tut13 combustion model

7/30/2019 Tut13 Combustion Model

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Tutorial 13. Using the Non-Premixed Combustion Model

Introduction

A pulverized coal combustion simulation involves modeling a continuous gas phase flowfield and its interaction with a discrete phase of coal particles. The coal particles, travelingthrough the gas, will devolatilize and undergo char combustion, creating a source of fuelfor reaction in the gas phase.

The reaction can be modeled using either the species transport model or the non-premixedcombustion model. In this tutorial you will model a simplified coal combustion furnaceusing the non-premixed combustion model for the reaction chemistry.

In this tutorial you will learn how to:

Define inputs for modeling non-premixed combustion chemistry

Prepare a Probability Density Function (PDF) table in FLUENT for pulverized coal

Define a discrete second phase of coal particles

Solve a simulation involving reacting discrete phase coal particles

Use the P-1 radiation model with particle/radiation interaction

The non-premixed combustion model uses a modeling approach that solves transportequations for one or two conserved scalars, the mixture fractions. Multiple chemicalspecies, including radicals and intermediate species, may be included in the problemdefinition. Their concentrations will be derived from the predicted mixture fractiondistribution.

Property data for the species are accessed through a chemical database and turbulence-chemistry interaction is modeled using a -function for the PDF. See Chapter 15 of theUsers Guide for details on the non-premixed combustion modeling approach.

Prerequisites

This tutorial assumes that you are familiar with the menu structure in FLUENT and thatyou have completed Tutorial 1. Some steps in the setup and solution procedure will notbe shown explicitly.

c Fluent Inc. January 11, 2005 13-1
http://tut01.pdf/http://../ug/fl62ug.pdfhttp://-/?-


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Using the Non-Premixed Combustion Model

Problem Description

The coal combustion system considered in this tutorial is a simple 10 m by 1 m two-dimensional duct depicted in Figure 13.1. As the model is symmetric, only half of thedomain width is modeled.

The inlet of the 2D duct is split into two streams. A high-speed stream near the centerof the duct enters at 50 m/s and spans 0.125 m. The other stream enters at 15 m/s andspans 0.375 m. Both streams are air at 1500 K. Coal particles enter the furnace near thecenter of the high-speed stream with a mass flow rate of 0.1 kg/s (total flow rate in thefurnace is 0.2 kg/s).

The duct wall has a constant temperature of 1200 K. The Reynolds number based onthe inlet dimension and the average inlet velocity is about 100,000. Thus, the flow isturbulent.

Details of the coal composition and size distribution are included in Step 3: Non AdiabaticPDF Table and Step 5: Materials.

0.5 m

10 m

SymmetryPlane

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 13.1: 2D Furnace with Pulverized Coal Combustion

13-2 c Fluent Inc. January 11, 2005


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Setup and Solution

Preparation

1. Download non_premix_combustion.zip from the Fluent Inc. User Services Centeror copy it from the FLUENT documentation CD to your working directory (as

described in Tutorial 1).

2. Unzip non_premix_combustion.zip .

coal.msh can be found in the /non premix combustion folder created after unzip-ping the file.

The mesh file, coal.msh is a quadrilateral mesh describing the system geometryshown in Figure 13.1.

3. Start the 2D version of FLUENT.

Step 1: Grid

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

File Read Case...

The FLUENT console window reports that the mesh contains 1357 quadrilateralcells.

2. Check the grid.

Grid Check

The grid check should not report any errors or negative volumes.

3. Display the grid (Figure 13.2).

Display Grid...

Due to the grid resolution and the size of the domain, you may find it more usefulto display just the outline, or to zoom in on various portions of the grid display.

You can use the mouse zoom button (middle button, by default) to zoom in tothe display and the mouse probe button (right button, by default) to find out theboundary zone labels. As annotated in Figure 13.2, the upstream boundary containstwo velocity inlets (for the low-speed and high-speed air streams), the downstream

boundary is a pressure outlet, the top boundary is a wall, and the bottom boundaryis a symmetry plane.

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velocity-inlet-8

wall-7

symmetry

velocity-inlet-2

GridFLUENT 6.2 (2d, segregated, lam)

Figure 13.2: 2D Coal Furnace Mesh Display with Annotated Boundary Types



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Step 2: Models

1. Accept the default segregated solver.

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

Define Models Solver...

2. Turn on the Energy Equation.

Define Models Energy...

