Coal combustion simulation in FLUENT

Tutorial 12. Using the Non-Premixed

Combustion Model

Introduction: A pulverized coal combustion simulation involves mod-eling a continuous gas phase flow eld and its interaction with a dis-crete phase of coal particles. The coal particles, traveling throughthe gas, will devolatilize and undergo char combustion, creating asource of fuel for reaction in the gas phase. Reaction can be mod-eled using either the species transport model or the non-premixedcombustion model. In this tutorial you will model a simplied coalcombustion furnace using the non-premixed combustion model forthe reaction chemistry.

In this tutorial you will learn how to:

Prepare a PDF table for a pulverized coal fuel using theprePDF preprocessor

Dene FLUENT inputs for non-premixed combustion chem-istry modeling

Dene a discrete second phase of coal particles Solve a simulation involving reacting discrete phase coal par-

ticles

The non-premixed combustion model uses a modeling approachthat solves transport equations for one or two conserved scalars,the mixture fractions. Multiple chemical species, including radicalsand intermediate species, may be included in the problem deni-tion and their concentrations will be derived from the predictedmixture fraction distribution. Property data for the species areaccessed through a chemical database and turbulence-chemistryinteraction is modeled using a Beta or double-delta probabilitydensity function (PDF). See the Users Guide for more detail onthe non-premixed combustion modeling approach.

c Fluent Inc. November 27, 2001 12-1

Using the Non-Premixed Combustion Model

Prerequisites: This tutorial assumes that you are familiar with themenu structure in FLUENT, and that you have solved Tutorial 1or its equivalent. Some steps in the setup and solution procedurewill not be shown explicitly.

Problem Description: The coal combustion system considered in thistutorial is a simple 10 m by 1 m two-dimensional duct depicted inFigure 12.1. Only half of the domain width is modeled becauseof symmetry. The inlet of the 2D duct is split into two streams.A high-speed stream near the center of the duct enters at 50 m/sand spans 0.125 m. The other stream enters at 15 m/s and spans0.375 m. Both streams are air at 1500 K. Coal particles enter thefurnace near the center of the high-speed stream with a mass flowrate of 0.1 kg/s (total flow rate in the furnace is 0.2 kg/s). The ductwall has a constant temperature of 1200 K. The Reynolds numberbased on the inlet dimension and the average inlet velocity is about100,000. Thus, the flow is turbulent.

Details regarding the coal composition and size distribution areincluded in Step 5: Models: Continuous (Gas) Phase and Step 8:Materials: Discrete Phase.

12-2 c Fluent Inc. November 27, 2001


0.5 m

10 m

Symmetry Plane

Air: 50 m/s, 1500 K

Air: 15 m/s, 1500 K

0.125 m

Coal Injection: 0.1 kg/s

T = 1200 Kw

Figure 12.1: 2D Furnace with Pulverized Coal Combustion

Preparation for prePDF

1. Start prePDF.

When you use the non-premixed combustion model, you preparea PDF le with the preprocessor, prePDF. The PDF le containsinformation that relates species concentrations and temperatures tothe mixture fraction values, and is used by FLUENT to obtain thesescalars during the solution procedure.



Step 1: Dene the Preliminary Adiabatic Systemin prePDF

1. Dene the prePDF model type.

You can dene either a single fuel stream, or a fuel stream plus asecondary stream. Enabling a secondary stream allows you to keeptrack of two mixture fractions. For coal combustion, this wouldallow you to track volatile matter (the secondary stream) separatelyfrom the char (fuel stream). In this tutorial, we will not followthis approach. Instead, we will model coal using a single mixturefraction.

Setup !Case...

(a) Under Heat transfer options, keep the default setting of Adia-batic.

The coal combustor studied in this tutorial is a non-adiabaticsystem, with heat transfer at the combustor wall and heat



transfer to the coal particles from the gas. Therefore, a non-adiabatic combustion system must be considered in prePDF.

Because non-adiabatic calculations are more time-consumingthan those for adiabatic systems, you will start the prePDFsetup by considering the results of an adiabatic system. Bycomputing the PDF/equilibrium chemistry results for the adi-abatic system, you will determine appropriate system param-eters that will make the non-adiabatic calculation more ef-cient. Specically, the adiabatic calculation will provide in-formation on the peak (adiabatic) flame temperature, the stoi-chiometric mixture fraction, and the importance of individualcomponents to the chemical system. This process of begin-ning with an adiabatic system calculation should be followedin all PDF calculations that ultimately require a non-adiabaticmodel.

(b) Under Chemistry models, keep the default setting of Equilib-rium Chemistry.

In most PDF-based simulations, the Equilibrium Chemistry op-tion is recommended. The Stoichiometric Reaction (mixed isburned) option requires less computation but is generally lessaccurate. The Laminar Flamelets option oers the ability toinclude aerodynamic strain induced non-equilibrium eects,such as super-equilibrium radical concentration andsub-equilibrium temperatures. This can be important for NOxprediction, but is excluded here.

(c) Keep the default setting of the PDF models.

The Beta PDF integration is always recommended because itis more accurate than the Delta PDF approach.

(d) Under Empirically Dened Streams, enable the Fuel stream op-tion.

This will allow you to dene the fuel stream using the empir-ical input option. The empirical input option allows you todene the composition in terms of atom fractions of H, C, N,and O, along with the lower heating value and heat capacity



of the fuel. This is a useful option when the ultimate analysisand heating value of the coal are known.

(e) Click Apply and close the panel.



