Tut 12 multiple char reactions tutorial

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  • Tutorial: Multiple Char Reactions

    Introduction

    The purpose of this tutorial is to provide guidelines and recommendations to set up and solvemultiple char reactions for coal combustion or gasification using finite-rate/eddy-dissipationmodel.

    This tutorial demonstrates how to do the following:

    Use discrete phase model to set up and solve multiple char reactions for coal combus-tion.

    Activate and set up the finite-rate/eddy-dissipation model for the reactions occurringduring combustion.

    Solve the case using appropriate solver settings. Postprocess the resulting data. Include the radiation model and study its effect on reaction temperature.

    Prerequisites

    This tutorial is written with the assumption that you have completed Tutorial 1 fromANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENTnavigation pane and menu structure. Some steps in the setup and solution procedure willnot be shown explicitly. In this tutorial, you will use turbulence and combustion models,so you should have some experience with them. This tutorial will focus on the applicationof these models in coal combustion and will not cover the mechanics of using these models.

    Problem Description

    The coal combustion system considered in this tutorial is a simple 10 m 1 m two-dimensional duct as shown in Figure 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 streamnear the center of the duct enters at 50 m/s and spans 0.125 m. The other stream en-ters at 15 m/s and spans 0.375 m. Both streams are air at 1500 K. Coal particles enterthe furnace near the center of the high-speed stream with a mass flow rate of 0.1 kg/s(total flow rate in the furnace is 0.2 kg/s). The duct wall has a constant temperature of

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    Figure 1: Problem Schematic

    1200 K. The Reynolds number, based on the inlet dimension and the average inlet velocity,is approximately 100,000. Thus, the flow is turbulent.

    Setup and Solution

    Preparation

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

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

    For more information about FLUENT Launcher see Section 1.1.2 StartingANSYS FLUENT Using FLUENT Launcher in ANSYS FLUENT 13.0 Users Guide.

    3. Enable Double-Precision in the Options list.

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

    Step 1: Mesh

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

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

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    Step 2: General Settings

    1. Retain the default solver settings.

    2. Check the mesh (Figure 2).

    General Check

    Figure 2: Mesh Display

    Step 3: Models

    1. Enable the Energy Equation.

    Models Energy Edit...2. Enable the Realizable k-epsilon (2 eqn) turbulence model.

    Models Viscous Edit...

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    3. Define the species model.

    Models Species Edit...

    (a) Select Species Transport in the Model group box.

    (b) Enable Volumetric and Particle Surface in the Reactions group box.

    (c) Select coal-mv-volatiles-air from the Mixture Material drop-down list.

    (d) Select Finite Rate/Eddy-Dissipation in the Turbulence-Chemistry Interaction groupbox and click OK.

    Click OK in the Information dialog box.

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    Step 4: Materials

    Materials Create/Edit...

    1. Copy the fluids from FLUENT Database....

    (a) Click FLUENT Database... and select fluid from the Material Type drop-down list.

    (b) Select carbon-monoxide (co) from the FLUENT Fluid Materials list and click Copy.

    (c) Similarly copy carbon-solid(c) and hydrogen(h2) from the fluid database.

    Note: There are two types of solid material definitions in FLUENT.

    SolidIt is used for conducting walls and solid bodies where only energyequation is solved.

    Fluid-SolidIn this case, solids like granular materials are defined asfluids to facilitate solution of flow as well as energy equations.

    Further, there are two different fluids available in the database, carbon(c) andcarbon-solid(c). For defining granular carbon, select carbon-solid(c).

    (d) Close the FLUENT Database Materials dialog box.

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    2. Ensure that piecewise-polynomial is selected from the Cp drop-down list for co2, co,c, h2, n2, o2, and h2o species.

    Retain the default values in the Piecewise-Polynomial Profile dialog box.

    3. Retain Cp as constant for the species coal mv volatiles.

    4. Modify the properties of coal-mv-volatiles-air mixture.

    (a) Select mixture from the Material Type drop-down list.

    (b) Click Edit... for Mixture Species to open the Species dialog box.

    i. Add h2 and co to the Selected Species list.

    Make sure that nitrogen is the last species in the list. If not, remove nitrogenand add it again.

    ii. Add c to the Selected Solid Species list.

    iii. Click OK to close the Species dialog box.

    (c) Click Edit... for the Reaction to open the Reactions dialog box.

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    i. Set the Total Number of Reactions to 6.

