Tutorial: Simulate Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model Introduction This tutorial describes the simulation process of non-premixed combustion in a direct injection natural gas engine. Direct injection natural gas engines are used in many heavy duty vehicles. Similar to diesel engines, high thermal efficiency and power density is main- tained in such direct injection natural gas engines. In such engines, natural gas is injected directly into the combustion chamber. Then the gas mixes with the high pressure air in the combustion chamber and combustion occurs. Due to the non-premixed nature of the combustion occurring in such engines, non-premixed combustion model of ANSYS FLUENT can be used to simulate the combustion process. The tutorial demonstrates how to do the following: • Set up an in-cylinder (IC) case involving only a part of compression and power stroke with only a sector of mesh. • Set up IC non-premixed combustion in ANSYS FLUENT. • Use user-defined functions (UDF) to specify initial swirl and injection flow rate. Prerequisites This tutorial is written with the assumption that you have completed 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. In this tutorial, you will use the non-premixed combustion model and UDFs. For more information about this model, see Chapter 16, Modeling Non-Premixed Combustion in the ANSYS FLUENT 13.0 User’s Guide. If you have not used UDFs before, see ANSYS FLUENT 13.0 UDF Manual. Problem Description The problem to be solved in this tutorial is shown in Figure 1. A 30 degree sector of a 4-stroke engine which corresponds to one fuel injector hole is modeled. Since combustion simulation is to be studied, the simulation starts at 20 degree crank angle (CA) before the c ANSYS, Inc. October 11, 2010 1
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Tutorial: Simulate Combustion in Direct Injection Natural
Gas Engine Using Non-Premixed Combustion Model
Introduction
This tutorial describes the simulation process of non-premixed combustion in a directinjection natural gas engine. Direct injection natural gas engines are used in many heavyduty vehicles. Similar to diesel engines, high thermal efficiency and power density is main-tained in such direct injection natural gas engines. In such engines, natural gas is injecteddirectly into the combustion chamber. Then the gas mixes with the high pressure air inthe combustion chamber and combustion occurs. Due to the non-premixed nature of thecombustion occurring in such engines, non-premixed combustion model of ANSYS FLUENTcan be used to simulate the combustion process.
The tutorial demonstrates how to do the following:
• Set up an in-cylinder (IC) case involving only a part of compression and power strokewith only a sector of mesh.
• Set up IC non-premixed combustion in ANSYS FLUENT.
• Use user-defined functions (UDF) to specify initial swirl and injection flow rate.
Prerequisites
This tutorial is written with the assumption that you have completed
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.
In this tutorial, you will use the non-premixed combustion model and UDFs. For moreinformation about this model, see Chapter 16, Modeling Non-Premixed Combustion in theANSYS FLUENT 13.0 User’s Guide. If you have not used UDFs before, see ANSYS FLUENT13.0 UDF Manual.
Problem Description
The problem to be solved in this tutorial is shown in Figure 1. A 30 degree sector of a4-stroke engine which corresponds to one fuel injector hole is modeled. Since combustionsimulation is to be studied, the simulation starts at 20 degree crank angle (CA) before the
Simulate Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model
start of injection (SOI) and ends at 50 degree CA after top dead center (TDC). A simplifiedmodel of the engine with no valves is modeled since during the entire combustion process,both the valves remain closed.
Figure 1: Sector Geometry of a Direct Injection Natural Gas Engine
Setup and Solution
Preparation
1. Copy the mesh file (natural gas-comb-CA0360.msh.gz) and the UDF source files(initialize.c, injection ch4.c) to your working folder.
2. Use FLUENT Launcher to start the 3D 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 UDF Compiler tab and ensure that the Setup Compilation Environment forUDF is enabled.
The path to the .bat file which is required to compile the UDF will be displayed as soonas you enable Setup Compilation Environment for UDF.
If the UDF Compiler tab does not appear in the FLUENT Launcher dialog box by default,click the Show Additional 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 Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model
Step 1: Mesh
1. Read the mesh file (natural gas-comb-CA0360.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 −→ Transient
2. Check the mesh (see Figure 2).
General −→ Check
Warnings will be displayed regarding unassigned interface zones, resulting in the failureof the mesh check. You do not need to take any action at this point, as this issue willbe rectified when you define the mesh interfaces in a later step.
Simulate Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model
(b) Create periodic boundary out of face zone period outer1 and period outer2 usingthe text command /grid/mz/make-periodic.
Periodic zone [()] period outer1
Shadow zone [()] period outer2
Rotational periodic? (if no, translational) [yes]
Create periodic zones? [yes]
all 779 faces matched for zones 14 and 13.
zone 13 deleted
created periodic zones.
5. Check the mesh.
General −→ Check
This ensures that the periodic boundary is correctly created. ANSYS FLUENT showsthe progress in the console (as shown below). Ensure that the stored rotation angle isequal to the average rotation angle.
Checking periodic boundaries.
Periodic Zone 12: average rotation angle (deg) = -30.000 (-30.000 to -30.000)
Simulate Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model
i. Click Display and close the Mesh Display dialog box.
(b) Preview the mesh motion.
Dynamic Mesh −→ Preview Mesh Motion...
i. Enter 1360 for Number of Time Steps .
ii. Click Apply and then click Preview.
(c) Save the case file (natural gas-comb-CA0700.cas.gz).
File −→ Write −→Case...
(d) Examine the UDF inputs as shown in Appendix A.
(e) Examine the UDF inputs for injection ch4.c as shown in Appendix B.
Open initialize.c, injection ch4.c in text editor. For this tutorial, the swirlratio is 2, swirl axis is z coordinate, and the swirl origin is (0, 0, 0).
Step 4: Models: Combustion Model Setup
1. Set up the combustion model.
(a) Read the case file (natural gas-comb-CA0700.cas.gz).
Simulate Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model
(d) Select Time Step from the Every drop-down list.
(e) Select Mass-Average from the Report Type drop-down list.
(f) Select Pressure... and Static Pressure from the Field Variable drop-down list.
(g) Select fluid-inj, fluid-outer, and fluid-inner from the Cell Zones list.
(h) Click OK to close the Volume Monitor dialog box.
Solution Initialization
Initialize the variables carefully because the simulation will start at CA = 700 degrees,which is almost the end of the compression stroke. Assuming adiabatic compressionand atmospheric conditions (i.e. atmospheric pressure = 1 and temperature = 300k) at the start of compression stroke (i.e. at BDC or 360 CA), calculate the pressureand the temperature at 700 CA. For this engine, the Gause Pressure and Temperatureat 700 CA can be assumed as 1898675 pascal and 690 K respectively.
Simulate Combustion in Direct Injection Natural Gas Engine Using Non-Premixed Combustion Model
Summary
In this tutorial, you used ANSYS FLUENT to model combustion process in a direct injectionnatural gas engine. The tutorial explained entire process from setting up the dynamic meshmodel for an IC engine to setting up the non-premixed model for combustion in the engine.It also demonstrated the process of creating and using PDF tables.