fluent6.3非预混燃烧算例

Tutorial 14. Using the Non-Premixed Combustion Model Introduction A 300KW BERL combustor simulation is modeled using the PDF mixture fraction model. The reaction can be modeled using either the species transport model or the non-premixed combustion model. In this tutorial you will set up and solve a natural gas combustion problem using the non-premixed combustion model for the reaction chemistry. This tutorial demonstrates how to do the following: • Define inputs for modeling non-premixed combustion chemistry. • Prepare a Probability Density Function (PDF) table in FLUENT. • Solve a natural gas combustion simulation problem. • Use the P-1 radiation model for combustion applications. • Use the k-� turbulence model. The non-premixed combustion model uses a modeling approach that solves transport equations for one or two conserved scalars and the mixture fractions. Multiple chemical species, including radicals and intermediate species, may be included in the problem definition. Their concentrations will be derived from the predicted mixture fraction distribution. Property data for the species are accessed through a chemical database and turbulence- chemistry interaction is modeled using a β-function for the PDF. See Chapter 15 of the User’s 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 that you have completed Tutorial 1. Some steps in the setup and solution procedure will not be shown explicitly. c© Fluent Inc. September 21, 2006 14-1 Using the Non-Premixed Combustion Model Problem Description The flow considered is an unstaged natural gas flame in a 300 kW swirl-stabilized burner. The furnace is vertically-fired and of octagonal cross-section with a conical furnace hood and a cylindrical exhaust duct. The furnace walls are capable of being refractory-lined or water-cooled. The burner features 24 radial fuel ports and a bluff centerbody. Air is introduced through an annular inlet and movable swirl blocks are used to impart swirl. The combustor dimensions are described in Figure 14.1, and Figure 14.2 shows a close- up of the burner assuming 2D axisymmetry. The boundary condition profiles, velocity inlet boundary conditions of the gas, and temperature boundary conditions are based on experimental data [1]. Figure 14.1: Problem Description 14-2 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model Do 1.15 Do 1.33 Do 1.66 Do 20o 0.66 Donatural gas swirling combustion air Do = 87 mm 24 holes ∅ 1.8 mm 195 mm Figure 14.2: Close-Up of the Burner Setup and Solution Preparation 1. Download non_premix_combustion.zip from the Fluent Inc. User Services Center or copy it from the FLUENT documentation CD to your working folder (as described in Tutorial 1). 2. Unzip non_premix_combustion.zip. berl.msh and berl.prof can be found in the non premix combustion folder, which will be created after unzipping the file. The mesh file, berl.msh is a quadrilateral mesh describing the system geometry shown in Figures 14.1 and 14.2. 3. Start the 2D (2d) version of FLUENT. c© Fluent Inc. September 21, 2006 14-3 Using the Non-Premixed Combustion Model Step 1: Grid 1. Read the mesh file berl.msh. File −→ Read −→Case... The FLUENT console will report that the mesh contains 9784 quadrilateral cells. A warning will be generated informing you to consider making changes to the zone type, or to change the problem definition to axisymmetric. You will change the problem to axisymmetric swirl in Step 2. 2. Check the grid. Grid −→Check FLUENT will perform various checks on the mesh and will reports the progress in the console. Make sure that the minimum volume reported is a positive number. 3. Scale the grid. Grid −→Scale... (a) Select mm (millimeters) from the Grid Was Created In drop-down list in the Unit Conversion group box. (b) Click Change Length Units. All dimensions will now be shown in millimeters. (c) Click Scale to scale the grid. (d) Close the Scale Grid panel. 14-4 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 4. Display the grid (Figure 14.3). Display −→Grid... (a) Retain the default settings. (b) Click Display and close the Grid Display panel. Grid FLUENT 6.3 (2d, pbns, lam) Figure 14.3: 2D BERL combustor Mesh Display Due to the grid resolution and the size of the domain, you may find it more useful to display just the outline, or to zoom in on various portions of the grid display. c© Fluent Inc. September 21, 2006 14-5 Using the Non-Premixed Combustion Model Extra: You can use the mouse zoom button (middle button, by default) to zoom in to the display and the mouse probe button (right button, by default) to find out the boundary zone labels. The zone labels will be displayed in the console. 