    The first reaction for volatile oxidation has already been set. The remainingfive reactions are as follows:

    C(s) + 0.5O2 CO (1)C(s) + CO2 2CO (2)

    C(s) +H2O H2 + CO (3)H2 + 0.5O2 H2O (4)CO + 0.5O2 CO2 (5)

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    ii. Specify the reactions as shown in the following table:

    ID 2 3 4 5 6

    Reaction Type ParticleSurface

    ParticleSurface

    ParticleSurface

    Volumetric Volumetric

    Number of Reactants 2 2 2 2 2

    Species c, o2 c,co2

    c,h2o

    h2, o2 co, o2

    Stoich. Coefficient c=1,o2=0.5

    c=1,co2=1

    c=1,h2o=1

    h2=1,o2=0.5

    co=1,o2=0.5

    Rate Exponent default default default default defaultArrhenius Rate default default default default defaultNumber of Products 1 1 2 1 1

    Species co co h2, co h2o co2Stoich. Coefficient co=1 co=2 h2=1,

    co=1h2o=1 co2=1

    Rate Exponent default default default default defaultParticle Surface Reac-tion / Mixing Rate

    default default default default default

    iii. Click OK to close the Reactions dialog box.

    (d) Click Edit... for Mechanism to open the Reaction Mechanisms dialog box.

    i. Retain the selection of all the reactions from the Reactions list.

    ii. Click OK to close the Reaction Mechanisms dialog box.

    (e) Retain the selection of incompressible-ideal-gas from the Density drop-down list.

    (f) Retain the selection of mixing law from the Cp drop-down list.

    (g) Click Change/Create and close the Create/Edit Materials dialog box.

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    Step 5: Discrete Phase Model

    1. Define the discrete phase model.

    Models Discrete Phase... Edit...

    (a) Enable Interaction with Continuous Phase in the Interaction group box.

    (b) Enter 40 for Number of Continuous Phase Iterations per DPM Iteration.

    (c) Enter 10000 for Max. Number of Steps in the Tracking Parameters group box.

    (d) Enable Specify Length Scale.

    (e) Retain default value of 0.01 m for Length Scale.

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

    2. Create the discrete phase injections.

    Define Injections...(a) Click Create to open the Set Injection Properties dialog box.

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    i. Select group from the Injection Type drop-down list.

    ii. Enter 10 for Number of Particle Streams.

    iii. Select Combusting in the Particle Type group box.

    iv. Select coal-mv from the Material drop-down list.

    v. Select rosin-rammler from the Diameter Distribution drop-down list.

    vi. Enter the following values for First Point under Point Properties tab.

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

    vii. Enter the same values of X-Position, Y-Position, X-Velocity, Y-Velocity, andTemperature for the Last Point.

    viii. Click the Turbulent Dispersion tab.

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    A. Enable Discrete Random Walk Model in the Stochastic Tracking groupbox.

    B. Enter 10 for Number of Tries.

    ix. Click OK to close the Set Injection Properties dialog box.

    (b) Close the Injections dialog box.

    3. Set the properties for the Combusting Particle, coal-mv.

    Materials coal-mv Create/Edit...

    Parameter Value

    Density (kg/m3) 1300Cp (j/kg-K) 1000Volatile Component Fraction(%)

    28

    Binary Diffusivity (m2/s) 5e-4Combustible Fraction (%) 64Combustion Model multiple-surface-

    reactions

    (a) Click Change/Create and close the Create/Edit Materials dialog box.

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    A new tab Multiple Reactions will appear in the Set Injection Properties dialog box.You can click the Multiple Reactions tab and check that the Species Mass Fractionsof c is set to 1.

    Step 6: Boundary Conditions

    Boundary Conditions

    1. Set the boundary conditions for velocity-inlet-2.

    Boundary Conditions... velocity-inlet-2

    Parameter ValueVelocity Magnitude 15 m/sSpecification Method Intensity and Hydraulic Diameter

    Turbulence Intensity 10%Hydraulic Diameter 0.75 mTemperature 1500 KSpecies Mass Fractions o2=0.23

    2. Set the boundary conditions for velocity-inlet-8.

    Boundary Conditions... velocity-inlet-8

    Parameter ValueVelocity Magnitude 50 m/sSpecification Method Intensity and Hydraulic Diameter

    Turbulence Intensity 5%Hydraulic Diameter 0.25 mTemperature 1500 KSpecies Mass Fractions o2=0.23

    3. Set the boundary conditions for the wall-7.

    Boundary Conditions... wall-7(a) In the Thermal tab select Temperature from Thermal Conditions and enter 1200 K

    for Temperature.

    4. Set the boundary conditions for the pressure-outlet-6.

    Boundary Conditions... pressure-outlet-6

    Parameter ValueSpecification Method Intensity and Hydraulic Diameter

    Backflow Turbulence Intensity 5%Backflow Hydraulic Diameter 1 mBackflow Total Temperature 2000 KSpecies Mass Fractions o2=0.23

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    Step 7: Solution

    1. Enable the plotting of residuals during the calculation.

    Monitors Residuals Edit...(a) Retain the default settings.

    (b) Click OK to close the Residual Monitors dialog box.

    2. Initialize the flow field.

    Solution Initialization

    (a) Select velocity-inlet-2 from the Compute from drop-down list.

    (b) Click Initialize.

