution using the partially premixed combustion model.

• Obtain the final solution using the PDF transport model.

• Check for energy balance using heat flux reports.

• Postprocess the resulting data.

Experimental Overview

A simple, axi-symmetric jet diffusion flame was chosen because extensive and accurateexperimental measurements are available. The data was collected at Sandia National Labs,and includes simultaneous point measurements of T , N2, O2, CH4, CO2, H2O, H2 OH,NO, and CO. A set of flames ranging from laminar (denoted Flame A) to near globallyextinguished (Flame F) were measured and a flame exhibiting moderate local extinction(Flame D) was chosen for this tutorial. For more information about the experiment fromthe following link:

http://www.ca.sandia.gov/TNF/DataArch/FlameD.html

Prerequisites

This tutorial is written with the assumption that you have completed from ANSYS FLUENT14.5 Tutorial Guide, and that you are familiar with the ANSYS FLUENT navigation paneand menu structure. Some steps in the setup and solution procedure will not be shownexplicitly.

In this tutorial, you will use partially premixed combustion model and PDF transport model.If you have not used these models before, see Chapters 18, Modeling Partially PremixedCombustion and 19, Modeling a Composition PDF Transport Problem, respectively inANSYS FLUENT 14.5 User’s Guide.

c© ANSYS, Inc. November 7, 2012 1

http://www.ca.sandia.gov/TNF/DataArch/FlameD.html

PDF Transport Simulation of Sandia Flame

Problem Description

Flame D is an axisymmetric jet diffusion flame. The burner has a main jet nozzle of diameter7.2 mm, surrounded by a burnt pilot annulus with an outer diameter of 18.2 mm. The pilotis used to delay flame blow-off. The main jet composition is 25% CH4 and 75% air (byvolume), selected to minimize sooting, for easier modeling. The stoichiometric value ofthe mixture fraction is 0.351 and the flame length (defined as the point where the mixturefraction is stoichiometric on the axis) is about 47 jet nozzle diameters.

Strategy

The PDF transport solution is performed in two steps. In the first step, a chemical equi-librium solution is obtained using the partially premixed model in ANSYS FLUENT. In thesecond step, the final solution is obtained using PDF transport model.PDF transport simulations are computationally expensive. Hence, first use the partiallypremixed simulation which provides a good initial condition, and reduces the time andnumber of iterations required for convergence.

Setup and Solution

Preparation

1. Copy the mesh file (flameD.msh.gz) to the working folder.

2. Copy the CHEMKIN mechanism file (CH4-skel.che) and thermodynamic databasefile (therm.dat) to the working folder.

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

4. Enable Double-Precision in the Options list.

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

Step 1: Mesh

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

File −→ Read −→Mesh

As the mesh file is read, ANSYS FLUENT will report the progress in the console.

2 c© ANSYS, Inc. November 7, 2012


Step 2: General Settings

1. Select Axisymmetric in the 2D Space list.

General −→ Axisymmetric

2. Check the mesh (Figure 1).

General −→ Check

Figure 1: Mesh Display

Step 3: Models

1. Enable the Realizable k-epsilon (2 eqn) turbulence model.

Models −→ Viscous −→ Edit...

The realizable model is used to obtain the correct spreading of a round jet.

2. Select Partially Premixed Combustion model.

Models −→ Species −→ Edit...

Note: As the fuel is premixed with air, you cannot use the equilibrium non-premixedmodel. It will burn the inlet stream at chemical equilibrium.

(a) Select Partially Premixed Combustion from the list of Model.



(b) Create the PDF table.

i. Click the Chemistry tab.

A. Ensure that Chemical Equilibrium is selected from the State Relation groupbox.

B. Select Non-Adiabatic option from the Energy Treatment group box.

The Energy Model is automatically enabled.

C. Enter 0.7 for the Fuel Stream Rich Flammability Limit.

ii. Click the Boundary tab.



A. Enter 294 K for Fuel and 291 K for Oxid.

B. Select Mole Fraction in the Specify Species in list.

C. Set the composition of Fuel and Oxid for Species as shown in the followingtable:

Species Fuel Oxidch4 0.25 0n2 0.5925 0.79o2 0.1575 0.21

iii. Click the Table tab.



A. Retain the default settings.

B. Click Calculate PDF Table.

iv. Click the Premixed tab.

A. Examine the properties in Partially Premixed Mixture Properties groupbox.

For calculating the properties of unburnt mixture, third order polynomialfunctions (function of mixture fractions) are used for mixed unburnt den-sity, temperature, specific heat, and thermal diffusivity. Also, a piecewiselinear function (function of mass fraction) is used for laminar flamespeed.

v. Click OK to close the Species Model dialog box.

3. Examine the relationship between Mean Temperature and Mean Mixture Fraction.

Display −→PDF Tables/Curves...



4. Write the PDF file (flameD.pdf.gz).

File −→ Write −→PDF...

Step 4: Materials

Materials −→ Create/Edit...

ANSYS FLUENT sets the mixture material to pdf-mixture.

