Tutorial: Simulate In-Cylinder Premixed Combustion Using ECFM Model Introduction For spark-ignited (SI) engines and some natural gas combustion engines, fuel and air are mixed before entering into the combustion chamber. At the spark event, the mixture can be assumed to be homogeneous and the combustion process to be premixed. This tutorial demonstrates how to do the following: • Set up in-cylinder (IC) premixed combustion. • Set up the spark model. • Use the user-defined functions (UDFs) to modify laminar flame speed. • Use UDF for some IC postprocessing. Prerequisites Tutorial 1 from ANSYS FLUENT 13.0 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. It is also assumed that you are familiar with ANSYS FLUENT moving deforming mesh (MDM) layering approach. For more information see Section 11.6, Using Dynamic Meshes in ANSYS FLUENT 13.0 User’s Guide. Problem Description This tutorial considers a 2D axi-symmetric geometry of the IC engine cylinder configura- tion. Simulation starts at IVC and ends at EVO, hence there are no valves involved. The schematic is as shown in Figure 1. The initial configuration of the system has piston at TDC, hence first mesh motion is performed to bring the piston to IVC position. c Fluent Inc. February 28, 2011 1
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Tutorial: Simulate In-Cylinder Premixed Combustion Using
ECFM Model
Introduction
For spark-ignited (SI) engines and some natural gas combustion engines, fuel and air aremixed before entering into the combustion chamber. At the spark event, the mixture canbe assumed to be homogeneous and the combustion process to be premixed.
This tutorial demonstrates how to do the following:
• Set up in-cylinder (IC) premixed combustion.
• Set up the spark model.
• Use the user-defined functions (UDFs) to modify laminar flame speed.
• Use UDF for some IC postprocessing.
Prerequisites
Tutorial 1 from ANSYS FLUENT 13.0 Tutorial Guide, and that you are familiar with theANSYS FLUENT navigation pane and menu structure. Some steps in the setup and solutionprocedure will not be shown explicitly.
It is also assumed that you are familiar with ANSYS FLUENT moving deforming mesh(MDM) layering approach. For more information see Section 11.6, Using Dynamic Meshesin ANSYS FLUENT 13.0 User’s Guide.
Problem Description
This tutorial considers a 2D axi-symmetric geometry of the IC engine cylinder configura-tion. Simulation starts at IVC and ends at EVO, hence there are no valves involved. Theschematic is as shown in Figure 1.
The initial configuration of the system has piston at TDC, hence first mesh motion isperformed to bring the piston to IVC position.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
Figure 1: Schematic
Setup and Solution
Preparation
1. Copy the mesh file (IC premixed com.msh.gz) and the UDF source files (initialize.c,laminar flame speed.c, and work.c) 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 the ANSYS FLUENT 13.0 User’s Guide.
3. Enable Double-Precision in the Options list.
4. Click the Environment tab and ensure that the Setup Compilation Environment for UDFis enabled.
The path to the .bat file which is required to compile the UDF will be displayed assoon as you enable Setup Compilation Environment for UDF.
If the Environment tab does not appear in the FLUENT Launcher dialog box by default,click the Show More Options button to view the additional settings.
The Display Options are enabled by default. Therefore, after you read in the mesh, itwill be displayed in the embedded graphics window.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
Step 1: Mesh
1. Read the mesh file (IC premixed com.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
(a) Select Transient in the Time list.
(b) Select Axisymmetric Swirl in the 2D Space list.
2. Check the mesh (see Figure 2).
General −→ Check
ANSYS FLUENT will perform various checks on the mesh and will report the progressin the console. Ensure that the minimum volume reported is a positive number.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
(a) Enter 840 for Number of Time Steps.
(b) Ensure Display Mesh is enabled in the Options group box.
(c) Click Preview.
(d) Close the Mesh Motion dialog box after the preview is done.
At Crank Angle Step Size of 0.25 degree, a mesh motion of 840 steps will bring themesh to CA=570, the starting point of our combustion simulation.
4. Save the case file (IC premixed com CA570.cas.gz).
File −→ Write −→Case...
5. Examine the UDF inputs for initialize.c as shown in Appendix A.
6. Examine the UDF inputs for work.c as shown in Appendix B.
7. Examine the UDF inputs for laminar flame speed.c as shown in Appendix C.
Open the files, initialize.c, work.c, and laminar flame speed.c using a text ed-itor. For this tutorial, the swirl ratio is 3, swirl axis is x coordinate, and the swirlorigin is (0, 0). For this tutorial, there is no need to modify the UDF input.
Step 4: Models
1. Set up the combustion model.
(a) Compile and load the UDF library.
Define −→ User-Defined −→ Functions −→Compiled...
(b) Click Add... and select the files, initialize.c, laminar flame speed.c, andwork.c.
(c) Click OK to close the Select File dialog box.
(d) Click Build to build the library.
Note: Make sure that the UDF source files are in same directory that containthe case and data files.
(e) Click Load to close the Compiled UDFs dialog box.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
(a) Select Premixed Combustion in the Model group box.
(b) Select Non-Adiabatic in the Premixed Combustion Model Options group box.
(c) Select Extended Coherent Flame Model from the Premixed Model group box.
(d) Select meneveau from the ITNFS Treatment drop-down list.
(e) Click OK.
The information dialog box will appear informing that available material proper-ties or methods have changed, asking you to confirm the property values. ClickOK to close the Information dialog box.
6. Set up the spark ignition model.
Models −→ Spark Ignition −→ Edit...
(a) Set Number of Sparks to 1.
(b) Click Define... to open the Set Spark Ignition dialog box.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
(a) Select laminar flame speed from the User-Defined Functions dialog box.
This will hook Metghalchi and Keck model to calculate laminar flame speed. InANSYS FLUENT this model is now available internally. To use it, you have toselect metghalchi-keck-law from the drop-down list and then methane-air from thecombustion mixture dialog box.
(b) Click OK to close the User-Defined Functions dialog box..
4. Enter 5e+07 (j/kg) for Heat of Combustion.
5. Enter 0.0363 for Unburnt Fuel Mass Fraction.
6. Click Change/Create and close the Create/Edit Materials dialog box.
Step 6: Boundary Conditions
Boundary Conditions −→ axis
1. Select axis from the Type drop-down list.
The question dialog box will appear to confirm the change of type of axis from wall toaxis. Click Yes to open the Axis dialog box.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
Step 10: Postprocessing
At the end of the simulation, you have TIFF files for the contours of progress variable atdifferent crank angles. Figures 4–5 show some of the TIFF files.
Simulate In-Cylinder Premixed Combustion Using ECFM Model
Further Improvement
UDF will automatically create the file work.txt. This file has combustion chamber pressureand burnt fuel mass fraction as a function of Crank Angle (CA). These can be used togenerate plots as shown (see Figures 6, and 7,
Figure 6: Pressure as a Function of CA
Figure 7: Burnt Fuel Mass Fraction as a Function of CA