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08_Multiple Char Reactions Flipbook PDF
08_Multiple Char Reactions
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Tutorial: Multiple Char Reactions
Introduction The purpose of this tutorial is to provide guidelines and recommendations to set up and solve multiple char reactions for coal combustion or gasification using finite-rate/eddy-dissipation model. This tutorial demonstrates how to do the following: • Use discrete phase model to set up and solve multiple char reactions for coal combustion. • Activate and set up the finite-rate/eddy-dissipation model for the reactions occurring during 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 from ANSYS FLUENT 14.0 Tutorial Guide, and that you are familiar with the ANSYS FLUENT navigation pane and menu structure. Some steps in the setup and solution procedure will not 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 application of 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 twodimensional duct as shown in Figure 1. Only half of the domain width is modeled because of 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/s and spans 0.125 m. The other stream enters at 15 m/s and spans 0.375 m. Both streams are air at 1500 K. Coal particles enter the 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 Starting ANSYS FLUENT Using FLUENT Launcher in ANSYS FLUENT 14.0 User’s Guide. 3. Enable Double-Precision in the Options list. The Display Options are enabled by default. Therefore, after you read in the mesh, it will 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 group box 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. • Solid—It is used for conducting walls and solid bodies where only energy equation is solved. • Fluid-Solid—In this case, solids like granular materials are defined as fluids to facilitate solution of flow as well as energy equations. Further, there are two different fluids available in the database, carbon(c) and carbon-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 nitrogen and 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 remaining five reactions are as follows:
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C(s) + 0.5O2 → CO
(1)
C(s) + CO2 → 2CO
(2)
C(s) + H2 O → H2 + CO
(3)
H2 + 0.5O2 → H2 O
(4)
CO + 0.5O2 → CO2
(5)
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ii. Specify the reactions as shown in the following table: ID Reaction Type Number of Reactants Species Stoich. Coefficient Rate Exponent Arrhenius Rate Number of Products Species Stoich. Coefficient Rate Exponent Particle Surface Reaction / Mixing Rate
2 Particle Surface 2 c, o2 c=1, o2=0.5 default default 1 co co=1
3 Particle Surface 2 c, co2 c=1, co2=1 default default 1 co co=2
default default
default default
4 Particle Surface 2 c, h2o c=1, h2o=1 default default 2 h2, co h2=1, co=1 default default
5 Volumetric
6 Volumetric
2 h2, o2
2 co, o2
h2=1, o2=0.5 default default 1 h2o h2o=1
co=1, o2=0.5 default default 1 co2 co2=1
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 40 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 X-Position (m) Y-Position (m) X-Velocity (m/s) Y-Velocity (m/s) Temperature (K) Total Flow Rate (kg/s) Min. Diameter (m) Max. Diameter (m) Mean Diameter (m) Spread Parameter
Value 0.001 0.03124 10 5 300 0.1 70e-6 200e-6 134e-6 4.52
vii. Enter the same values of X-Position, Y-Position, X-Velocity, Y-Velocity, and Temperature for the Last Point. viii. Click the Turbulent Dispersion tab.
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A. Enable Discrete Random Walk Model in the Stochastic Tracking group box. 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 Density (kg/m3) Cp (j/kg-K) Volatile Component Fraction (%) Binary Diffusivity (m2/s) Combustible Fraction (%) Combustion Model
Value 1300 1000 28 5e-4 64 multiple-surfacereactions
(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 Fractions of 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 −→ Edit...
Parameter Velocity Magnitude Specification Method Turbulence Intensity Hydraulic Diameter Temperature Species Mass Fractions
Value 15 m/s Intensity and Hydraulic Diameter 10% 0.75 m 1500 K o2=0.23
2. Set the boundary conditions for velocity-inlet-8. Boundary Conditions −→
velocity-inlet-8 −→ Edit...