2. Dene the chemical species in the system.

The choice of which species to include depends on the fuel type andcombustion system. Guidelines on this selection are provided in theFLUENT Users Guide. Here, you will assume that the equilibriumsystem consists of 13 species: C, C(s), CH4, CO, CO2, H, H2,H2O, N, N2, O, O2, and OH.

C, H, O, and N are included because the fuel stream will be de-ned in terms of these atom fractions, using the \empirical" inputmethod.

! You should include both C and C(S) in the system when theempirical input option is used.

Setup ! Species !Dene...

(a) Set the Maximum # of Species to 13. Use the up and downarrows to set the maximum number of species, or enter thenumber in the text eld followed by .

(b) Select the top species in the Dened Species list (initially la-beled UNDEFINED).



(c) In the Database Species drop-down list, use the slider bar toscroll the list, and select C. The Dened Species list now showsC as the rst entry.

(d) Select the next species in the Dened Species list (or incrementthe Species # counter to 2).

(e) In the Database Species drop-down list, use the slider bar toscroll the list, and select the next species (C(S)).

(f) Repeat steps (d) and (e) until all 13 species are dened.

(g) Click Apply and then close the panel.

Note: In other combustion systems, you might want to include ad-ditional chemical species, but you should not add slow chemi-cal species like NOx.

3. Determine the fuel composition inputs.

The fuel considered here is known, from proximate analysis, toconsist of 28% volatiles, 64% char, and 8% ash. You will use thisinformation, along with the ultimate analysis given below, to denethe coal composition in prePDF. The fuel stream composition (charand volatiles) is derived as follows.

Begin by converting the proximate data to a dry-ash-free basis:

Proximate Analysis Wt % Wt %(dry) (DAF)

Volatiles 28 30.4Char (C(s)) 64 69.6Ash 8 -

The ultimate analysis, for the dry-ash-free coal, is known to be:

Element Wt % (DAF)C 89.3H 5.0O 3.4N 1.5S 0.8



For modeling simplicity, the sulfur content of the coal can be com-bined into the nitrogen mass fraction, to yield:

Element Wt % (DAF)C 89.3H 5.0O 3.4N 2.3S -

We can combine the proximate and ultimate analysis data to yieldthe following elemental composition of the volatile stream:

Element Wt % Moles Mole FractionC 89.3 7.44 0.581H 5.0 5 0.390O 3.4 0.21 0.016N 2.3 0.16 0.013Total 12.81

You will enter the mole fractions in the nal column, above, inorder to dene the fuel composition. prePDF will use this informa-tion, along with the coal heating value, to dene the species presentin the fuel.

The lower heating value of coal (DAF) is known to be:

LCVcoal,DAF = 35.3 MJ/kgThe specic heat and density of the coal are known to be 1000 J/kg-K and 1 kg/m3 respectively.



4. Enter the fuel and oxidizer compositions.

Setup ! Species !Composition...(a) Enable the input of the oxidizer stream composition.

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

i. Under Stream, select Oxidiser.

ii. Under Specify Composition In, retain the default selectionof Mole Fractions.

iii. Select O2 in the Dened Species list and enter 0.21 in theSpecies Fraction eld.



iv. Select N2 in the Dened Species list and enter 0.79 in theSpecies Fraction eld.

(b) Enable the input of the fuel stream composition.

Note: Because the empirical input option is enabled for thefuel stream, you will be prompted to enter atom mole frac-tions for C, H, O, and N, along with the heating value andheat capacity of the coal.

i. Under Stream, select Fuel.

ii. Under Specify Composition In, retain the default selectionof Mole Fractions.



iii. Select C in the Dened Species list and enter 0.581 in theAtom Fraction eld.

iv. Select H in the Dened Species list and enter 0.390 in theAtom Fraction eld.

v. Select N in the Dened Species list and enter 0.016 in theAtom Fraction eld.

vi. Select O in the Dened Species list and enter 0.013 in theAtom Fraction eld.

vii. Enter 3.53e7 J/kg for the Lower Caloric Value and 1000J/kg-K for the Specic Heat.

viii. Click Apply and close the panel.

5. Dene the density of the solid carbon.

Here, a value of 1300 kg/m3 is assumed.

Setup ! Species !Density...

(a) Select C(S) in the Dened Species list.

(b) Set the Density to 1300.

(c) Click Apply and close the panel.



Note: prePDF will use this information during computation of themixture density for the fuel. You should enter the density ofsolid char. This input will dier from the coal density de-ned in FLUENT, which is the apparent density of the ash-containing coal particles.

6. Dene the system operating conditions.

The system pressure and inlet stream temperatures are required forthe equilibrium chemistry calculation. The fuel stream inlet temper-ature for coal combustion should be the temperature at the onset ofdevolatilization. The oxidizer inlet temperature should correspondto the air inlet temperature. In this tutorial, the coal devolatiliza-tion temperature will be set to 400 K and the air inlet temperatureis 1500 K. The system pressure is one atmosphere.

Setup !Operating Conditions...



(a) Enter 400 K and 1500 K as the Fuel and Oxidiser inlet tem-peratures.

(b) Click Apply and close the panel.



Step 2: Compute and Review the Adiabatic Sys-tem prePDF Look-Up Tables

1. Accept the default PDF solution parameters.

Setup !Solution Parameters...

The look-up table calculation performed by prePDF will result in atable of values for species mole fractions and temperature at a setof discrete mixture fraction values. You control the number anddistribution of these discrete points using the Solution Parameterspanel. You can also set the Fuel Rich Flamability Limit in this panel.

