Tutorial: Premixed Flow in a Conical Chamber using the Finite-Rate Chemistry Model

Transcription

1 Tutorial: Premixed Flow in a Conical Chamber using the Finite-Rate Chemistry Model Introduction The purpose of this tutorial is to provide guidelines and recommendations for setting up and solving the combustion of a lean, premixed gaseous fuel mixture of methane and air in a conical reactor. Combustion is modeled using the finite-rate chemistry model in ANSYS Fluent. This tutorial demonstrates how to do the following: Set up and solve the combustion problem of a premixed gaseous fuel mixture of methane and air in a conical reactor. Use the species transport model with finite-rate chemistry. Set up and solve the case with appropriate solver settings. Postprocess the resulting data. Prerequisites This tutorial is written with the assumption that you have completed Tutorial 1 from ANSYS Fluent 14.5 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. This tutorial uses the species transport model with finite-rate chemistry and volumetric reactions. A basic understanding of the combustion processes is desirable. For more information on species transport and finite-rate chemistry relating to volumetric reactions, see Section 15.1 Volumetric Reactions in the ANSYS Fluent 14.5 User s Guide. Problem Description The conical combustor considered is shown in Figure 1. A small nozzle at the center of the combustor introduces the lean methane-air mixture (equivalence ratio = 0.6) at 60 m/s and 650 K. Combustion involves several complex reactions between CH 4, O 2, CO 2, CO, H 2 O, and N 2. The high-speed flow reverses direction in the combustor and exits through the co-axial outlet. c Fluent Inc. January 28,

2 Figure 1: Schematic Figure Setup and Solution Preparation 1. Copy the file (conreac.msh) to your working folder. 2. Use FLUENT Launcher to start the 2D version of ANSYS Fluent. 3. Enable Double-Precision in the Options list. Step 1: Mesh 1. Read the mesh file (conreac.msh). File Read Mesh... As the mesh file is read, ANSYS Fluent will report the progress in the console. Step 2: General Settings 1. Define the solver settings. General Axisymmetric (a) Select Axisymmetric from the 2D Space list. 2 c ANSYS, Inc. January 28, 2013

3 2. Check the mesh (see Figure 2). General Check Figure 2: Mesh Display ANSYS Fluent will perform various checks on the mesh and will report the progress in the console. Make sure the minimum volume reported is a positive number. Step 3: Models 1. Enable the Energy Equation. Models Energy Edit Define the k-epsilon turbulence model. Models Viscous Edit... (a) Select k-epsilon in the Model list to open Viscous Model dialog box. (b) Retain the default settings and click OK to close Viscous Model dialog box. c ANSYS, Inc. January 28,

4 3. Define the species model. Models Species Edit... (a) Select Species Transport from the Model list to open Species Model dialog box. i. Enable Volumetric from the Reactions group box. ii. Select methane-air-2step from the Mixture Material drop-down list. iii. Select Finite-Rate/Eddy Dissipation from the Turbulence-Chemistry Interaction group box. iv. Click OK to close the Species Model dialog box. An Information dialog box will appear informing that the material properties are changed. Click OK. 4 c ANSYS, Inc. January 28, 2013

5 Step 4: Materials Materials Fluid Create/Edit Copy nitrogen-oxide (no) from the database. (a) Click the Fluent Database... to open the Fluent Database Materials dialog box. i. Select fluid from the Material Type drop-down list. ii. Select nitrogen-oxide (no) from the Fluent Fluid Materials list. iii. Click Copy and close the Fluent Database Materials dialog box. iv. Click Change/Create and close the Create/Edit Materials dialog box. 2. Modify the mixture material, methane-air-2step. Materials Mixture Create/Edit... (a) Enter for Thermal Conductivity in the Properties group box. (b) Click the Edit... to the right of the Mixture Species to open the Species dialog box. i. Add nitrogen-oxide (no) to the Selected Species list. ii. Reorder the species so that nitrogen (n2) appears last in the Selected Species selection list. c ANSYS, Inc. January 28,

6 iii. Click OK to close the Species dialog box. (c) Specify the reactions in the Reactions dialog box. i. Click the Edit... to the right of the Reaction to open the Reactions dialog box. A. Increase the Total Number of Reactions to 5. B. Define the reactions as shown in the following table: where, PEF = Pre-Exponential Factor AE = Activation Energy TE = Temperature Exponent 6 c ANSYS, Inc. January 28, 2013