5. Mirror the display about the symmetry plane. Display −→Views... (a) Select axis-2 from the Mirror Planes list. (b) Click Apply and close the Views panel. The full geometry will be displayed, as shown in Figure 14.4. 14-6 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model Grid FLUENT 6.3 (2d, pbns, lam) Figure 14.4: 2D BERL Combustor Mesh Display Including the Symmetry Plane c© Fluent Inc. September 21, 2006 14-7 Using the Non-Premixed Combustion Model Step 2: Models 1. Change the spatial definition to axisymmetric swirl. Define −→ Models −→Solver... (a) Retain the default selection of Pressure Based in the Solver list. The non-premixed combustion model is available only with the pressure-based solver. (b) Select Axisymmetric Swirl in the Space list. (c) Click OK to close the Solver panel. 2. Enable the Energy Equation. Define −→ Models −→Energy... Since heat transfer occurs in the system considered here, you will have to solve the energy equation. 14-8 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 3. Select the standard k-epsilon turbulence model. Define −→ Models −→Viscous... (a) Select k-epsilon (2 eqn) from the Model list. For axisymmetric swirling flow, the RNG k-epsilon model can also be used. (b) Retain all other default settings. (c) Click OK to close the Viscous Model panel. c© Fluent Inc. September 21, 2006 14-9 Using the Non-Premixed Combustion Model 4. Select the P1 radiation model. Define −→ Models −→Radiation... (a) Select P1 from the Model list. (b) Click OK to close the Radiation Model panel. The FLUENT console will list the properties that are required for the model you have enabled. An Information dialog box will open, reminding you to confirm the property values. (c) Click OK to close the Information dialog box. The DO radiation model produces a more accurate solution than the P1 radiation model but it can be CPU intensive. The P1 model will produce a quick, acceptable solution for this problem. See Chapter 13 of the User’s Guide for details on the different radiation models available in FLUENT. 14-10 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 5. Select the Non-Premixed Combustion model. Define −→ Models −→ Species −→Transport & Reaction... (a) Select Non-Premixed Combustion from the Model list. The panel will expand to show the related inputs. You will use this panel to create 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 with turbulence. FLUENT interpolates the PDF during the solution of the non-premixed combustion model. c© Fluent Inc. September 21, 2006 14-11 Using the Non-Premixed Combustion Model Step 3: Non Adiabatic PDF Table 1. Enable the Create Table option, in the PDF Options group box of the Species Model panel. This will update the panel to display the inputs for creating the PDF table. The Inlet Diffusion option enables the mixture fraction to diffuse out of the domain through inlets and outlets. 2. Click the Chemistry tab to define chemistry models. (a) Retain the default selection of Equilibrium and Non-Adiabatic. In most non-premixed combustion simulations, the Equilibrium chemistry model is recommended. The Steady Flamelets option can model local chemical non- equilibrium due to turbulent strain. (b) Retain the default value for Operating Pressure. (c) Enter 0.064 for Fuel Stream in the Rich Flammability Limit box. For combustion cases, a value larger than 10% – 50% of the stoichiometric mixture fraction can be used for the rich flammability limit of the fuel stream. In this case, the stoichiometric fraction is 0.058, therefore a value that is 10% greater is 0.064. 14-12 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model The Fuel 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 of full equilibrium. 3. Click the Boundary tab to add and define the boundary species. (a) Add c2h6, c3h8, c4h10, and co2. i. Enter c2h6 in the Boundary Species text-entry field and click Add. ii. Similarly, add c3h8, c4h10, and co2. All four species will appear in the table. (b) Select Mole Fraction from the Species Units list. (c) Retain default values for n2 and o2 under Oxid. The oxidizer (air) consists of 21% O2 and 79% N2 by volume. (d) Specify the fuel composition by entering the following values under Fuel: The fuel composition is entered in mole fractions of the species, c2h6, c3h8, c4h10, and co2. c© Fluent Inc. September 21, 2006 14-13 Using the Non-Premixed Combustion Model Species Mole Fraction ch4 0.965 n2 0.013 c2h6 0.017 c3h8 0.001 c4h10 0.001 co2 0.003 Hint: Scroll down to see all the species. Note: All boundary species with a mass or mole fractions of zero will be ig- nored. (e) Enter 315 for Fuel and Oxid each in the Temperature group box. (f) Click Apply. 