The Fuel Rich Flamability Limit allows you to perform a \partialequilibrium" calculation, suspending equilibrium calculations whenthe mixture fraction exceeds the specied rich limit. This increasesthe eciency of the PDF calculation, allowing you to bypass thecomplex equilibrium calculations in the fuel-rich region, and is morephysically realistic than the assumption of full equilibrium. Forempirically dened streams, the rich limit is always 1.0 and cannotbe altered.



(a) Keep the default setting for Automatic Distribution.

This feature allows you to improve the prePDF prediction byoptimizing the distribution of the discrete mixture fraction val-ues, clustering them around the peak temperature value. If youchoose not to use the Automatic Distribution, you should setthe distribution center point on the rich side of the stoichio-metric scale mixture fraction.


2. Save your inputs (coal ad.inp).

File ! Write !Input...3. Calculate the adiabatic system chemistry.

Calculate !PDF TableDuring the calculation, prePDF rst retrieves thermodynamic datafrom the database. Then the time-averaged values of temperature,composition, and density at the discrete mixture-fraction/mixture-fraction-variance points (21 points as dened in the Solution Pa-rameters panel) are calculated. The result will be a set of tablescontaining time-averaged values of species mole fractions, density,and temperature at each discrete value of these two parameters.prePDF reports the progress of the look-up table construction in theconsole window.

When the calculations are complete, prePDF will warn you thatequilibrium calculations have been performed for the fuel inlet. Youcan simply acknowledge this warning, as the equilibrium conditionspredicted do not impact your modeling inputs unless the fuel streamis representing a gaseous fuel inlet.



4. Save the adiabatic PDF le (coal ad.pdf).

File ! Write !PDF...(a) Under File Type, select Write Formatted File.

When you write a PDF le, prePDF will save a binary leby default. If you are planning to use the PDF le on thesame machine, you can save the le using the default WriteBinary File option. However, if you are planning to use thePDF le on a dierent machine, you should save an ASCII(formatted) le from prePDF. Note that ASCII les take upmore disk space than binary les.

(b) Under Solver, select FLUENT 6.

(c) Enter coal ad.pdf as the Pdf File name.

(d) Click OK to write the le.

5. Examine the temperature/mixture-fraction relationship in the adi-abatic system.

The results of the adiabatic calculation provide insight into the sys-tem description that will be used for the non-adiabatic calculation.

Display !PDF Table...

(a) Select TEMPERATURE from the Plot Variable list and thenclick Display to generate the table (Figure 12.2).



The temperature display shows how the time-averaged sys-tem temperature varies with the mean mixture fraction andits variance.

The temperature/mixture-fraction relationship shows that thepeak flame temperature is about 2750 K at fuel stoichiomet-ric mixture fractions of approximately 0.1. The relatively highflame temperature is a result of the high pre-heat in the com-bustion air.

Note: The adiabatic flame temperature predicted by the adi-abatic system calculation will be used to select the maxi-mum temperature in the non-adiabatic system calculation.

2.50E-01

2.00E-01

1.50E-01

1.00E-01

5.00E-02

0.00E+00

SCALED-F-VARIANCE

1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00

TEMPERATURE

K

prePDF V4.00

2.8E+03

2.4E+03

2.0E+03

1.6E+03

1.2E+03

7.6E+02

Fluent Inc.

F-MEAN

MEAN FLAME TEMPERATUREPDF TABLE - CHEMICAL EQUILIBRIUM

Figure 12.2: Time-Averaged Temperature: Adiabatic prePDF Calcula-tion



Step 3: Create and Compute the Non-AdiabaticprePDF System

Creating a non-adiabatic PDF system description requires that you dothe following:

Redene the system as non-adiabatic. Set the peak system temperature (based on the adiabatic result of

2750 K).

After these modications, you will recompute the system chemistry andsave a non-adiabatic PDF le for use in FLUENT.



1. Dene the prePDF model type as non-adiabatic.

Setup !Case...

(a) Select Non-Adiabatic under Heat transfer options and click Ap-ply.



2. Set the system temperature limits.

Minimum and maximum temperatures in the system are requiredwhen the PDF calculation is non-adiabatic.

The minimum temperature should be a few degrees lower than thelowest boundary condition temperature (e.g., the inlet temperatureor wall temperature). In coal combustion systems, the minimumsystem temperature should also be set below the temperature atwhich the volatiles begin to evolve from the coal. Here, the va-porization temperature at which devolatilization begins will be setto 400 K. Thus, the minimum system temperature is set to 298 K(the default).

The maximum temperature should be at least 100 K higher thanthe peak flame temperature found in the preliminary adiabatic cal-culation. Here, the maximum temperature will be taken as 3000 K,well above the peak adiabatic system temperature of 2750 K.

Setup !Operating Conditions...



(a) Enter 298 for Min. Temperature and 3000 for Max. Tempera-ture.


3. Save the non-adiabatic system inputs (coal.inp).

File ! Write !Input...4. Compute the non-adiabatic PDF look-up tables.

Calculate !PDF TableThe non-adiabatic prePDF calculation requires much more compu-tation than the adiabatic calculation. prePDF begins by accessingthe thermodynamic data from the database. Next, the enthalpy



eld is initialized and the enthalpy grid adjusted to account forinlet conditions and solution parameters. Time-averaged valuesof temperature, composition, and density at the discrete mixture-fraction/mixture-fraction-variance/enthalpy points (21 points, asdened in the Solution Parameters panel) are then calculated. Theresult will be a set of tables containing time-averaged values ofspecies mole fractions, density, and temperature at each discretevalue of these three parameters.