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

3. Turn on the standard k-epsilon turbulence model.

Define Models Viscous...

Reynolds number of the flow is approximately 105. Hence the flow is turbulent and

the high-Rek- model is suitable.



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4. Turn on the P1 radiation model.

Define Models Radiation...

Coal particles can radiate significantly, and the P-1 radiation model is appropriatefor combustors larger than 1m, so the optical thickness is higher than 1.



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5. Turn on the Non-Premixed Combustion model.

Define Models Species Transport & Reaction...

(a) Under Model, select Non-Premixed Combustion.

The panel will expand to show the related inputs. You will use this panel tocreate the PDF table.

When you use the non-premixed combustion model, you need to create a PDF table.This table contains information on the thermo-chemistry and its interaction withturbulence. FLUENT interpolates the PDF during the solution of the non-premixed

combustion model.



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Step 3: Non Adiabatic PDF Table

1. In the Species Model panel, under PDF Options, turn on the Create Table option.

This will update the panel to display the inputs for the creating the PDF table.The Inlet Diffusion option enables the mixture fraction to diffuse out of the domainthrough inlets and outlets.

2. Define the chemistry models in the Chemistry tab.

For single mixture fraction cases, the assumed shape -function is used since itaccurately represents the experimentally observed PDFs of mixture fraction.

(a) Retain the default options, Equilibrium and Non Adiabatic.

In most non-premixed combustion simulations, theEquilibrium chemistry modelis recommended. The Laminar Flamelets option can model local chemical non-equilibrium due to turbulent strain, such as radical super-equilibrium, but can-not model slow chemistry like NOx.

The coal combustor considered in this tutorial is a non-adiabatic system, with

heat transfer at the combustor wall and heat transfer to the coal particles fromthe gas. Therefore, a non-adiabatic combustion system must be consideredwhile creating a PDF table.

(b) Retain the default value for Operating Pressure.

(c) Under Options, turn on Empirical Fuel Stream.



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You can define either a single fuel stream, or a fuel stream plus a secondarystream. Enabling a secondary stream allows you to keep track of two mixture

fractions. For coal combustion, this will allow you to track volatile matter (thesecondary stream) separately from the char (fuel stream). However, the twomixture fraction has a substantially greater computational expense than thesingle mixture fraction model.

For complex hydrocarbons, like coal, the individual species components are mostoften unknown. The empirical model allows you to define the fuel compositionin terms of the ultimate analysis (atomic fractions of C, H, O, N, and S),along with the lower heating value and heat capacity.

(d) Specify the Empirical Parameters for fuel.

i. Specify the Fuel Lower Calorific Value as 3.53e+07 j/kg.

ii. Specify the value of Fuel Specific Heat as 1000 j/kg-k.

TheFuel Rich Flammability Limit allows you to perform a partial equilibrium

calculation, suspending equilibrium calculations when the mixture fraction ex-ceeds the specified rich limit. This increases the efficiency of the PDF cal-culation, allowing you to bypass the complex equilibrium calculations in the

fuel-rich region. This is also more physically realistic than the assumption offull equilibrium. For empirically defined streams, the rich limit is always 1.0and cannot be altered.

3. Define the boundary species in the Boundary tab.

For an empirically-defined fuel, you need to define the atom fraction of C, H, O,N, and S elements. The intermediate and product species will be determined auto-matically.



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(a) Under Species Units, select Mole Fraction.

(b) Enter the oxidizer composition.

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

i. Under Oxid, retain default values for n2 and o2.

(c) Enter the fuel composition.

The fuel composition is entered in mole fractions of the elements C, H, O, and

N. The mole fractions are obtained from the proximate and ultimate analysisof coal.

The calculation for the proximate and ultimate analysis that yields the elemen-tal composition of the volatile stream is given in the Appendix: Analysis forElemental Composition of Coal.

i. Under Fuel, enter the following values:

Species Mole FractionC 0.581H 0.390

O 0.016N 0.013

FLUENT will use this information, along with the coal heating value, to definethe species present in the fuel.

Note: All boundary species with a mass or mole fractions of zero will be ig-nored.



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(d) Under Temperature, enter the inlet temperatures for Fuel as 400 and Oxid as1500 and click Apply.

