Steady state simulation of flow over a throttle body

Steady-state simulation of flow over a throttle body

Aim: Set up and run the steady-state simulation for flow over the throttle body.

Objectives: Post process the results and show pressure and velocity contours.

Show the mesh (i.e surface with edges)

Show the plots for pressure, velocity, mass flow rate, and total cell count.

Create an animation.

Theory:

In this steady-state Simulation, the throttle is fixed and we are interested to simulate the flow around the fixed throttle.

The main function of a throttle body assembly is to control the airflow into the engine based on vehicle demand. The throttle body is mounted between the air cleaner and the intake manifold. Following the angle of the throttle valve (butterfly valve) changes, it restricts the amount of airflow into the engine cylinder.

It has a venturi to reduce the pressure of the air flowing through it. The intake flow is throttled by reducing the flow area. This is done by providing a circular shaft known as throttle shaft and is mounted with a butterfly valve at the downstream of the venture. The main challenge is the change in throttle position during transient operation of the engine, which introduces additional problems as the butterfly position is frequently changed as per driver’s demand. The airflow can be considered as unaffected by the fuel flow. However, the reverse is not true and fuel flow strongly depends upon the airflow. The schematic of a throttle body assembly describing the airflow path is shown in Fig 1. Filtered air enters into the throttle body and moves downstream. Butterfly valve (or throttle valve), restricts the amount of airflow into the engine based on the accelerator position. There exists a bypass passage (Fig. 2), which is used for adjusting the airflow at idling conditions.

The angle of the throttle opening position when the air flows through the throttle valve plays an important role for reducing the exhaust gas, since the throttle body can control the flow of air into the engine cylinder and the fuel Equivalent ratio), control of air and fuel fully mixed HC, CO exhaust reduction, reduce noise and vibration.

Governing equations

The following governing equations are solved. Unsteady three-dimensional continuity equation in Conservation is given as:

where drho/dt is the rate of change of density with respect to time.

div(rho*u)= net mass rate out of the element across its boundaries.

Momentum Equation in Non-conservation form is given as:

where S_M(x,y,z) is the source momentum per unit volume per unit time in x, y, z directions respectively.

Boundary conditions

In this study, three types of boundaries are involved, viz., inlet, outlet and wall. Inlet pressure boundary conditions are used to define the fluid pressure at the flow inlet. Pressure inlet boundary conditions are used when the inlet pressure is known, but the flow rate or velocity is not known. Outlet pressure boundary conditions require the specification of static pressure at the outlet boundary.

Simulation:

The following Simulation tools will be incorporated:

1. Converge studio:- To pre-process the data that needs to simulate the case. It plays an important role in generating important 3d input files only using GUI. It does not run simulation directly, however it generates inputs files which use to run the simulation.

2. Cygwin:- Cygwin is a collection of GNU and open-source tools that provide functionality similar to Linux distribution on windows.
3. Converge msmpi 3.0:- To run the simulation using these input files in Cygwin in series or in parallel.
4. Post convert:- To convert all the output files which are processed by converge msmpi into the files which will be readable for post-processing in Paraview.
5. Paraview:- To post-processing to obtain the contour, plots, etc. It is an open-source, multi-platform data analysis and visualization software.

Geometry Setup:

• Import the “stl” file.
• Create an automatic Boundary fence. Boundary->Find/clean->Find.
• Delete the outer thickness of the Elbow by “by Angle” option.
• Patch the Open edges by Patch tool. Select Patch(under Repair)-> “By open edges”-> select inlet edges -> enter. Same for outlet.
• Boundary Flagging

Case Setup/Simulation Setup:

MATERIAL:

Gas Simulation:

• As only air is our working medium hence uncheck Reaction Mechanism.
• Predefined mixture: air

Global Transport Properties

• Turbulent Prandtl number: 0.9
• Turbulent Schmidt number: 0.78

SIMULATION PARAMETER:

Run Parameter:

• Solver: Pressure-based Steady-state
• Simulation Mode: Full Hydrodynamic
• Gas flow Solver: Compressible (as air is working medium)

Simulation Time Parameter:

• End time: 15000 cyc
• Start time: 0cyc
• Initial time step: 1e-09 s
• Minimum time step: 1e-09 s
• Maximum time step: 1 s

Solver Type:

• Pressure based

BOUNDARY:

• Naming: Select faces and name as inlet, outlet, Elbow wall, Throttle.
• Boundary condition:

Inlet:

• Boundary Type: Inflow
• Total Pressure: 1.5e5 Pa
• Specified Boundary condition: Enable Total Pressure
• Species Boundary condition: air

Outlet:

• Boundary type: OUTFLOW
• Static Pressure: 1e5 Pa
• Convert law of wall into no-slip for top-bottom-wall under velocity boundary condition.

Elbow wall:

• Boundary type: Wall

Throttle:

• Boundary type: wall
• Velocity BT: Law of wall (as the flow is Turbulence)
• Temperature BT: Law of wall.

REGION & INITIALIZATION:

• Click on Region and Initialization
• Add species: +air
• O2=0.77 & N2=0.3

Base grid:

• dx=0.002m; dy=0.002m; dz=0.002m

Post-variable selection:

Output files:

The time interval for writing 3d output data files=40cyc

The time interval for writing restarting output=40cyc

Export input files and Running the Simulation:

Now we have successfully set up the Case and need to export the input files to run the Simulation.

Run the Simulation in Cygwin using a parallel processor.

Post Converting:

Now we need to convert all the output files which are processed by converge msmpi into the files which will be readable for post-processing in Paraview

Paraview:

We need to Post-process the output files to obtain the contour.

Results:

Contour:

Pressure Contour:

Inlet Total pressure is 1e5 Pa and outlet static pressure is 1e Pa.

Pressure distribution around the Throttle:

From above contour, we can see the high pressure and low-pressure region around the throttle. The upstream region area of the throttle where the flow is impinging has higher pressure and on the back portion of the throttle, the pressure is minimum due to flow separation and vacuum creation.

Velocity Contour:

From the above velocity contour, we can see that, as the flow progresses through the elbow, the velocity increases.

Mass flow rate:

From the above plot, we can see that the Mass flow rate of 0.065 kg/s converges at around 6000 cycles.

Velocity plot:

From the above plot, we can see that the velocity of 200m/s converges at around 8000 cycles.

Pressure Plot:

Inlet Total pressure is 1e5 Pa and outlet static pressure is 1e Pa.

Total cell counts:

The total number of cells is constant 36,000.

Mesh:

Mesh size is 2mm.

Animation:

Conclusion:

• As the Throttle valve is fixed, hence the mass flow rate around the throttle valve will be constant.
• As the flow progresses through the Elbow, the velocity increases and the pressure decreases.
• The cell counts and the mesh size is constant