When the calculations are complete, prePDF will warn you thatequilibrium calculations have been performed for the fuel inlet. Asnoted above, you can simply acknowledge this warning, which hasno impact on your inputs when you are modeling coal or liquidfuels.

5. Write the PDF output le (coal.pdf).

File ! Write !PDF...(a) Under File Type, select Write Formatted File.

(b) Select FLUENT 6 under Solver.

(c) Enter coal.pdf as the Pdf File name.

(d) Click OK to write the le.



6. Review one slice of the 3D look-up table prepared by prePDF.

Display !Nonadiabatic Table...

(a) Select TEMPERATURE from the Plot Variable drop-down listand click Display (Figure 12.3).

Note: Review of the 3D look-up tables is accomplished on a slice-by-slice basis. By default, the slice selected is that correspond-ing to the adiabatic enthalpy values. This display should lookvery similar to the look-up table created during the adiabaticcalculation. You can select other slices of constant enthalpyfor display, as well.



2.50E-01

2.00E-01

1.50E-01

1.00E-01

5.00E-02

0.00E+00

SCALED-F-VARIANCE

1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00

TEMPERATURE

K

prePDF V4.00

2.8E+03

2.4E+03

2.0E+03

1.6E+03

1.2E+03

7.6E+02

Fluent Inc.

F-MEAN

MEAN FLAME TEMPERATURE FROM 3D-PDF-TABLEMEAN ENTHALPY SLICE NUMBER 23

Figure 12.3: Non-Adiabatic Temperature Look-Up Table on the SliceCorresponding to Adiabatic Enthalpy



7. Examine the species/mixture-fraction relationship in the non-adiabaticsystem.

Display !Nonadiabatic Table...

(a) Select SPECIES from the Plot Variable drop-down list.

The Species Selection panel will open automatically.

(b) In the Species Selection panel, select C(S) in the Species drop-down list and click OK.

(c) Click Display in the Nonadiabatic-Table panel to generate thetable (Figure 12.4).

8. Follow the steps above to plot the instantaneous mole fractions forCO (Figure 12.5).



2.50E-01

2.00E-01

1.50E-01

1.00E-01

5.00E-02

0.00E+00

SCALED-F-VARIANCE

1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00

MOLE

FRACTION

prePDF V4.00

7.6E-01

6.1E-01

4.6E-01

3.1E-01

1.5E-01

0.0E+00

Fluent Inc.

F-MEAN

SPECIES C(S) FROM 3D-PDF-TABLEMEAN ENTHALPY SLICE NUMBER 23

Figure 12.4: Time-Averaged C(S) Mole Fractions: Non-AdiabaticprePDF Calculation

2.50E-01

2.00E-01

1.50E-01

1.00E-01

5.00E-02

0.00E+00

SCALED-F-VARIANCE

1.00E+00 8.00E-01 6.00E-01 4.00E-01 2.00E-01 0.00E+00

MOLE

FRACTION

prePDF V4.00

3.1E-01

2.4E-01

1.8E-01

1.2E-01

6.1E-02

0.0E+00

Fluent Inc.

F-MEAN

SPECIES CO FROM 3D-PDF-TABLEMEAN ENTHALPY SLICE NUMBER 23

Figure 12.5: Time-Averaged CO Mole Fractions: Non-Adiabatic prePDFCalculation



9. Exit from prePDF.

File !Exit

Preparation for FLUENT Calculation

With the PDF le creation completed, you are ready to use the non-premixed combustion model in FLUENT to predict the combusting flowin the coal furnace.

1. Copy the le coal/coal.msh from the FLUENT documentation CDto your working directory (as described in Tutorial 1).

The mesh le coal.msh is a quadrilateral mesh describing the sys-tem geometry shown in Figure 12.1.

2. Start the 2D version of FLUENT.



Step 4: Grid

1. Read the 2D mesh le, coal.msh.

File ! Read !Case...The FLUENT console window reports that the mesh contains 1357quadrilateral cells.

2. Check the grid.

Grid !CheckThe grid check should not report any errors or negative volumes.

3. Display the grid (Figure 12.6).

Display !Grid...Due to the grid resolution and the size of the domain, you may ndit more useful to display just the outline, or to zoom in on variousportions of the grid display.

Note: You can use the mouse probe button (right button, by de-fault) to nd out the boundary zone labels. As annotated inFigure 12.7, the upstream boundary contains two velocity in-lets (for the low-speed and high-speed air streams), the down-stream boundary is a pressure outlet, the top boundary is awall, and the bottom boundary is a symmetry plane.



GridFLUENT 6.0 (2d, segregated, lam)

Aug 28, 2001

Figure 12.6: 2D Coal Furnace Mesh Outline Display

wall-7

symmetry-5

velocity-inlet-8

velocity-inlet-2

GridFLUENT 6.0 (2d, segregated, lam)

Aug 28, 2001

Figure 12.7: Mesh Display with Annotated Boundary Types



Step 5: Models: Continuous (Gas) Phase

1. Accept the default segregated solver.

The non-premixed combustion model is available only with the seg-regated solver.

Dene ! Models !Solver...



2. Turn on the standard k- turbulence model.

Dene ! Models !Viscous...

Note: As indicated in the problem description, the Reynolds num-ber of the flow is about 105. Thus, the flow is turbulent andthe high-Re k- model is suitable.



3. Turn on the non-premixed combustion model.

Dene ! Models !Species...(a) Select Non-Premixed Combustion under Model.

The panel will expand to show the related inputs.

When you click OK, FLUENT will open the Select File dialogbox, requesting input of the PDF le to be used in the simu-lation.

(b) In the Select File dialog box, select and read the non-adiabaticPDF le (coal.pdf).