1. Ensure that pdf is selected from the Density a drop-down list.

2. Ensure that prepdf-polynomial is selected from the Laminar Flame Speed drop-downlist.

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

Step 5: Boundary Conditions

1. Set the boundary conditions for coflow.

Boundary Conditions −→ coflow −→ Edit...

(a) Enter 0.9 m/s for Velocity Magnitude.

(b) Retain selection of Intensity and Viscosity Ratio from the Specification Methoddrop-down list.

(c) Enter 10 for Turbulent Intensity.

(d) In the Thermal tab enter 291 for Temperature.

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



2. Set the boundary conditions for jet.

Boundary Conditions −→ jet −→ Edit...


(b) Select Intensity and Hydraulic Diameter from the Specification Method drop-downlist.


(d) Enter 0.0072 m for Hydraulic Diameter.

(e) In the Thermal tab enter 294 for Temperature.

(f) Enter 1 for the Mean Mixture Fraction in the Species tab.

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

3. Set the boundary conditions for pilot.

Boundary Conditions −→ pilot −→ Edit...


(b) Select Intensity and Hydraulic Diameter from the Specification Method drop-downlist.


(d) Enter 0.0165 m for Hydraulic Diameter.

(e) In the Thermal tab enter 1908 for Temperature.

(f) Enter 1 for Progress Variable and 0.2755 for Mean Mixture Fraction in the Speciestab.

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

4. Set the boundary conditions for outlet.

Boundary Conditions −→ outlet −→ Edit...

Reversed flow may occur at the outlet, either in the final steady solution or during theiteration process. Hence, it is always a good idea to set the backflow pressure outletproperties.

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

(b) Retain 5% for Backflow Turbulent Intensity.

(c) Enter 0.36 m for Backflow Hydraulic Diameter, respectively.

(d) In the Thermal tab enter 291 for Temperature.

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



Step 6: Solution for the Partially Premixed Combustion Model

1. Select PRESTO! from the Pressure drop-down list in the Spatial Discretization groupbox.

Solution Methods

2. Initialize the flow field.

Solution Initialization −→ Initialize

Hybrid Initialization is the default Initialization Method in ANSYS FLUENT 14.5. Referto the section 28.11 Hybrid Initialization, in the ANSYS FLUENT 14.5 User’s Guide.

3. Run the calculation for 250 iterations (see Figure 2).

Run calculation

Figure 2: Scaled Residuals After 250 Iterations

4. Save the case and data files (flameD-1.cas/dat.gz).

5. Display contours of static temperature as a visual check of the flame.

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-downlist.

(c) Click Display (see Figure 3).



Figure 3: Contours of Static Temperature

The equilibrium partially premixed solution will be used as an initial condition for thePDF transport simulation.

Step 7: PDF Transport Model

1. Import a chemical mechanism in CHEMKIN format.

The mechanism (CH4-skel.che) has 16 species and 41 reactions.

File −→ Import −→CHEMKIN Mechanism...

(a) Enter ch4-skel for Material Name.

(b) Click Browse... and select CH4-skel.che for Gas Phase CHEMKIN MechanismFile.

(c) Click Browse... and select therm.dat for Gas Phase Thermodynamic DatabaseFile.



(d) Click Import and close the CHEMKIN Mechanism Import dialog box.

2. Enable the Composition PDF Transport model.


(a) Select Composition PDF Transport from the Model list.

(b) Select ch4-skel from the Mixture Material drop-down list.

(c) Enable Volumetric and click Apply.

(d) Click the Edit... for the Mixture Material.

i. Ensure that mixing-law is selected from the Cp drop-down list.

ii. Click Edit... for Reaction and ensure that the Total Number of Reactions are41.

iii. Click OK to close the Reactions dialog box.

iv. Click Change and close the Edit Material dialog box.

(e) Click Integration Parameters....



i. Enable Chemistry Agglomeration under Options group box.

ii. Close the Integration Parameters dialog box.

Note: Chemistry agglomeration is another method to speed up the detailed chem-istry model calculation. The chemistry agglomeration option works as fol-lows:

i. Cells are collected that are close in compositions i.e similar temperaturesand mass fractions.

ii. These cells are averaged in a single composition.

iii. ISAT is called to calculate the reaction step.

iv. The reaction step is mapped back to the original cells.

This will improve the run time but will reduce the accuracy of the solution.For details refer to Sections 20.1 Using ISAT and 20.2 Using Chemistry Ag-glomeration in ANSYS FLUENT 14.5 User’s Guide. In this tutorial you willuse chemistry agglomeration to get an initial solution. You will later disablethis to get the final solution.

ANSYS FLUENT uses ISAT (In-Situ Adaptive Tabulation) to speed up thechemistry calculations. Since integration of the stiff chemical mechanismis computationally expensive, ISAT builds a chemistry table immediately.Hence, the initial iterations are slow, but the speed increases as the tableis filled. A good understanding about what ISAT is and how it works isimperative for efficient PDF transport calculations. For more details aboutISAT, see Section 20.1, Using ISAT in ANSYS FLUENT 14.5 User’s Guide.

Detailed chemistry is computationally expensive and the iterations requiresa few hours to complete. It is therefore suggested that you perform a coupleof iterations to ensure that the setup is okay.