The system pressure and inlet stream temperatures are required for the equi-librium chemistry calculation. The fuel stream inlet temperature for coal com-bustion should be the temperature at the onset of devolatilization. The oxidizer

inlet temperature should correspond to the air inlet temperature. In this tuto-rial, the coal devolatilization temperature will be set to 400 K and the air inlettemperature is 1500 K. The system pressure is one atmosphere.

4. In the Control tab, retain default species to be excluded from the equilibrium cal-culation.

5. Specify the table parameters and calculate the PDF table.

(a) In the Table tab, retain the default values for all the Table Parameters andclick Apply.

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

The minimum temperature should be a few degrees lower than the lowest bound-ary condition temperature (e.g., the inlet temperature or wall temperature).In coal combustion systems, the minimum system temperature should also beset below the temperature at which the volatiles begin to evolve from the coal.Here, the vaporization temperature at which devolatilization begins will be setto 400 K. Thus, the minimum system temperature is set to 298 K (the default).



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

The non-adiabatic calculation requires much more computation than the adiabaticcalculation. FLUENT begins by accessing the thermodynamic data from the database.The enthalpy field is initialized, the temperature limits are calculated, the stoichio-metric mixture fractions are calculated, and the enthalpy grid adjusted to account

for the solution parameters.Mean values of temperature, composition, and density at the discrete mixture-

fraction/mixture-fraction-variance/enthalpy points are then calculated. The resultis a set of tables containing mean values of species mole fractions, density, andtemperature at each discrete value of these three parameters.

7. Click OK to close the Species Model panel.

8. Save the PDF output file (coal.pdf).

File Write PDF...

(a) Enter coal.pdf as the Pdf File name.(b) Click OK to write the file.

By default, the file will be saved as formatted (ASCII, or text). To save abinary (unformatted) file, turn on the Write Binary Files option in the SelectFile panel.

9. Review one slice of the 3D look-up table.

Display PDF Tables/Curves...



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(a) Retain default parameters and click Display (Figure 13.3).

The 3D look-up tables are reviewed on a slice-by-slice basis. By default, theslice selected is that corresponding to the adiabatic enthalpy values. You canselect other slices of constant enthalpy for display, as well.

Mean Temperature(K)FLUENT 6.2 (2d, segregated, pdf20, ske)

ZYX

Figure 13.3: Non-Adiabatic Temperature Look-Up Table on the Adiabatic Enthalpy Slice

The maximum and minimum values for mean temperature and the correspondingmean mixture fraction are also reported in the console. The maximum mean tem-

perature is reported as 2782 K at a mean mixture fraction of 0.09.10. Examine the species/mixture-fraction relationship in the non adiabatic system.

(a) Select Mole Fraction of c as the Plot Variable and click Display (Figure 13.4).

(b) Similarly, plot the mean mole fractions for CO (Figure 13.5).



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Mole Fraction of cFLUENT 6.2 (2d, segregated, pdf20, ske)

ZY

X

Figure 13.4: Mole Fractions of C(s)

Mole Fraction of coFLUENT 6.2 (2d, segregated, pdf20, ske)

ZY

X

Figure 13.5: Mole Fractions of CO



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11. Generate a 2D curve of species/mixture-fraction at the adiabatic slice.

(a) Select Mole Fraction of co2 as the Plot Variable.

(b) Select 2D Curve on 3D Surface as the Plot Type and click Display.

ZY

X

Mole Fraction of co2FLUENT 6.2 (2d, segregated, pdf20, ske)

Mean Mixture Fraction

co2of

FractionMole

10.90.80.70.60.50.40.30.20.10

1.20e-01

1.00e-01

8.00e-02

6.00e-02

4.00e-02

2.00e-02

0.00e+00

Figure 13.6: Mole Fractions of CO2 (2D Curve)



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Step 4: Models: Discrete Phase

The flow of pulverized coal particles will be modeled by FLUENT using the discrete phasemodel. This model predicts the trajectories of individual coal particles, each represent-ing a continuous stream (or mass flow) of coal. Heat, momentum, and mass transferbetween the coal and the gas will be included by alternately computing the discrete phase

trajectories and the gas phase continuum equations.

1. Enable the discrete phase coupling to the continuous phase flow prediction.

Define Models Discrete Phase...



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(a) Under Interaction, turn on the Interaction with Continuous Phase option.

This option enables coupling, in which the discrete phase trajectories (alongwith heat and mass transfer to the particles) are allowed to impact the gasphase equations.

If you leave this option turned off, you can track particles but they will have

no impact on the continuous phase flow.

