-
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.
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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.
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Using the Non-Premixed Combustion Model
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.
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Using the Non-Premixed Combustion Model
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
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Using the Non-Premixed Combustion Model
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
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Using the Non-Premixed Combustion Model
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.
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Using the Non-Premixed Combustion Model
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).
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(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
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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.
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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.
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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.
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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.
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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...
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(a) Enter 400 K and 1500 K as the Fuel and Oxidiser inlet
tem-peratures.
(b) Click Apply and close the panel.
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Using the Non-Premixed Combustion Model
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.
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(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.
(b) Click Apply and close the panel.
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.
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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).
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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
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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.
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Using the Non-Premixed Combustion Model
1. Dene the prePDF model type as non-adiabatic.
Setup !Case...
(a) Select Non-Adiabatic under Heat transfer options and click
Ap-ply.
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Using the Non-Premixed Combustion Model
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...
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Using the Non-Premixed Combustion Model
(a) Enter 298 for Min. Temperature and 3000 for Max.
Tempera-ture.
(b) Click Apply and close the panel.
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
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Using the Non-Premixed Combustion Model
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.
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Using the Non-Premixed Combustion Model
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.
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Using the Non-Premixed Combustion Model
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
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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).
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Using the Non-Premixed Combustion Model
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
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Using the Non-Premixed Combustion Model
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.
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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.
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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
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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...
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Using the Non-Premixed Combustion Model
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.
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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.
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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.
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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.
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(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-
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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.
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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.
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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.
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(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.
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(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.
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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.
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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.
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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...
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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.
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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.
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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.)
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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.)
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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.
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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.
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2. Set the following conditions for the velocity-inlet-8 zone
(the high-speed inlet boundary).
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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.
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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.
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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
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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...
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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.
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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
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2. Display the Mean Mixture Fraction distribution (Figure
12.9).
Display !Contours...
The mixture-fraction distribution shows where the char and
volatilesreleased from the coal exist in the gas phase.
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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
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3. Display the devolatilization rate (Figure 12.10).
Display !Contours...
(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.
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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
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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.
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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
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6. Display the oxygen distribution (Figure 12.13).
Display !Contours...
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).
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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
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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
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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
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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.
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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:
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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
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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.
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