Week 4.2: Project - Transient simulation of flow over a throttle body

                                                                               Transient  State Simulation of Flow Over a Throttle body - Using Converge.

Aim: To setup and simulate the  Transient simulation of flow over throttle body.

Objective: 

• Setting up Transient state simulation for flow over a throttle body.
• To generate the mesh that gives more accurate results.
• Post processing the obtain results using paraview and show pressure and velocity contour. 

Description:

A throttle is the mechanism by which fluid flow is managed by constriction or obstruction.The throttle is a means of controlling an engines power by regulating the amount of fuel or the air entering the engine. The throttle lever acts as  a direct mechanical linkage. The butterfly valve of the throttle is operated by means of an arm piece, loaded by spring. This arm is usuall directly linked to the accelerator cable and operates in accordance with the driver, who hits it. Further the pedal is pushed , the wider the throttle valve opens.

Theory:

In this Transient Simulation, the throttle is rotating and we are interested to simulate the flow at a different opening angle of the 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 Transient solver.
• Simulation Mode: No Hydrodynamic
• Gas flow Solver: Compressible (as air is working medium)

Simulation Time Parameter:

• End time: 0.01 second
• Start time: 0sec
• Initial time step: 1e-06 s
• Minimum time step: 1e-06 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
• Wall motion type: Rotating
• 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 the 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.

Velocity streamlines:

The velocity is maximum at the side of the throttle due to creation of venturi(low area region) and there is negative pressure behind the throttle due to sudden breaking of flow.

Behind the throttle, the velocity is minimum because there is a flow separation, leaving vacuum or low-pressure space in which air is not able to fill properly. This leads to the formation of vortices due to changing of velocity gradient from positive into a negative value at this point. Hence this point is called a Point of separation. In this point of Separation, the velocity profile du/dy is 0 at the wall. If the adverse pressure gradient acts over a sufficiently extended distance, the deceleration in the flow will be sufficient to change the direction of the flow in the Boundary layer. Hence the Boundary layer develops the Point of Inflection, known as the point of Boundary layer Separation beyond which a circular flow pattern is established.

Mass flow rate:

Velocity plot:

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

Pressure Plot:

From the above Pressure, velocity and mass flow rate plots, we can see that the Mass flow rate and velocity increase rapidly for the time of 0-2 millisecond for the Valve angle of 25 degrees whereas the pressure reduces rapidly. This is due to the sudden decrease in the area which acts as a venturi or as a nozzle. Further increases in time from 2-4 milliseconds, the valve is idle. And for 4-8 millisecond, the valve regains its original position which creates increases in the area hence pressure reduces gradually, and velocity and mass flow rate increase gradually. After 8milliseconds, the mass flow rate, pressure, and velocity is constant. 

 Total cell counts:

The cell counts fluctuate due to the movement of the throttle. It is also called as the Dynamic meshing. For 0-2 ms, the area is reducing, therefore cell counts have to increases to capture the physics accurately at the minimum region. For 2-4 ms the valve is idle hence the cell counts are not changing Further reducing the area from 4-8 ms, the cell counts have to decrease. After 8ms the throttle position is constant hence no changes in cell counts.

Mesh:

For throttle position of 0 degrees.                                            For throttle position of 25 degrees.                         

Animation Video :-

Results:

• From the above contours it is clear that, on giving the fine mesh to the valve the contours appear more details and the results are highly accurate. 
• The flow through the throttle position is more accurate as the mesh generated along the throttle position is refined (having more cells surroding the valve) i.e. by providing 2embeded layers and a scaling of 3 to the mesh.
• The contours show that, there is a restriction to the flowof intake air that is caused by throttle plate.The inflow mass flow rate is balanced by the outflow rate.
• If there is less restriction in the flow , then there is a lower loss in energy. Hence because of this the velocity at outlet increases.
• As we have a moving mesh, number of cells generated are more and hence simulation time is high.