(b) Set the coupling parameter, the Number of Continuous Phase Iterations perDPM Iteration, to 20.

Use higher values of this parameter in problems that include a high particlemass loading or a larger grid size. Less frequent trajectory updates can bebeneficial in such problems, in order to converge the gas phase equations morecompletely before repeating the trajectory calculation.

(c) Under Tracking Parameters, set the Max. Number of Steps to 10000.

The limit on the number of trajectory time steps is used to abort trajectories

of particles that are trapped in the domain (e.g., in a recirculation).(d) Retain the default Step Length Factor of 5.

(e) In the Physical Models tab, turn on Particle Radiation Interaction.

(f) Click OK to close the Discrete Phase Model panel.

2. Create the discrete phase coal injections.

The flow of the pulverized coal is defined by the initial conditions that describe thecoal as it enters the gas. FLUENT uses these initial conditions as the starting point

for the time integration of the particle equations of motion (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 are assumed to obey a Rosin-Rammler sizedistribution between 70 and 200 micron diameter.

Define Injections...

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

This will open theSet Injection Properties panel where you will define the initialconditions defining the flow of coal particles.



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The particle stream will be defined as a group of 10 distinct initial conditions,

all identical except for diameter, which will obey the Rosin-Rammler size dis-tribution 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 specified initial conditionsby 10 discrete particle streams, each with its own set of discrete initial condi-tions. Here, this will result in 10 discrete particle diameters, as the diameterwill be varied within the injection group.

(d) Select Combusting under Particle Type.

By selecting Combusting you are activating the submodels for coal devolatiliza-tion and char burnout. Similarly, selecting Droplet enables the submodels fordroplet evaporation and boiling.



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

The Material list contains the combusting particle materials in the FLUENTdatabase. You can select an appropriate coal from this list and review ormodify its properties in the Materials panel (see Step 5-2: Materials).

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

The coal particles have a nonuniform size distribution with diameters rangingfrom 70m to 200m. The size distribution fits the Rosin-Rammler equation,with a mean diameter of 134 m and a spread parameter of 4.52.

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

(h) Specify the range of initial conditions under Point Properties starting with thefollowing values 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 values for position, velocity, and tempera-ture.

(j) Click on the Turbulent Dispersion tab to define the turbulent dispersion.

The panel will change to show the related inputs.



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i. Under Stochastic Tracking, turn on Discrete Random Walk Model.

Stochastic tracks model the effect of turbulence in the gas phase on theparticle trajectories. Including stochastic tracking is important in coalcombustion simulations, to simulate realistic particle dispersion.

ii. Set the Number of Tries to 10 and click OK.

The new injection (named injection-0, by default) appears in the Injectionspanel.



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The Injections panel can be used to copy and delete injection definitions. Youlist the initial conditions of particle streams defined by an injection in theconsole window.

The listing for the injection-0 group will show 10 particle streams, each with aunique diameter between the specified minimum and maximum value, obtained

from the Rosin-Rammler distribution, and a unique mass flow rate.



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Step 5: Materials

Define Materials...

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

All thermodynamic data for the continuous phase, including density, specific heat,and formation enthalpies are extracted from the chemical database when the non-premixed combustion model is used. These properties are transferred as the pdf-mixture material, for which only transport properties, such as viscosity and thermalconductivity, need to be defined.

(a) Set Thermal Conductivity to 0.025 (constant).

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

(c) Select wsggm-cell-based in the drop-down list for the Absorption Coefficient.

This specifies a composition-dependent absorption coefficient, using the weighted-sum-of-gray-gases model.

See Section 12.3.9 of the Users Guide for more details.

http://../ug/fl62ug.pdfhttp://-/?-


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(d) Click the Change/Create button.

You can click on the View... button next to Mixture Species to view the speciesincluded in the pdf-mixture material. These are the species included during thesystem chemistry setup. TheDensity andCp laws cannot be altered: these propertiesare stored in the non-premixed combustion look-up tables.

FLUENT uses the gas law to compute the mixture density and a mass-weightedmixing law to compute the mixture cp. When the non-premixed combustion modelis used, do not alter the properties of the individual species. This will create aninconsistency with the PDF look-up table.

2. Define the discrete phase material.

(a) In the Material Type drop-down list, select combusting-particle .

Thecombusting-particle material type appears because you have activated com-busting particles using the Set Injection Properties panel. Other discrete phasematerial types (droplets, inert particles) will appear in this list if you have

created injections of those types.