FLUENT reports in the console window that it is reading thenonadiabatic PDF le containing 13 species. It also reportsthat a new material, called pdf-mixture, has been created. Thismixture contains the 13 species that you dened in prePDF andtheir thermodynamic properties.

FLUENT will present an Information dialog box telling you thatavailable material properties have changed. You will be settingproperties later, so you can simply click OK in the dialog boxto acknowledge this information.

Note: FLUENT will automatically activate solution of the en-ergy equation when it reads the non-adiabatic PDF le, soyou do not need to visit the Energy panel to enable heattransfer.



4. Turn on radiation by selecting the P1 radiation model.

Dene ! Models !Radiation...

The P-1 model is one of the radiation models that can account forthe exchange of radiation between gas and particulates.

After you click OK, FLUENT will present an Information dialog boxtelling you that available material properties have changed. Youwill be setting properties later, so you can simply click OK in thedialog box to acknowledge this information.



Step 6: Models: Discrete Phase

The flow of pulverized coal particles will be modeled by FLUENT using thediscrete phase model. This model predicts the trajectories of individualcoal particles, each representing a continuous stream (or mass flow) ofcoal. Heat, momentum, and mass transfer between the coal and the gaswill be included by alternately computing the discrete phase trajectoriesand the gas phase continuum equations.

1. Enable the discrete phase coupling to the continuous phase flowprediction.

Dene ! Models !Discrete Phase...(a) Under Interaction, turn on the Interaction with Continuous Phase

option.

This option enables coupling, in which the discrete phase tra-jectories (along with heat and mass transfer to the particles)are allowed to impact the gas phase equations. If you leavethis option turned o, you can track particles but they willhave no impact on the continuous phase flow.



(b) Set the coupling parameter, the Number of Continuous PhaseIterations per DPM Iteration, to 20.

You should use higher values of this parameter in problemsthat include a high particle mass loading or a larger grid size.Less frequent trajectory updates can be benecial in such prob-lems, in order to converge the gas phase equations more com-



pletely prior to repeating the trajectory calculation.

(c) Under Tracking Parameters, set the Max. Number of Steps to10000.

The limit on the number of trajectory time steps is used toabort trajectories of particles that are trapped in the domain(e.g., in a recirculation).

(d) Retain the default Length Scale of 0.01 m.

The Length Scale controls the time step size used for integra-tion of the discrete phase trajectories. The value of 0.01 mused here implies that roughly 1000 time steps will be used tocompute trajectories along the 10 m length of the domain.

(e) Under Options, turn on Particle Radiation Interaction.



2. Create the discrete phase coal injections.

The flow of the pulverized coal is dened by the initial conditionsthat describe the coal as it enters the gas. FLUENT will use theseinitial conditions as the starting point for its time integration ofthe particle equations of motion (the trajectory calculations).

Here, the total mass flow rate of coal (in the half-width of the duct)is 0.1 kg/s (per unit meter depth). The particles will be assumed toobey a Rosin-Rammler size distribution between 70 and 200 microndiameter. Other initial conditions (velocity, temperature, position)are detailed below along with the appropriate input procedures.

Dene ! Injections...

(a) Click the Create button in the Injections panel.

This will open the Set Injection Properties panel where you willdene the initial conditions dening the flow of coal particles.



In the Set Injection Properties panel you will dene the initialconditions of the flow of coal particles. The particle streamwill be dened as a group of 10 distinct initial conditions,all identical except for diameter, which will obey the Rosin-Rammler size distribution law.

(b) Select group in the Injection Type drop-down list.



(c) Set the Number of Particle Streams to 10.

These inputs tell FLUENT to represent the range of speciedinitial conditions by 10 discrete particle streams, each with itsown set of discrete initial conditions. Here, this will result in10 discrete particle diameters, as the diameter will be variedwithin the injection group.

(d) Select Combusting under Particle Type.

By selecting Combusting you are activating the submodels forcoal devolatilization and char burnout. Similarly, selectingDroplet would enable the submodels for droplet evaporationand boiling.

(e) Select coal-mv in the Material drop-down list.

The Material list contains the combusting particle materialsin the FLUENT database. You can select an appropriate coalfrom this list and then review or modify its properties in theMaterials panel (see Step 8: Materials: Discrete Phase).

(f) Select rosin-rammler in the Diameter Distribution drop-downlist.

The coal particles have a nonuniform size distribution withdiameters ranging from 70 m to 200 m. The size distribu-tion ts the Rosin-Rammler equation, with a mean diameterof 134 m and a spread parameter of 4.52.

(g) Select o2 (the default) in the Oxidizing Species drop-down list.



(h) Specify the range of initial conditions under Point Propertiesstarting with the following inputs for First Point:

X-Position: 0.001 m Y-Position: 0.03124 m X-Velocity: 10 m/s Y-Velocity: 5 m/s Temperature = 300 K Total Flow Rate: 0.1 kg/s Min. Diameter: 70e-6 m Max. Diameter: 200e-6 m Mean Diameter: 134e-6 m Spread Parameter: 4.52

(i) Under Last Point, specify identical inputs for position, veloc-ity, and temperature.

(j) Dene the turbulent dispersion.

i. Click on Turbulent Dispersion.

The panel will change to show the related inputs.



ii. Under Stochastic Tracking, turn on Stochastic Model.

Stochastic tracks model the eect of turbulence in the gasphase on the particle trajectories. Including stochastictracking is important in coal combustion simulations, tosimulate realistic particle dispersion.

iii. Set the Number of Tries to 10.