    3. Run the calculation for 500 iterations. The residuals are as shown in Figure 3.

    The solution converges in approximately 240 iterations.

    Run calculation

    Figure 3: Scaled Residuals

    4. Save the case and data files (mchar.cas/dat.gz).

    File Write Case & Data...

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    Step 8: Postprocessing

    1. Change the view to mirror the display across the symmetry plane.

    Graphics and Animations Views...(a) Select symmetry-5 from the Mirror Planes list and click Apply.

    (b) Close the Views dialog box.

    2. Display the temperature contours.

    Graphics and Animations Contours Set Up...(a) Enable Filled in the Options group box.

    (b) Select Temperature... and Static Temperature from the Contours of drop-downlist.

    (c) Click Display. The temperature contours are as shown in Figure 4.

    Figure 4: Contours of Static Temperature

    3. Display filled contours of species mass fraction of h2o (Figure 5).

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    Figure 5: Contours of Mass Fraction of h2o

    4. Display filled contours of species mass fraction of co2 (Figure 6).

    Figure 6: Contours of Mass Fraction of co2

    5. Close the Contours dialog box.

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    6. Display particle trajectory for one of the streams.

    Graphics and Animations Particle Tracks Set Up...

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

    (b) Enable Track Single Particle Stream.

    (c) Set the Stream ID to 5.

    (d) Click Display. The particle tracks are as shown in Figure 7.

    Figure 7: Particle Tracks

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    (e) Close the Particle Tracks dialog box.

    7. Generate a summary for the injections using the TUI commands.

    Hint: You may need to enter press the key to get the > prompt.

    > report

    /report> dpm-summary

    The summary for the injections will be displayed in the console window.

    Fate Number Elapsed Time (s) Injection, IndexMin Max Avg Std Dev Min Max

    ---- ---- --------- --------- --------- ---------- ---------------------------Escaped - Zone 6 10 2.286e-01 2.950e-001 2.705e-01 2.277e-02 injection-0 59injection-0 50

    (*)- Mass Transfer Summary -(*)

    Fate Mass Flow (kg/s)Initial Final Change

    ---- ---------- ---------- ----------Escaped - Zone 6 1.665e-02 1.332e-03 -1.532e-02

    (*)- Energy Transfer Summary -(*)

    Fate Change of Heat (W)Sensible Latent Reaction Total

    ---- ---------- ---------- ---------- ------------Escaped - Zone 6 2.510e+03 -4.311e+00 2.074e+04 2.324e+04

    (*)- Combusting Particles -(*)

    Fate Volatile Content (kg/s) Char Content (kg/s)Initial Final %Conv Initial Final

    %Conv---- ---------- ---------- ------- ---------- -----------------Escaped - Zone 6 4.661e-03 0.000e+00 100.00 1.065e-02 0.000e+00100.00

    (*) - Multiple Surface Reactions -(*)

    Fate Species Species Content (kg/s)Names Initial Final %Conv

    ---- ------- ---------- ---------- -------Escaped - Zone 6 c 1.065e-02 1.066e-06 99.99

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    Step 9: Radiation Model

    1. Enable the P1 radiation model.

    Models Radiation Edit...2. Select wsggm-domain-based from the Absorption Coefficient drop-down list.

    Materials coal-mv-volatiles-air Create/Edit...3. Ensure that the Under-Relaxation Factors for P1 is set to 1.

    Solution Controls

    4. Run the solution for 100 iterations.

    Run Calculation

    The solution converges in approximately 30 additional iterations. The scaled residualsare as shown in Figure 8.

    Figure 8: Scaled Residuals Using the Radiation Model

    5. Save the case and data files (mchar-rad.cas/dat.gz).

    6. Display the contours of temperature (Figure 9).

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    Figure 9: Contours of Static Temperature Using Radiation Model

    Results

    In this tutorial, an injection group is introduced at the inlet. The coal-mv particles travela distance before they start releasing volatiles. The reaction starts at this point and thetemperature increases. The radiation model lowers the peak temperature by taking theheat away from the reaction zone. In finite-rate/eddy-dissipation coal combustion, coalparticles release volatiles that react with oxygen and produce combustion products. Thestoichiometric coefficients can be calculated once chemical composition of coal volatiles isknown. For information on determining coal volatile composition, see tutorial, Using theNon-Premixed Combustion Model in the ANSYS FLUENT 13.0 Tutorial Guide.

    Summary

    Application of multiple char reactions and finite-rate/eddy-dissipation model in a coal com-bustion case has been demonstrated.

    Further Improvements

    This tutorial demonstrates an initial first order solution. You may be able to obtain amore accurate solution by using an appropriate higher-order discretization scheme and byadapting the mesh. Mesh adaption ensures that the solution is independent of the mesh. Inmore realistic/complex cases, you can obtain non-reacting solution, reacting flow solution,and then solution with radiation similar to the tutorial, Coal Combustion with Eddy BreakUp (EBU) Model.

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