(b) Retain the default selection (coal-mv) in the Fluent Combusting Particle Mate-rials list.



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This is the combusting particle material type that you selected from the list ofdatabase options in the Set Injection Properties panel. Additional combustingparticle materials can be copied from the property database, if required. Clickthe Fluent Database... button to view the combusting-particle materials thatare available. Modify the property settings for the selected material, coal-mv.How FLUENTuses these inputs are defined in the Appendix: Discrete PhaseMaterial Properties.

(c) Modify the coal-mv material properties as per following constant values:

Properties Values

Density kg/m3 1300Cp J/kg-K 1000Thermal Conductivity w/m-k 0.0454Latent Heat 0Vaporization Temperature K 400Volatile Component Fraction (%) 28

Binary Diffusivity w/m-k 5e-4Particle Emissivity 0.9Particle Scattering Factor 0.6Swelling Coefficient 2Burnout Stoichiometric Ratio 2.67Combustible Fraction (%) 64

Note: The values for the Vaporization Temperature should be consistent withthe fuel temperature considered in Step 3-3: Non Adiabatic PDF Table. TheVolatile Component Fraction andCombustible Fraction should be consistent withthe volatiles and char ratios in the proximate analysis of the coal shown in the

Appendix: Analysis for Elemental Composition of Coal.(d) For Devolatilization Model, select single rate in the drop-down list and accept

default values in the Single Rate Devolatilization Model panel.

(e) For the Combustion Model, select kinetics/diffusion limited and accept defaultvalues in the Kinetics/Diffusion-Limited Combustion Model panel.



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(f) Click Change/Create and close the Materials panel.



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Step 6: Operating Conditions

1. Keep the default operating conditions.

Define Operating Conditions...



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Step 7: Boundary Conditions

Define Boundary Conditions...

Hint: You can click the mouse probe button (right button, by default) on the requiredboundary zone in the graphics display window to select that zone in the Boundary

Conditions panel.

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

Properties Values

Velocity Specification Method Magnitude and DirectionVelocity Magnitude(m/s) 15Temperature(k) 1500Turbulent Specification Method Intensity and Hydraulic DiameterTurbulence Intensity(%) 10Hydraulic Diameter(m) 0.75



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Turbulence parameters are defined based on intensity and hydraulic diameter. Therelatively large turbulence intensity of 10% may be typical for combustion air flows.The hydraulic diameter has been set to twice the height of the 2D inlet stream.

For the non-premixed combustion calculation, you have to define the inlet MeanMixture Fraction and Mixture Fraction Variance. For coal combustion, all fuel comes

from the discrete phase and thus the gas phase inlets have zero mixture fraction.Therefore, you can accept the zero default settings.

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

Properties Values

Velocity Specification Method Magnitude and DirectionVelocity Magnitude(m/s) 50Temperature(k) 1500Turbulent Specification Method Intensity and Hydraulic DiameterTurbulence Intensity(%) 10Hydraulic Diameter(m) 0.25



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

Properties ValuesBackflow Total Temperature 2000Turbulent Specification Method Intensity and Hydraulic DiameterTurbulence Intensity(%) 10Hydraulic Diameter(m) 1

The exit gauge pressure of zero defines the system pressure at the exit to be theoperating pressure. The backflow conditions for scalars (temperature, mixture frac-tion, turbulence parameters) will be used only if flow is entrained into the domainthrough the exit. It is a good idea to use reasonable values in case flow reversal

occurs at the exit at some point during the solution process.



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4. Set conditions for the wall-7 zone (furnace wall).

The furnace wall will be treated as an isothermal boundary with a temperature of1200 K.

(a) Under Thermal Conditions, select Temperature.

(b) Enter 1200 in the Temperature field.

Note: The default boundary condition for particles that hit the wall is reflect, asshown under DPM. Alternate treatments can be selected, using the BC Typelist, for particles that hit the wall.



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Step 8: Solution

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

Solve Controls Solution...

2. Initialize the flow field 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 field, and then close the panel.

!Use the Init button to initialize the flow field data.The Apply button doesnot initialize the flow field data. It only allows you to store your initializa-tion parameters for later use.