7 Reaction ID Number of Reactants Species ch4, o2 co, o2 co2 n2, o2, co n2, o2 Stoich. ch4 = 1 co = 1 co2 = 1 n2 = 1 n2 = 1 Coefficient o2 = 1.5 o2 = 0.5 o2 = 1 o2 = 1 co = 0 Rate ch4 = 1.46 co = co2 = 1 n2 = 0 n2 = 1 Exponent o2 = o2 = 1.57 o2 = o2 = 0.5 co = Arrhenius PEF=1.6596e+15 PEF=7.9799e+14 PEF=2.2336e+14 PEF=8.8308e+23 PEF=9.2683e+14 Rate AE=1.72e+08 AE=9.654e+07 AE=5.1774e+08 AE=4.4366e+08 AE=5.7276e+08 TE = 0.5 Number of Products Species co, h2o co2 co, o2 no, co no Stoich. co = 1 co2 = 1 co = 1 no = 2 no = 2 Coefficient h2o = 2 o2 = 0.5 co = 0 Rate co = 0 co2 = 0 co = 0 no = 0 no = 0 Exponent h2o = 0 o2 = 0 co = 0 Mixing default default default A = 1e+11 A = 1e+11 Rate values values values B = 1e+11 B = 1e+11 ii. Click OK to close the Reactions dialog box. (d) Click the Edit... to the right of the Mechanism to open the Reaction Mechanisms dialog box. i. Select all the reactions from the Reactions list. ii. Click OK to close the Reaction Mechanisms dialog box. (e) Ensure that mixing law is selected from the Cp (Specific Heat) drop-down list for mixture. (f) Ensure that piecewise-polynomial is selected from the Cp (Specific Heat) dropdown list for all the species. 3. Click Change/Create and close the Create/Edit Materials dialog box. c ANSYS, Inc. January 28,

8 Step 5: Boundary Conditions Boundary Conditions 1. Set the boundary conditions for pressure-outlet-4. Boundary Conditions pressure-outlet-4 Edit... (a) Enter the values as shown in the following table: Parameters Values Specification Method Intensity and Length Scale Backflow Turbulent Intensity 10 % Backflow Turbulent Length Scale m Backflow Total Temperature 2500 K Species Mass Fractions o2 = 0.05, co2 = 0.1, and h2o = 0.1 (b) Click OK to close the Pressure Outlet dialog box. 2. Set the boundary conditions for velocity-inlet-5. Boundary Conditions velocity-inlet-5 Edit... (a) Enter the values as shown in the table: Parameters Values Velocity Magnitude 60 m/s Specification Method Intensity and Length Scale Turbulent Intensity 10 % Turbulent Length Scale m Temperature 650 K Species Mass Fractions ch4 = o2 = (b) Click OK to close the Velocity Inlet dialog box. 3. Retain the default boundary conditions for wall-1. 8 c ANSYS, Inc. January 28, 2013

9 Step 6: Solution 1. Modify the species model. Models Species Edit... (a) Disable Volumetric from the Reactions list. (b) Click OK to close the Species Model dialog box. This is a non-reacting simulation. It is good practice to solve for flow and mixing reactions (species transport) and then enable the reactions. 2. Set the under-relaxation factors. Solution Controls (a) Enter 0.95 for all the species and for Energy in the Under-Relaxation Factors group box. 3. Initialize the solution. Solution Initialization Hybrid Initialization is the default Initialization Method in ANSYS Fluent Refer to the section Hybrid Initialization, in the ANSYS Fluent 14.5 User s Guide. c ANSYS, Inc. January 28,

10 (a) Click More Settings... i. In the Species Settings tab enable Specify Species Parameters. ii. Enter 0.01 for all Species Fraction. iii. Click OK and close the Hybrid Initialization dialog box. (b) Click Initialize. 4. Save the case file (5step cold.cas.gz). File Write Case Solve for 200 iterations (see Figure 3). Run Calculation Calculate 10 c ANSYS, Inc. January 28, 2013

11 Figure 3: Scaled Residuals 6. Save the data file (5step cold.dat.gz). File Write Data... Step 7: Postprocessing 1. Display the velocity vectors in the domain (see Figure 4). Graphics and Animations Vectors Set Up... (a) Enter 10 for Scale and click Display. (b) Close the Vectors dialog box. Figure 4: Velocity Vectors c ANSYS, Inc. January 28,