Note: The new injection (named injection-0, by default) nowappears in the Injections panel.

This panel can be used to copy and delete injection deni-tions. You can also select an existing injection and list theinitial conditions of particle streams dened by that injectionin the console window. The listing for the injection-0 groupwill show 10 particle streams, each with a unique diameterbetween the specied minimum and maximum value, obtainedfrom the Rosin-Rammler distribution, and a unique mass flowrate.



Step 7: Materials: Continuous Phase

All thermodynamic data including density, specic heat, and formationenthalpies are extracted from the prePDF chemical database when thenon-premixed combustion model is used. These properties are transferredto FLUENT as the pdf-mixture material, for which only transport prop-erties, such as viscosity and thermal conductivity, need to be dened.

Dene !Materials...



1. Set Thermal Conductivity to 0.025 (constant).

2. Set Viscosity to 2e-5 (constant).

3. Select wsggm-cell-based in the drop-down list for the AbsorptionCoecient.

This species a composition-dependent absorption coecient, usingthe weighted-sum-of-gray-gases model. See the Users Guide fordetails.

4. Click the Change/Create button.

Note: You can click on the View... button next to Mixture Species toview the species included in the pdf-mixture material. These are thespecies included during the system chemistry setup in prePDF. Notethat the Density and Cp laws cannot be altered: these properties arestored in the non-premixed combustion look-up tables. prePDF usesthe gas law to compute the mixture density and a mass-weightedmixing law to compute the mixture cp. Although it is possible foryou to alter the properties of the individual species, you should notdo so when the non-premixed combustion model is used. This wouldcreate an inconsistency with the look-up table created in prePDF.



Step 8: Materials: Discrete Phase

Dene !Materials...

1. Select combusting-particle from the Material Type list.

The combusting-particle material type appears because you have ac-tivated combusting particles using the Set Injection Properties panel.Other discrete phase material types (droplets, inert particles) willappear in this list if you have created injections of those types.

2. Keep the current selection (coal-mv) in the Combusting Particle Ma-terials list.



This is the combusting particle material type that you selected fromthe list of database options in the Set Injection Properties panel.Additional combusting particle materials can be copied from theproperty database, if desired. You can click the Database... buttonin order to view the combusting-particle materials that are available.Here, you will simply modify the property settings for the selectedmaterial, coal-mv.

3. Set the following constant property values for the coal-mv material:

Density 1300 kg/m3

Cp 1000 J/kg-KThermal Conductivity 0.0454 w/m-kLatent Heat 0Vaporization Temperature 400 KVolatile Component Fraction (%) 28Binary Diusivity 5e-4 m2/sParticle Emissivity 0.9Particle Scattering Factor 0.6Swelling Coecient 2Burnout Stoichiometric Ratio 2.67Combustible Fraction (%) 64

FLUENT uses these inputs as follows:

Density impacts the particle inertia and body forces (when thegravitational acceleration is non-zero).

Cp determines the heat required to change the particle temper-ature.

Latent Heat is the heat required to vaporize the volatiles. Thiscan usually be set to zero when the non-premixed combustionmodel is used for coal combustion. If the volatile compositionhas been selected in order to preserve the heating value of thefuel, the latent heat has been eectively included. (You would,however, use a non-zero latent heat if water content had beenincluded in the volatile denition as vapor phase H2O.)



Vaporization Temperature is the temperature at which the coaldevolatilization begins. It should be set equal to the fuel inlettemperature used in prePDF.

Volatile Component Fraction determines the mass of each coalparticle that is devolatilized.

Binary Diusivity is the diusivity of oxidant to the particlesurface and is used in the diusion-limited char burnout rate.

Particle Emissivity is the emissivity of the particles. It is usedto compute radiation heat transfer to the particles.

Particle Scattering Factor is the scattering factor due to parti-cles.

Swelling Coecient determines the change in diameter duringcoal devolatilization. A swelling coecient of 2 implies thatthe particle size will double as the volatile fraction is released.

Burnout Stoichiometric Ratio is used in the calculation of thediusion-controlled burnout rate. Otherwise, this parameterhas no impact when the non-premixed combustion model isused. When nite-rate chemistry is used instead, the stoichio-metric ratio denes the mass of oxidant required per mass ofchar. The default value represents oxidation of C(s) to CO2.

Combustible Fraction is the mass fraction of char in the coalparticle. It determines the mass of each coal particle that isconsumed by the char burnout submodel.

! The settings for the Vaporization Temperature, Combustible Frac-tion, and Volatile Component Fraction inputs should all beconsistent with your prePDF inputs. (See Step 1: Dene thePreliminary Adiabatic System in prePDF.)



4. Select the Single Rate Devolatilization Model for DevolatilizationModel.

(a) Select the single-rate option in the Devolatilization Model drop-down list.

This opens the Single Rate Devolatilization Model panel.

(b) Accept the default devolatilization model parameters.

5. Select kinetics/diusion-limited for the Combustion Model.

(a) Select the kinetic/diusion-limited option in the CombustionModel drop-down list.

This opens the Kinetics/Diusion Limited Combustion Modelpanel.

(b) Accept the default values.

6. Click Change/Create and then close the Materials panel.



Step 9: Boundary Conditions

Dene !Boundary Conditions...

Hint: You can click your mouse probe button (the right button, by de-fault) on the desired boundary zone in the graphics display window.FLUENT will then select that zone in the Boundary Conditions panel.

1. Set the following conditions for the velocity-inlet-2 zone (the low-speed inlet boundary).

Note: Turbulence parameters are dened here based on intensityand hydraulic diameter. The relatively large turbulence in-tensity of 10% may be typical for combustion air flows. Thehydraulic diameter has been set to twice the height of the 2Dinlet stream.