4. Click the Control tab and retain default species to be excluded from the equilibrium calculation. 5. Click the Table tab to specify the table parameters and calculate the PDF table. (a) Retain the default values for all the paremeters in the Table Parameters group box. (b) Click Apply. The maximum number of species determines the number of most preponderant species to consider after the equilibrium calculation is performed. 14-14 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model (c) Click Calculate PDF Table to compute the non-adiabatic PDF table. (d) Click the Display PDF Table... button to open the PDF Table panel. i. Retain the default parameters and click Display (Figure 14.5). ii. Close the PDF Table panel. Mean Temperature(K) FLUENT 6.3 (axi, swirl, pbns, pdf20, ske) ZY X Figure 14.5: Non-Adiabatic Temperature Look-Up Table on the Adiabatic Enthalpy Slice The 3D look-up tables are reviewed on a slice-by-slice basis. By default, the slice selected is that corresponding to the adiabatic enthalpy values. You can select other slices of constant enthalpy for display, as well. c© Fluent Inc. September 21, 2006 14-15 Using the Non-Premixed Combustion Model The maximum and minimum values for mean temperature and the corresponding mean mixture fraction will also be reported in the console. The maximum mean temperature is reported as 2246 K at a mean mixture fraction of 0.058. 6. Save the PDF output file (berl.pdf). File −→ Write −→PDF... (a) Enter berl.pdf for 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 a binary (unformatted) file, enable the Write Binary Files option in the Select File dialog box. 7. Click OK to close the Species Model panel. Step 4: Materials 1. Specify the continuous phase (pdf-mixture) material. Define −→Materials... 14-16 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 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 thermal conductivity, need to be defined. (a) Select wsggm-domain-based from the Absorption Coefficient drop-down list. Hint: Scroll down to view the Absorption Coefficient option. This specifies a composition-dependent absorption coefficient, using the weighted- sum-of-gray-gases model. WSGGM-domain-based is a variable coefficient that uses a length scale, based on the geometry of the model. Note that WSGGM- cell-based uses a characteristic cell length and can be more grid dependent. See Section 13.3.8 of the User’s Guide for more details. (b) Click Change/Create and close the Materials panel. You can click the View... button next to Mixture Species to view the species included in the pdf-mixture material. These are the species included during the system chem- istry setup. The Density and Cp laws cannot be altered: these properties are stored in the non-premixed combustion look-up tables. FLUENT uses the gas law to compute the mixture density and a mass-weighted mixing law to compute the mixture cp. When the non-premixed combustion model is used, do not alter the properties of the individual species. This will create an inconsistency with the PDF look-up table. Step 5: Operating Conditions 1. Keep the default operating conditions. Define −→Operating Conditions... The Operating Pressure was already set in the PDF table generation in Step 3. c© Fluent Inc. September 21, 2006 14-17 Using the Non-Premixed Combustion Model Step 6: Boundary Conditions 1. Read the boundary conditions profile file. File −→ Read −→Profile... (a) Select berl.prof from the Select File dialog box. (b) Click OK. The CFD solution for reacting flows can be sensitive to the boundary conditions, in particular the incoming velocity field and the heat transfer through the walls. Here, you will use profiles to specify the velocity at air-inlet-4, and the wall temperature for wall-9. The latter approach of fixing the wall temperature to measurements is common in furnace simulations, to avoid modeling the wall convective and radia- tive heat transfer. The data used for the boundary conditions was obtained from experimental data [1]. 2. Define the boundary conditions for the zones. Define −→Boundary Conditions... 