12 2. Display contours of stream function (see Figure 5). Graphics and Animations Contours Set Up... (a) Select Velocity... and Stream Function from the Contours of drop-down lists and click Display. (b) Close the Contours dialog box. Figure 5: Contours of Stream Function Step 8: Reacting Flow Case Setup 1. Enable volumetric reactions. Models Species Edit... (a) Enable Volumetric from the Reactions list. (b) Click OK to close the Species Model dialog box. 2. Patch an initial temperature field to initiate the combustion process. Solution Initialization Patch... (a) Select Temperature from the Variable list. (b) Enter 1000 for Value. (c) Select fluid-6 from the Zones to Patch list. (d) Click Patch. (e) Close the Patch dialog box. 3. Save the case file (5step final.cas.gz). File Write Case c ANSYS, Inc. January 28, 2013

13 4. Solve for an additional 1000 iterations (see Figure 6). Run Calculation Calculate Figure 6: Scaled Residuals After 1000 Iterations 5. Save the data file (5step final.dat.gz). File Write Data Compute the gas phase mass fluxes through all the boundaries. Reports Fluxes Set Up... (a) Calculate the Mass Flow Rate with velocity-inlet-5 as boundary. i. Select velocity-inlet-5 from the Boundaries list. ii. Click Compute. (b) Calculate Mass Flow Rate with pressure-outlet-4 as boundary. i. Select Pressure-outlet-4 from the Boundaries list. ii. Click Compute. Both these figures should be equal and opposite in sign to each other. 7. Compute the gas phase energy fluxes through all the boundaries. Reports Fluxes Set Up... (a) Select Total Heat Transfer Rate from the Options list. (b) Select all the zones from the Boundaries list and click Compute. (c) Close the Flux Reports dialog box. c ANSYS, Inc. January 28,

14 Step 9: Postprocessing 1. Display velocity vectors in the domain with a scale factor of 10 (see Figure 7). Figure 7: Velocity Vectors 2. Display contours of stream function (see Figure 8). Figure 8: Contours of Stream Function 14 c ANSYS, Inc. January 28, 2013

15 3. Display filled contours of static temperature (see Figure 9). Figure 9: Contours of Static Temperature 4. Display contours of species mass fractions of ch4 (see Figure 10). Figure 10: Contours of Mass Fraction of CH 4 c ANSYS, Inc. January 28,

16 5. Display contours of species mass fractions of co2 (see Figure 11). Figure 11: Contours of Mass Fraction of CO 2 6. Display contours of species mass fractions of co (see Figure 12). Figure 12: Contours of Mass Fraction of CO 16 c ANSYS, Inc. January 28, 2013

17 7. Display contours of species mass fractions of h2o (see Figure 13). Figure 13: Contours of Mass Fraction of H 2 O 8. Display contours of species mass fractions of no (see Figure 14). Figure 14: Contours of Mass Fraction of NO c ANSYS, Inc. January 28,

18 9. Display contours of species mass fractions of o2 (see Figure 15). Figure 15: Contours of Mass Fraction of O 2 Results The finite-rate chemistry model in ANSYS Fluent can be used to predict the temperature field and species mass fractions. Summary Application of the finite-rate chemistry model using a 5-step global chemical reaction mechanism has been demonstrated. The 5-step global mechanism by Nicol [1] for methane oxidation and NO formation is tuned specifically for lean, premixed combustion applications. The mechanism is valid for a pressure of 1 Atm, inlet temperature of 650 K, and a fuel-air equivalence ratio range of 0.45 to Quantities presented below are in units of kmoles, cubic meters, seconds, and Kelvin. 1. CH O 2 CO + 2H 2 O R 1 = [CH 4 ] 1.46 [O 2 ] exp( 20643/T ) 2. CO + 0.5O 2 CO 2 R 2 = [CO] [O 2 ] 1.57 exp( 11613/T ) 18 c ANSYS, Inc. January 28, 2013

19 3. CO 2 CO + 0.5O 2 R 3 = [CO 2 v]exp( 62281/T ) 4. N 2 + O 2 2NO R 4 = [CO] [O 2 ] exp( 53369/T ) 5. N 2 + O 2 2NO R 5 = T 0.5 [N 2 ][O 2 ] 0.5 exp( 68899/T ) References [1] Nicol, D.G. A Chemical and Numerical Study of NO x and Pollutant Formation in Low-Emissions Combustion, Ph.D. Dissertation, University of Washington (1995). c ANSYS, Inc. January 28,