Parameter Velocity Magnitude Specification Method Turbulence Intensity Hydraulic Diameter Temperature Species Mass Fractions
Value 50 m/s Intensity and Hydraulic Diameter 5% 0.25 m 1500 K o2=0.23
3. Set the boundary conditions for the wall-7. Boundary Conditions −→
wall-7 −→ Edit...
(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 −→ Edit...
Parameter Specification Method Backflow Turbulence Intensity Backflow Hydraulic Diameter Backflow Total Temperature Species Mass Fractions
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Value Intensity and Hydraulic Diameter 5% 1m 2000 K o2=0.23
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Step 7: Solution 1. Use coupled solver scheme. Solution Methods (a) Select Coupled from the Scheme drop-down list. (b) Select PRESTO! from the Pressure drop-down list. (c) Enable Pseudo Transient. 2. Initialize the flow field. Solution Initialization −→ Initialize Hybrid Initialization is the default Initialization Method in ANSYS FLUENT 14.0. Refer to the section 29.11 Hybrid Initialization, in the ANSYS FLUENT 14.0 User’s Guide. 3. Run the calculation for 500 iterations. The residuals are as shown in Figure 3. The solution converges in approximately 355 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. Change the colormap position. Graphics and Animations −→ Options... (a) Select Bottom from the Colormap Alignment drop-down list. (b) Close the Display Options dialog box. 3. 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-down list. (c) Click Display. The temperature contours are as shown in Figure 4.
Figure 4: Contours of Static Temperature
4. Display filled contours of species mass fraction of h2o (Figure 5).
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Figure 5: Contours of Mass Fraction of h2o
5. Display filled contours of species mass fraction of co2 (Figure 6).
Figure 6: Contours of Mass Fraction of co2
6. Close the Contours dialog box.
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7. 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. 8. 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, Index Min Max Avg Std Dev Min Max --------- --------- --------- ---------- -------------
-------------------Escaped - Zone 6 10 2.224e-01 3.037e-01 2.451e-01 2.303e-02 injection-0 injection-0 52 (*)- Mass Transfer Summary -(*) Fate ---Escaped - Zone 6
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Mass Flow (kg/s) Initial Final Change ---------- ---------- ---------2.217e-03 1.773e-04 -2.039e-03 (*)- Energy Transfer Summary -(*)
Fate ---Escaped - Zone 6
Change of Heat (W) Sensible Latent Reaction Total ---------- ---------- ---------- -----------3.650e+02 -5.741e-01 -1.119e+04 -1.083e+03 (*)- Combusting Particles -(*)
Fate
Volatile Content (kg/s) Initial Final %Conv
Char Content (kg/s) Initial Final
%Conv ------------- ---------- ------- ---------- ---------------Escaped - Zone 6 6.207e-04 0.000e+00 100.00 1.419e-03 0.000e+00 100.00 (*) - Multiple Surface Reactions -(*) Fate ---Escaped - Zone 6
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Species Species Content (kg/s) Names Initial Final %Conv ------- ---------- ---------- ------c 1.419e-03 1.419e-07 99.99
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Step 9: Radiation Model 1. Select 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 300 iterations. Run Calculation The solution converges in approximately 85 additional iterations. The scaled residuals are 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). File −→ Write −→Case & Data... 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 travel a distance before they start releasing volatiles. The reaction starts at this point and the temperature increases. The radiation model lowers the peak temperature by taking the heat away from the reaction zone. In finite-rate/eddy-dissipation coal combustion, coal particles release volatiles that react with oxygen and produce combustion products. The stoichiometric coefficients can be calculated once chemical composition of coal volatiles is known. For information on determining coal volatile composition, see tutorial, Using the Non-Premixed Combustion Model in the ANSYS FLUENT 14.0 Tutorial Guide.
Summary Application of multiple char reactions and finite-rate/eddy-dissipation model in a coal combustion case has been demonstrated.
Further Improvements This tutorial demonstrates a second order solution. You may be able to obtain a more accurate solution by adapting the mesh. Mesh adaption ensures that the solution is independent of the mesh. In more 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 Break Up (EBU) Model.
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