For the non-premixed combustion calculation, you need to de-ne the inlet Mean Mixture Fraction and Mixture Fraction Vari-ance. For coal combustion, all fuel comes from the discretephase and thus the gas phase inlets have zero mixture frac-tion. Therefore, you can accept the zero default settings.



2. Set the following conditions for the velocity-inlet-8 zone (the high-speed inlet boundary).



3. Set the following conditions for the pressure-outlet-6 zone (the exitboundary).

The exit gauge pressure of zero simply denes the system pressureat the exit to be the operating pressure. The backflow conditionsfor scalars (temperature, mixture fraction, turbulence parameters)will be used only if flow is entrained into the domain through theexit. 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 conditions for the wall-7 zone (the furnace wall).

The furnace wall will be treated as an isothermal boundary with atemperature of 1200 K.

(a) Under Thermal Conditions, select Temperature.

(b) Enter 1200 in the Temperature eld.

Note: The default boundary condition for particles that hit thewall is reflect, as shown under DPM. Alternate treatmentscan be selected, using the BC Type list, for particles that hitthe wall.



Step 10: Solution

1. Set the P1 under-relaxation factor to 1.

Solve ! Controls !Solution...2. Initialize the flow eld using conditions at velocity-inlet-2.

Solve ! Initialize !Initialize...

(a) Select velocity-inlet-2 in the Compute From list.

(b) Click the Init button to initialize the flow eld, and then closethe panel.

! The Apply button does not initialize the flow eld data. Youmust use the Init button. (Apply simply allows you to storeyour initialization parameters for later use.)

Note: Here, with very high pre-heat of the oxidizer stream, youcan start the combustion calculation from the inlet-based ini-tialization. In general, you may need to start your coal com-bustion calculations by patching a high-temperature region and



performing a discrete phase trajectory calculation. This pro-vides the initial volatile and char release required to initiatecombustion. The Solve/Initialize/Patch... menu item and thesolve/dpm-update text command can be used to perform thisinitialization.

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

Solve ! Monitors !Residual...4. Save the case le (coal.cas).

File ! Write !Case...5. Begin the calculation by requesting 400 iterations.

Solve !Iterate...

Note: The default convergence criteria will be met in about 170iterations.

6. Save the converged flow data (coal.dat).

File ! Write !Data...



Step 11: Postprocessing

1. Display the predicted temperature eld (Figure 12.8).

Display !Contours...

The peak temperature in the system is about 2260 K.

Hint: Use the Views panel (Display/Views...) to mirror the displayabout the symmetry plane.



Contours of Static Temperature (k)FLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

2.26e+03

2.16e+03

2.05e+03

1.94e+03

1.84e+03

1.73e+03

1.63e+03

1.52e+03

1.41e+03

1.31e+03

1.20e+03

Figure 12.8: Temperature Contours



2. Display the Mean Mixture Fraction distribution (Figure 12.9).


The mixture-fraction distribution shows where the char and volatilesreleased from the coal exist in the gas phase.



Contours of Mean Mixture FractionFLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

3.72e-02

3.35e-02

2.98e-02

2.61e-02

2.23e-02

1.86e-02

1.49e-02

1.12e-02

7.45e-03

3.72e-03

0.00e+00

Figure 12.9: Mixture-Fraction Distribution



3. Display the devolatilization rate (Figure 12.10).


(a) Select Discrete Phase Model... and DPM Evaporation/Devola-tilization in the drop-down lists under Contours Of.

4. Display the char burnout rate (Figure 12.11) by selecting DPMBurnout from the lower drop-down list.

Note: The display of devolatilization rate shows that volatiles arereleased after the coal travels about one eighth of the fur-nace length. (The onset of devolatilization occurs when thecoal temperature reaches the specied value of 400 K.) Thechar burnout occurs following complete devolatilization. Fig-ure 12.11 shows that burnout is complete at about three-quartersof the furnace.



Contours of DPM Evaporation/Devolatilization (kg/s)FLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

2.95e-03

2.66e-03

2.36e-03

2.07e-03

1.77e-03

1.48e-03

1.18e-03

8.86e-04

5.90e-04

2.95e-04

0.00e+00

Figure 12.10: Devolatilization Rate

Contours of DPM Burnout (kg/s)FLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

4.42e-04

3.97e-04

3.53e-04

3.09e-04

2.65e-04

2.21e-04

1.77e-04

1.32e-04

8.83e-05

4.42e-05

0.00e+00

Figure 12.11: Char Burnout Rate



5. Display the particle trajectory of one particle stream (Figure 12.12).

Display !Particle Tracks...

(a) Select injection-0 in the Release From Injections list.

(b) Select Particle Residence Time in the Color By drop-down list.

(c) Turn on Track Single Particle Stream and set the Stream ID to5.

(d) Click Display.



Particle Traces Colored by Particle Residence Time (s)FLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

3.63e-01

3.27e-01

2.90e-01

2.54e-01

2.18e-01

1.81e-01

1.45e-01

1.09e-01

7.26e-02

3.63e-02

0.00e+00

Figure 12.12: Trajectories of Particle Stream 5 Colored by Particle Res-idence Time



6. Display the oxygen distribution (Figure 12.13).


Note: Although transport equations are solved only for the mixturefraction and its variance, you can still display the predictedchemical species concentrations. These are predicted by thePDF equilibrium chemistry model.

