STEADY STATE SIMULATION OF AIR FLOWING OVER A THROTTLE BODY IN AN ELBOW PIPE USING CONVERGE CFD
Aim: To simulate steady state flow of air over a throttle body
1. Setup and run the simulation
2. Plot the Velocity and Pressure contours
3. Study the results and analyse the flow happening over and around the throttle body
Introduction and Theory:
I. Introduction:
In this Project, A Steady state simulation is done on a Flow through an Elbow joint consisting of a throttle valve. The steady state case is solved with the Throttle valve fully open until convergence is reached. The physics of the Flowing Fluid is captured using various contours and plots.
II. Introduction to Throttle valve :
Butterfly Valve
In a traditional spark ignition gasoline engine, the throttle body is the part of the air intake system that controls the amount of air that flows into an engine's combustion chamber. It consists of a housing unit that contains a throttle plate (butterfly valve) that rotates on a shaft.
When the accelerator (gas pedal) is pushed down, the throttle plate opens and allows air into the engine. When the gas pedal is released, the butterfly closes and effectively chokes off (throttles) air flow into the combustion chamber. This process effectively controls the speed of the engine and ultimately, the speed of the vehicle.
Throttle
A throttle is a mechanism through which fluid flow is controlled by obstructing the flow. In an IC engine, the throttle controls the engine power in response to driver accelerator pedal input by regulating the amount of air entering the engine cylinder. It is a butterfly valve to control the air flow.
While designing a fuel management system for an SI engine, the amount of air flowing in the cylinders is very important. Performance and emission reduction can be improved by designing and testing a fuel management system in a computer environment and it can also save valuable setup time in an engine test cell. One important part of the modeling is the throttle which regulates the air. In this project, steady state air flow over a throttle body is simulated in Converge CFD.
Case Setup:
1. First thing to be done is geometry cleanup. The flow is to be simulated inside the elbow so the thickness of the elbow must be converted to the flow region.
For that to be done, delete the outer face and the two thickness faces of the elbow. Use the "By Angle" option shown below to select the faces and then press D to delete the faces.
To use the HotKeys, the Key icon must be enabled.
2. Create patches for inlet and outlet faces.
In the geometry editing toolbox, go to Repair > Patch > Free Edge Loop > By Open Edge > Select the edges and press apply.
The patches should be formed on both the ends of the elbow as shown below.
The Geometry gas to be scaled as the imported STL file is in meters so the scaling has to be done otherwise the cell count will be greatly increased.
3. Create and assign boundary flags
Go to Boundary > Flag > Plus icon and create 4 boundary flags and assign the flags to their respective geometry.
4. Run a diagnosis check to ensure that there are no errors. Enable "Check that all the normals are pointing to the interior"
5. Case Setup:
5.1 Application Type
Select Time Based
5.2 Materials
For predefined mixtures select Air and keep Gas Simulation enabled.
5.3 Gas Simulation
Keep the default values and Press OK
5.4 Global Transport Properties
Keep all the values default and press OK
5.5 Run Parameters
Select the solver as Steady State Solver
under Misc. tab disable use shared memory and press OK.
5.6 Simulation Time Parameters
Select start time as 0 cycles, end time as 15000 cycles, Initial and Minimum time step as 1e-9 seconds and Maximum time-step as 1 second. Keep all the other values default and Press OK.
5.7 Solver Parameters
Change the Navier-stokes solver type as Pressure-based, keep all other settings as default and press OK.
5.8 Regions and Initializations
Select Add to create a new region. Select Air for Species initial conditions and keep the other values default and Press OK.
5.9 Turbulence Modeling
Keep all the settings default and press OK.
5.10 Grid Control
Enable Fixed Embedding.
5.11 Base Grid
Keep the mesh size as 2mm.
5.12 Fixed Embedding
Select Add, set Entity type as BOUNDARY, set Boundary ID as Throttle, scale as 3 and Embed Layers as 2.
5.13 Boundary
For Elbow Wall, select the boundary type as WALL, keep Law of wall for both Velocity and Temperature boundary condition and keep the other settings default as shown below.
Keep the Region Name as Region 0 for all the boundaries.
For Inlet boundary select the Boundary Type as INFLOW, specify the Pressure boundary condition as specified value of 1.5e5, keep the temperature boundary condition as default, and for species boundary condition select air
For Outlet set the Pressure Boundary condition as Specified Value of 1e5 and for species backflow select Air, keep the other settings default.
For Throttle boundary select the boundary type as WALL, and keep the velocity and temperature boundary condition as law of wall and keep all the other settings default.
5.14 Post Variable Selection
Keep all the settings default and press OK.