14-18 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 3. Set the boundary conditions for pressure outlet (poutlet-3). (a) Click the Momentum tab. (b) Select Intensity and Hydraulic Diameter from the Specification Method drop- down list in the Turbulence group box. (c) Enter 5% for Backflow Turbulent Intensity. (d) Enter 600 mm for Backflow Hydraulic Diameter. (e) Click the Thermal tab and enter 1300 for the Backflow Total Temperature. (f) Click OK to close the Pressure Outlet panel. The exit gauge pressure of zero defines the system pressure at the exit to be the operating pressure. The backflow conditions for scalars (temperature, mixture frac- tion, turbulence parameters) will be used only if flow is entrained into the domain through 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. c© Fluent Inc. September 21, 2006 14-19 Using the Non-Premixed Combustion Model 4. Set the boundary conditions for the velocity inlet (air-inlet-4). (a) Select Components from the Velocity Specification Method drop-down list. (b) Select vel-prof u from the Axial-Velocity(m/s) drop-down list. (c) Select vel-prof w from the Swirl-Velocity(m/s) drop-down list. (d) Select Intensity and Hydraulic Diameter from the Specification Method drop- down list in the Turbulence group box. (e) Enter 17% for Turbulent Intensity. (f) Enter 29 mm for Hydraulic Diameter. (g) Click the Thermal tab and enter 312 for Temperature. Turbulence parameters are defined based on intensity and length scale. The relatively large turbulence intensity of 17% may be typical for combustion air flows. For the non-premixed combustion calculation, you have to define the inlet Mean Mixture Fraction and Mixture Fraction Variance in the Species tab. In this case, the gas phase air inlet has a zero mixture fraction. Therefore, you can accept the zero default settings. (h) Click OK to close the Velocity Inlet panel. 14-20 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 5. Set the boundary conditions for velocity inlet (fuel-inlet-5). (a) Click the Momentum tab. (b) Select Components from the Velocity Specification Method drop-down list. (c) Enter 157.25 m/s for the Radial-Velocity. (d) Select Intensity and Hydraulic Diameter from the Specification Method drop- down list in the Turbulence group box. (e) Enter 5% for Turbulent Intensity. (f) Enter 1.8 mm for Hydraulic Diameter. The hydraulic diameter has been set to twice the height of the 2D inlet stream. (g) Click the Thermal tab and enter 308 for Temperature. (h) Click the Species tab and enter 1 for Mean Mixture Fraction for the fuel inlet. (i) Click OK to close the Velocity Inlet panel. c© Fluent Inc. September 21, 2006 14-21 Using the Non-Premixed Combustion Model 6. Set the boundary conditions for wall-6. (a) Click the Thermal tab. i. Select Temperature from the Thermal Conditions list. ii. Enter 1370 K for Temperature. iii. Enter 0.5 for Internal Emissivity. (b) Click OK to close the Wall panel. 7. Similarly, set the boundary conditions for wall-7 through wall-13 using the following values: Zone Name Temperature Internal Emissivity wall-7 312 0.6 wall-8 1305 0.5 wall-9 temp-prof t (from the drop-down list) 0.6 wall-10 1100 0.5 wall-11 1273 0.6 wall-12 1173 0.6 wall-13 1173 0.6 14-22 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model 8. Close the Boundary Conditions panel. Step 7: Solution 1. Set the solution control parameters. Solve −→ Controls −→Solution... (a) Set the following parameters in the Under-Relaxation Factors group box: Under-Relaxation Factor Value Pressure 0.5 Density 0.8 Momentum 0.3 Turbulent Kinetic Energy 0.7 Turbulent Dissipation Rate 0.7 P1 1 The default under-relaxation factors are considered to be too aggressive for reacting flow cases with high swirl velocity. (b) Select PRESTO! from the Pressure drop-down list in the Discretization group box. (c) Retain the default selection of First Order Upwind for other parameters. (d) Click OK to close the Solution Controls panel. c© Fluent Inc. September 21, 2006 14-23 Using the Non-Premixed Combustion Model 2. Initialize the flow field using the conditions at air-inlet-4. Solve −→ Initialize −→Initialize... (a) Select air-inlet-4 from the Compute From drop-down list. (b) Enter 0 for the Axial Velocity and Swirl Velocity each. (c) Enter 1300 for the Temperature. (d) Click Init and close the Solution Initialization panel. 3. Enable the display of residuals during the solution process. Solve −→ Monitors −→Residual... (a) Enable Plot in the Options group box. (b) Click OK to close the Residual Monitors panel. 4. Save the case file (berl-1.cas). File −→ Write −→Case... 5. Start the calculation by requesting 1500 iterations. Solve −→Iterate... 14-24 c© Fluent Inc. September 21, 2006 Using the Non-Premixed Combustion Model The solution will converge in approximately 1100 iterations. 6. Save the first-order converged solution (berl-1.dat). File −→ Write −→Data... 7. Switch to second-order upwind for improved accuracy. Solve −→ Controls −→Solution