Note: Here, with very high pre-heat of the oxidizer stream, you can start the com-bustion calculation from the inlet-based initialization. In general, you mayneed to start your coal combustion calculations by patching a high-temperatureregion and performing a discrete phase trajectory calculation. This provides theinitial volatile and char release required to initiate combustion. TheSolve/Initialize/Patch... menu item and the solve/dpm-update text command can be used to

perform this initialization.

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

Solve Monitors Residual...

(a) Enable Plot under Options and then click OK.



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4. Save the case file (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 140 iterations.

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

File Write Data...



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Step 9: Postprocessing

1. Display the predicted temperature field (Figure 13.7).

Display Contours...

Hint: Use the Views panel (Display/Views...) to mirror the display about the sym-

metry plane.The peak temperature in the system is about 2280 K.

2. Display the devolatilization rate (Figure 13.8).

Display Contours...

(a) Select Discrete Phase Model... and DPM Evaporation/Devolatilization in theContours of drop-down lists.

Figure 13.8shows that volatiles are released after the coal travels about one eighth ofthe furnace length. The onset of devolatilization occurs when the coal temperature

reaches the specified value of 400 K.

3. Display the char burnout rate (Figure 13.9) by selecting DPM Burnout from thelower drop-down list.

The char burnout occurs after complete devolatilization. Figure 13.9 shows thatburnout is complete at about three-quarters of the furnace.



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Contours of Static Temperature (k)FLUENT 6.2 (2d, segregated, pdf20, ske)

2.25e+03

2.20e+03

2.15e+03

2.09e+03

2.04e+03

1.99e+031.94e+03

1.88e+03

1.83e+03

1.78e+03

1.73e+03

1.67e+03

1.62e+03

1.57e+03

1.52e+03

1.46e+03

1.41e+03

1.36e+03

1.31e+03

1.25e+03

1.20e+03Z

Y

X

Figure 13.7: Temperature Contours

Contours of DPM Evaporation/Devolatilization (kg/s) FLUENT 6.2 (2d, segregated, pdf20, ske)

2.54e-03

2.41e-03

2.29e-03

2.16e-032.03e-03

1.91e-03

1.78e-03

1.65e-03

1.52e-03

1.40e-03

1.27e-03

1.14e-03

1.02e-03

8.89e-04

7.62e-04

6.35e-04

5.08e-04

3.81e-04

2.54e-04

1.27e-04

0.00e+00Z

Y

X

Figure 13.8: Devolatilization Rate



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Contours of DPM Burnout (kg/s)FLUENT 6.2 (2d, segregated, pdf20, ske)

3.89e-04

3.70e-04

3.51e-04

3.31e-04

3.12e-042.92e-04

2.73e-04

2.53e-04

2.34e-04

2.14e-04

1.95e-04

1.75e-04

1.56e-04

1.36e-04

1.17e-04

9.74e-05

7.79e-05

5.84e-05

3.89e-05

1.95e-05

0.00e+00Z

Y

X

Figure 13.9: Char Burnout Rate



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4. Display the Mean Mixture Fraction distribution (Figure 13.10).

Display Contours...

(a) Select Pdf... and Mean Mixture Fraction in the Contours of drop-down lists.

The mixture-fraction distribution shows where the char and volatiles released from

the coal exist in the gas phase.

Contours of Mean Mixture FractionFLUENT 6.2 (2d, segregated, pdf20, ske)

3.71e-02

3.52e-02

3.34e-02

3.15e-02

2.96e-02

2.78e-02

2.59e-02

2.41e-02

2.22e-02

2.04e-02

1.85e-02

1.67e-02

1.48e-02

1.30e-02

1.11e-029.26e-03

7.41e-03

5.56e-03

3.71e-03

1.85e-03

0.00e+00Z

Y

X

Figure 13.10: Mixture-Fraction Distribution

5. Display the oxygen distribution (Figure 13.11).

Display Contours...

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

Note: Although transport equations are solved only for the mixture fraction and itsvariance, you can display the chemical species concentrations as predicted bythe PDF equilibrium chemistry model.

6. Similarly, display mass fraction distributions for other species CO2 (Figure 13.12),H2O (Figure 13.13), CO (Figure 13.14).