5.15 Output Files
Change the time interval for writing 3D output data files and interval for writing restarting output to 100 and keep all the other values default and press OK.
6. Export all the input files
Export all the input files into a separate folder
Top Panel Files > Export > Export Input Files
Set the export directory and press OK.
7. Run the simulation
Copy the mpiexec.exe file to the directory in which the input file was exported in the previous step.
Open Cygwin and navigate to that directory and enter the below command to start the simulation.
mpiexec.exe -n 3 "C:Program FilesConvergent_ScienceCONVERGE3.0.12binconverge-intelmpi.exe" restricted </dev/null> logfile.txt &
8. Post Convert the Output Files to Paraview VTK inline binary format
After the simulation run is complete navigate to the output folder and enter the below command in cygwin.
mpiexec.exe -n 4 post_convert_30_msmpi_64.exe
After that enter the Case Name press Enter
Then type 8 and press enter to select the paraview format.
For boundary surface output type 1 and press enter
Type all and press enter to post convert all the files. Repeat the same for the Cell variable selection menu. All the files will be converted to paraview format.
9. Launch Paraview and open the .vtm file.
Results of Simulation:
By reducing the opacity the throttle can be seen clearly in paraview
Mesh
A closer look at the mesh near the throttle region.
Observe how the mesh is refined near the throttle boundary and how the mesh size gradually increases as we move away from the throttle. This is the benefit of using Fixed Embedding in Converge.
Pressure and Velocity Contours
Pressure Contour
A closer look at the pressure contour plot near the throttle region
Observation from the Plot:
There are basically two regions which we can see
1. Red Region: This region is the region of high pressure (between 1.4 to 1.5 bar). See how the flowing fluid imposes high pressure on the throttle.
2. Blue region: In this region the pressure is low. The pressure drops because the flow is constricted.
Pressure Contour Plot on the Throttle
Observation from the Plot
On the throttle also there are two regions. High pressure and low pressure region. In the red, high pressure region the fluid imposes pressure on the throttle as it flows over the throttle body.
And the low pressure (blue region) is due to the flow separation happening near the axis of rotation (shaft) of the throttle.
Velocity Contour Plot
A closer look at the velocity contour plot near the throttle region.
Observation from the above plot:
We can see three regions in the plot.
1. Top left blue region: This is the region before the fluid approaches the throttle. The velocity decreases as the fluid approaches the throttle.
2. Top right red region: In this region the flow area is very small that is why the fluid velocity increases to maintain the mass flow rate. This is the region where the fluid has the maximum velocity. The max velocity goes as high as 310 m/s.
3. Bottom right blue region: This is the recirculation region. Eddies form here and velocity drops to as low as 20 m/s.
Velocity Contour Plot on the Throttle
Velocity Contour on the throttle is similar to the velocity contour plot in the elbow slice and similar observations are made.
Velocity:
The velocity vector and the plane of throttle exposes the maximum velocity of 3.2e2m/s at the inlet section. While at the outlet section the velocity drops to 100m/s. The static pressure increases and the flow becomes seperated. the outlet velocity is low which supports low mass flow rate at the outlet.
Pressure:
The inlet pressure is high and encountered at the throttlle front region and the magnitude is about 1.5e5Pa. The air while passing through the corners reduces the total pressure and flow seperation takes place. The pressure reaches around 80000Pa across the throttle corner sectiona and back portion.
Pressure Convergence Plot
The above plot shows how the pressure converges after around 8000 cycles in the steady state simulation.
Velocity Convergence Plot
Inlet average velocity = 192.8 m/s
Outlet average velocity = 224.06 m/s
Mass Flow Rate Plot
Inlet
Inlet Mass Flow rate = 0.066 kg/s
Outlet
Outlet Mass Flow Rate = 0.066 kg/s
Inlet and Outlet mass flow rates are equal, hence mass conservation is achieved.
Total Cell Count Plot
Total number of cells = 35518.
velocity animation :
Pressure animation :
Discussion:
From the steady state simulation of throttle valve, it is seen that pressure at the inlet is higher than the outlet. The mass flow variation is encountered at the throttle valve front position. The velocity at the inlet position is higher, but some region of high velocity is seen to leak from the corner of the throttle section. At the outlet the flow seeration takes place from the throttle surface and the outlet velocity reeduces down to attain a steady state value . The density variation also a function of mass flow variation and pressure difference. The flow seperation can be seen to take place when pressure tends to drop down.
Conclusion
1. Steady state simulation of flow over a throttle body was setup and conducted in Converge CFD.
2. Various contour plots of velocity and pressure over the throttle body and a slice of the elbow were analysed and studied.