7. Select other species and display their mass fraction distributions(e.g., Figures 12.14{12.16).



Contours of Mass fraction of o2FLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

2.33e-01

2.22e-01

2.11e-01

2.00e-01

1.89e-01

1.78e-01

1.67e-01

1.56e-01

1.45e-01

1.34e-01

1.23e-01

Figure 12.13: O2 Distribution

Contours of Mass fraction of co2FLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

1.19e-01

1.07e-01

9.54e-02

8.35e-02

7.15e-02

5.96e-02

4.77e-02

3.58e-02

2.38e-02

1.19e-02

0.00e+00

Figure 12.14: CO2 Distribution



Contours of Mass fraction of h2oFLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

1.60e-02

1.44e-02

1.28e-02

1.12e-02

9.62e-03

8.02e-03

6.42e-03

4.81e-03

3.21e-03

1.60e-03

0.00e+00

Figure 12.15: H2O Distribution

Contours of Mass fraction of coFLUENT 6.0 (2d, segregated, pdf13, ske)

Sep 10, 2001

6.99e-03

6.29e-03

5.59e-03

4.89e-03

4.19e-03

3.49e-03

2.79e-03

2.10e-03

1.40e-03

6.99e-04

0.00e+00

Figure 12.16: CO Distribution



Step 12: Energy Balances and Particle Report-ing

FLUENT can provide many useful reports, including overall energy ac-counting and detailed information regarding heat and mass transfer fromthe discrete phase. Here, you will examine these reports.

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

Report !Fluxes...

(a) Select Total Heat Transfer Rate under Options.

(b) Under Boundaries, select the pressure-outlet-6, velocity-inlet-2,velocity-inlet-8, and wall-7 zones.

(c) Click Compute.

Note: Positive flux reports indicate heat addition to the domain.Negative values indicate heat leaving the domain. In reactingflows, the heat report uses total enthalpy (sensible heat plus



heat of formation of the chemical species). Here, the net \im-balance" of total enthalpy (about 14 KW) represents the totalenthalpy addition from the discrete phase.

2. Compute the volume sources of heat transferred between the gasand discrete particle phase.

Report !Volume Integrals...

(a) Select Sum under Options.

(b) Select Discrete Phase Model... and DPM Enthalpy Source inthe drop-down lists under Field Variable.

(c) Select fluid-1 under Cell Zones.

(d) Click Compute.

The total enthalpy transfer to the discrete phase from the gas isabout -13.2 KW, as expected based on the boundary flux reportabove. This represents the total enthalpy addition from the discretephase to the gas during the devolatilization and char combustionprocesses.



3. Obtain a summary report on the particle trajectories.

The discrete phase model summary report provides detailed infor-mation about the particle residence time, heat and mass transferbetween the continuous and discrete phases, and (for combustingparticles) char conversion and volatile yield.

Display !Particle Tracks...(a) Select Summary under Report Type.

(b) Select injection-0.

(c) Click Track.

FLUENT will report the summary in the console window. (Youcan write the report to a le by selecting File under Report to.

(d) Review the summary printed in the console window:



DPM Iteration ....

number tracked = 100, escaped = 0, aborted = 0, trapped = 0, evaporated = 0, incomp

Fate Number Elapsed Time (s) Inj

Min Max Avg Std Dev

---- ------ ---------- ---------- ---------- ---------- -------

Incomplete 100 2.398e-01 4.653e-01 3.096e-01 4.818e-02 inj

(*)- Mass Transfer Summary -(*)

Fate Mass Flow (kg/s)

Initial Final Change

---- ---------- ---------- ----------

Incomplete 1.000e-01 8.005e-03 -9.200e-02

(*)- Energy Transfer Summary -(*)

Fate Heat Content (W)

Initial Final Change

---- ---------- ---------- ----------

Incomplete -3.712e+03 9.532e+03 1.324e+04

(*)- Combusting Particles -(*)

Fate Volatile Content (kg/s) Char Content (kg/s)

Initial Final %Conv Initial Final %Con

---- ---------- ---------- ------- ---------- ---------- ------

Incomplete 2.800e-02 0.000e+00 100.00 6.400e-02 5.351e-06 99.9

Done.

The report shows that the average residence time of the coal parti-cles is about 0.33 seconds. Volatiles are completely released withinthe domain and the char conversion is 100% .

Extra: You can obtain a detailed report of the particle position, velocity,diameter, and temperature along the trajectories of individual par-ticles. This type of detailed track reporting can be useful if you aretrying to understand unusual or important details in the discretemodel behavior. To generate the report, visit the Particle Trackspanel. Select Step By Step under Report Type, and File under Re-port to. Enable the Track Single Particle Stream option, and set theStream ID to the desired particle stream. Clicking Track will bring



up the Select File dialog box, where you will enter the name of thele to be written. This le can then be viewed with a text editor.

Summary: Coal combustion modeling involves the prediction of volatileevolution and char burnout from the pulverized coal along withsimulation of the combustion chemistry occuring in the gas phase.In this tutorial you learned how to use the non-premixed combus-tion model to represent the gas phase combustion chemistry. Inthis approach the fuel composition was dened in prePDF and thefuel was assumed to react according to the equilibrium system data.This equilibrium chemistry model can be applied to other turbu-lent, diusion-reaction systems. Note that you can also model coalcombustion using the nite-rate chemistry model.

You also learned how to set up and solve a problem involving adiscrete phase of combusting particles. You created discrete phaseinjections, activated coupling to the gas phase, and dened thediscrete phase material properties. These procedures can be usedto set up other simulations involving reacting or inert particles.


Documents

Coal combustion simulation in FLUENT