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Contours of Mass fraction of o2FLUENT 6.2 (2d, segregated, pdf20, ske)

2.33e-01

2.27e-01

2.22e-01

2.16e-01

2.11e-01

2.05e-012.00e-01

1.94e-01

1.89e-01

1.83e-01

1.78e-01

1.72e-01

1.67e-01

1.61e-01

1.55e-01

1.50e-01

1.44e-01

1.39e-01

1.33e-01

1.28e-01

1.22e-01Z

Y

X

Figure 13.11: O2 Distribution

Contours of Mass fraction of co2 FLUENT 6.2 (2d, segregated, pdf20, ske)

1.21e-01

1.15e-01

1.09e-01

1.03e-019.67e-02

9.07e-02

8.46e-02

7.86e-02

7.25e-02

6.65e-02

6.04e-02

5.44e-02

4.84e-02

4.23e-02

3.63e-02

3.02e-02

2.42e-02

1.81e-02

1.21e-02

6.04e-03

0.00e+00Z

Y

X

Figure 13.12: CO2 Distribution



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Contours of Mass fraction of h2oFLUENT 6.2 (2d, segregated, pdf20, ske)

1.64e-02

1.56e-02

1.48e-02

1.40e-02

1.31e-02

1.23e-021.15e-02

1.07e-02

9.85e-03

9.03e-03

8.21e-03

7.39e-03

6.57e-03

5.75e-03

4.93e-03

4.10e-03

3.28e-03

2.46e-03

1.64e-03

8.21e-04

0.00e+00Z

Y

X

Figure 13.13: H2O Distribution

Contours of Mass fraction of co FLUENT 6.2 (2d, segregated, pdf20, ske)

4.54e-03

4.31e-03

4.08e-03

3.86e-033.63e-03

3.40e-03

3.18e-03

2.95e-03

2.72e-03

2.50e-03

2.27e-03

2.04e-03

1.81e-03

1.59e-03

1.36e-03

1.13e-03

9.07e-04

6.81e-04

4.54e-04

2.27e-04

0.00e+00Z

Y

X

Figure 13.14: CO Distribution



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7. Display the particle trajectory of one particle stream (Figure 13.15).

Display Particle Tracks...

(a) Retain Particle Variables... and Particle Residence Time under Color by.

(b) Select injection-0 in the Release from Injections list.

(c) Turn on Track Single Particle Stream, set the Stream ID to 5 and click Display.



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Particle Traces Colored by Particle Residence Time (s)FLUENT 6.2 (2d, segregated, pdf20, ske)

4.21e-01

4.00e-01

3.79e-01

3.58e-01

3.37e-01

3.16e-01

2.95e-01

2.74e-01

2.53e-01

2.32e-01

2.11e-01

1.90e-01

1.69e-01

1.47e-01

1.26e-01

1.05e-01

8.43e-02

6.32e-02

4.21e-02

2.11e-02

0.00e+00Z

Y

X

Figure 13.15: Trajectories of Particle Stream 5 Colored by Particle Residence Time



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Step 10: Energy Balances and Particle Reporting

FLUENT can report the overall energy balance and details of the heat and mass transferfrom the discrete phase.

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

Report Fluxes...

(a) Select Total Heat Transfer Rate under Options.

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

(c) Click Compute.

Positive flux reports indicate heat addition to the domain. Negative values indicateheat leaving the domain. In reacting flows, the heat report uses total enthalpy(sensible heat plus heat of formation of the chemical species). The net imbalanceof total enthalpy (about 11.26 kW) represents the total enthalpy addition from thediscrete phase.

2. Compute the volume sources of heat transferred between the gas and discrete par-

ticle phase.Report Volume Integrals...



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(a) Select Sum under Report Type.

(b) Select Discrete Phase Model... and DPM Enthalpy Source in the Field Variabledrop-down lists.

(c) Select fluid-1 under Cell Zones and click Compute.

The total enthalpy transfer from gas to the discrete phase is about -11.05 kW, asexpected based on the boundary flux report above. This represents the total en-thalpy addition from the discrete phase to the gas during devolatilization and charcombustion processes.

3. Obtain a summary report on the particle trajectories.

The discrete phase model summary report provides detailed information about theparticle residence time, heat and mass transfer between the continuous and discretephases, and (for combusting particles) char conversion and volatile yield.

Display Particle Tracks...

(a) Select Summary under Report Type.

(b) Select injection-0 and click Track.

FLUENT will report the summary in the console window. You can write thereport to a file by selecting File under Report to.

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



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DPM Iteration ....