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

3. Modify the boundary conditions for the PDF transport model.

(a) Set the boundary conditions for coflow.

Boundary Conditions −→ coflow −→ Edit...

i. Enter 0.233 for o2 in the Species tab.

ii. Click OK to close the Velocity Inlet dialog box.

(b) Set the boundary conditions for jet.

Boundary Conditions −→ jet −→ Edit...

i. Enter 0.1965 and 0.15637 for o2 and ch4 respectively, in the Species tab.


(c) Set the boundary conditions for pilot.

Boundary Conditions −→ pilot −→ Edit...

i. Click on Species tab and enter the values as shown in the following table:

Species Mass Frac-tion

h2o 0.092co2 0.11o2 0.056co 0.004


(d) Set the boundary conditions for outlet.

Boundary Conditions −→ outlet −→ Edit...

i. Enter 0.233 for o2 in the Species tab.

ii. Click OK to close the Pressure Outlet dialog box.

(e) Set jet wall and pilot wall to adiabatic by selecting the Heat Flux option in theThermal tab and retain the default value of zero.

The mesh has been built such that the upstream jet and pilot are resolved. Bydoing this, the turbulent flow is allowed to evolve, and the solution becomes lesssensitive to the specified inlet boundary conditions.



Step 8: Solution for the Composition PDF Transport Model

1. Set the solution parameters.

Solution Methods

(a) Select Second Order Upwind from the Turbulent Kinetic Energy and TurbulentDissipation Rate drop-down lists in the Spatial Discretization group box.

(b) Ensure that Second Order Upwind is selected from the Momentum drop-down list.

2. Create a surface monitor to track the co mass fraction near the outlet.

This monitor is used to confirm the convergence of the solution.

(a) Create a point surface.

Surface −→Point...

i. Enter 0.7 m and 0.01 m for x0 and y0, respectively.

This is the point near the outlet.

ii. Enter point-outlet as the New Surface Name.

iii. Click Create and close the Point Surface dialog box.

(b) Set the surface monitor for the co mass fraction at the point-outlet.

Monitors (Surface Monitors)−→ Create...

i. Enable Plot and Write options.

ii. Select Area-Weighted Average from the Report Type drop-down list.

iii. Select Species... and Mean co Mass fraction from the Field Variable drop-downlists.

iv. Select point-outlet from the Surfaces list.

v. Click OK to close the Surface Monitors dialog box.

3. Run the calculation for 250 iterations (see Figure 4).



Figure 4: Scaled Residuals

The surface monitor for Mean co Mass fraction is as shown in Figure 5.

Figure 5: History of Mean co Mass Fraction on point-outlet



4. Save the case and data files (flameD-2.cas.gz and flameD-2.dat.gz).

5. Change the ISAT tolerance.


(a) Click Integration Parameters... to open the Integration Parameters dialog box.

i. Enter 1e-04 for ISAT Error Tolerance.

ii. Enter 500 mb for Max. Storage.

iii. Disable Chemistry Agglomeration from Options group box.

Disabling chemistry agglomeration will slow down the simulation drastically,but it will move towards a stable solution. If you plan to use chemistryagglomeration for the final solution, the agglomeration error tolerance mayneed to be reduced to obtain an accurate solution.

iv. Click OK to close the Integration Parameters dialog box.

A Warning is displayed which reminds you that the ISAT table is not empty.Click OK.

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



6. Solve for another 350 iterations.

The scaled residuals and the convergence history of Mean co Mass fraction on point-outlet are as shown in Figures 6 and 7 respectively.

Figure 6: Scaled Residuals

Figure 7: Convergence History of Mean co Mass fraction on point-outlet



7. Check the heat flux reports to ensure that energy is balanced.

Reports −→ Fluxes −→ Set Up...

This is an excellent way to check for convergence in the PDF transport model. Thenet heat flux should be a few percent of the heat coming in from the inlets/walls, orescaping from the outlets/walls.

8. Save the case and data files (flameD-3.cas/dat.gz).

Step 9: Postprocessing

1. Display contours of Mean Static Temperature (see Figure 8).

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

Figure 8: Contours of Mean Static Temperature

2. Display contours of RMS Static Temperature (see Figure 9).



Figure 9: Contours of RMS Static Temperature

3. Display particle tracks of mass fraction of co (see Figure 10).

Graphics and Animations −→ Particle Tracks −→ Set Up...

(a) Select Particle Variables... and Particle Mass Fraction of co from the Color Bydrop-down lists.

(b) Enable Track PDF Transport Particles.

(c) Click Display and close the Particle Tracks dialog box.

Figure 10: Particle Tracks of Mass Fraction of co



Summary

In this tutorial, a simulation of an experimental turbulent jet diffusion flame was performedwith the PDF transport model. Finite rate chemical effects such as non-equilibrium OHand CO were captured. The equilibrium partially premixed solution was used as an initialcondition for the PDF transport simulation, in order to reduce the time and number ofiterations required for convergence.


introduction - mr-cfddl.mr-cfd.com/tutorials/ansys-fluent/03-pdf-transport-sandia-flame.pdf ·...

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