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

Fate Number Elapsed Time (s) Injection,

Min Max Avg Std Dev Min Max

---- ----- -------- --------- --------- ---------- -------- --------Escaped-Zone6 100 2.511e-01 4.793e-01 3.31e-01 5.248e-02 inj..n-0 1 inj..n-0 7

(*)- Mass Transfer Summary -(*)

Fate Mass Flow (kg/s)

Initial Final Change

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

Escaped - Zone 6 1.000e-01 8.003e-03 -9.200e-02

(*)- Energy Transfer Summary -(*)

Fate Heat Content (W)

Initial Final Change

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

Escaped - Zone 6 -1.463e+03 9.808e+03 1.127e+04

(*)- Combusting Particles -(*)

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

Initial Final %Conv Initial Final %Conv

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

Escaped - Zone 6 2.800e-02 0.000e+00 100.00 6.400e-02 3.803e-06 99.99

The report shows that the average residence time of the coal particles is about 0.33seconds. Volatiles are completely released within the domain and the char conver-sion is 99.99% .



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Extra: You can obtain a detailed report of the particle position, velocity, diameter, andtemperature along the trajectories of individual particles. A detailed track reportingis useful if you are trying to understand unusual or important details in the discretemodel behavior. To generate the report, visit the Particle Tracks panel and do the

following:

1. Select Step By Step under Report Type.2. Select File under Report to.

3. Enable the Track Single Particle Stream option, and set the Stream ID to thedesired particle stream.

4. Click Write... to bring up the Select File dialog box and enter the name of thefile to be written.

This file can be viewed with a text editor.

SummaryCoal combustion modeling involves the prediction of volatile evolution and char burnoutfrom pulverized coal along with simulation of the combustion chemistry occurring in thegas phase. In this tutorial you learned how to use the non-premixed combustion model torepresent the gas phase combustion chemistry. In this approach the fuel composition wasdefined empirically and the fuel was assumed to react according to the equilibrium systemdata. This equilibrium chemistry model can be applied to other turbulent, diffusion-reaction systems. You can also model coal combustion using the finite-rate chemistrymodel.

You also learned how to set up and solve a problem involving a discrete phase of com-busting particles. You created discrete phase injections, activated coupling to the gasphase, and defined the discrete phase material properties. These procedures can be usedto set up other simulations involving reacting or inert particles.



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Appendix

Coal Analysis for Elemental Composition

From proximate analysis, the fuel considered here consists of 28% volatiles, 64% char,and 8% ash. Use this information, along with the ultimate analysis, to define the coal

composition. Define the composition of the fuel stream (char) to be 100% C(S). The fuelstream composition (char and 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 -

For the dry-ash-free coal, the ultimate analysis is:

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

For simplifying modeling, the sulfur content of the coal can be combined into the nitrogenmass fraction, to yield:

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

Combine the proximate and ultimate analysis data to yield the following elemental com-position 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



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Discrete Phase Material Properties

FLUENT uses the material properties for the discrete phase as follows:

Density impacts the particle inertia and body forces (when the gravitational accel-eration is non-zero).

Cp determines the heat required to change the particle temperature.

Latent Heat is the heat required to vaporize the volatiles. This can usually be setto zero when the non-premixed combustion model is used for coal combustion. Ifthe volatile composition has been selected in order to preserve the heating value ofthe fuel, the latent heat has been effectively included. (You would, however, use anon-zero latent heat if water content had been included in the volatile definition asvapor phase H2O.)

Vaporization Temperature is the temperature at which the coal devolatilization be-

gins. It should be set equal to the fuel inlet temperature used in PDF table.

Volatile Component Fraction determines the mass of each coal particle that is de-volatilized.

Binary Diffusivity is the diffusivity of oxidant to the particle surface and is used inthe diffusion-limited char burnout rate.

Particle Emissivity is the emissivity of the particles. It is used to compute radiationheat transfer to the particles.

Particle Scattering Factor is the scattering factor due to particles.

Swelling Coefficient determines the change in diameter during coal devolatilization.A swelling coefficient of 2 implies that the particle size will double as the volatilefraction is released.

Burnout Stoichiometric Ratio is used in the calculation of the diffusion-controlledburnout rate. Otherwise, this parameter has no impact when the non-premixedcombustion model is used. When finite-rate chemistry is used instead, the stoichio-metric ratio defines the mass of oxidant required per mass of char. The defaultvalue represents oxidation of C(s) to CO2.

Combustible Fraction is the mass fraction of char in the coal particle. It determinesthe mass of each coal particle that is consumed by the char burnout submodel.

tut13 combustion model

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