Steady state simulation of flow over a throttle body

Aim: To perform a steady state simulation of flow over a Throttle body using Converge.

Theory: A throrttle valve is used to obstruct the fluid flow in IC engine in order to increase or decrease the power of the engine by restricting air-fuel mixture flow.A throttle valve represents a disc like device positioned inside a pipe and has a small shaft /rod through it which is connected to an actuator controlling the position of the disc as per the required flow. A throttle valve is usually an integral component of an intake manifold. Some vehicles use a single larger throttle body while some use dedicated throttle valves for each cylinder.

Methodology:

1) Pre-Processing:

We will be importing the elbow geometry in converge studio whose dimension will be scaled into meters and the dimensions are as follows:

• Length along x-direction (l_x) = 0.106 m.
• Length along y-direction (l_y) = 0.119 m.
• Length along z-direction (l_z) = 0.017 m.

We will be flagging the domain into 4 different boundaries namely Inlet, Outlet, Walls and Throttle Valve.

Next step is we will be moving on to the case setup where we will be setting up all the parameters.

• In this project we are interested in flow over the throttle body. Hence we perform a steady state analysis. Steady-state simulation computes the fully developed solution that does not change in time.
• This is a general flow problem and hence we will be selecting Time-based application type.
• Moving on to the Materials we will be using predefined air whose composition consists of nitrogen and oxygen. The transport properties are set to the default values.The species composition consist of N2 and O2.
• Simulation parameters are set in which the required steady state solver and steady state monitor is setup for the avg.velocity, Total Pressure and mass flowrate at the outlet boundary. Also simulation time parameters are set accordingly.

- Start time : 0 cycles.

- End time : 15000 cycles.

- Initial time step : 1e-9 s.

- Minimum time step : 1e-9 s.

- Maximum time step : 1 s.

• Now we will be creating a region in order to group various boundaries to define a volumetric region under initial conditions and initialization tab.
• Moving on, we flag different boundaries and assign respective boundary condition to create a meaningful Volumetric region.

- Inlet Boundary :

1) Boundary Type: Inflow.

2) Pressure (Dirichlet condition): 1.5e5 Pa.

3) Temperature (Dirichlet condition): 300 K.

4) Species BC: air.

- Outlet Boundary :

1) Boundary Type: Outflow.

2) Pressure(Dirichlet condition): 1e5 Pa.

3) Temperature (Dirichlet condition): 300 K.

4) Species BC: air.

- Elbow:

1) Boundary Type: Wall.

2) Wall motion type: Stationary.

3) Surface Movement: Fixed.

4) Law of Wall.

- Throttle valve:

1) Boundary Type: Wall.

2) Wall motion type: Stationary.

3) Surface Movement: Fixed.

4) Law of Wall.

• Turbulence Model : RNG k-epsilon.
• A grid size of 0.001 m was used along with fixed embedding of scale 3 and Embed layers of 2 around the throttle valve. Fixed embedding is applied for the throttle valve in order to refine the mesh and to capture the physics occuring around the valve.
• Finally we will be setting up the parameters for printing the 3D and other output files in the post-processing tab. - Time interval for writing 3D output data files :100 cycle. - Time interval for writing text ouput :1 cycle. - Time interval for writing restart output :1000 cycle.

2) Running the solver:

• Once the case is setup we export the input files from the converge studio and save in a designated location.
• Then using Cygwin we will run the simulation.

3) Post-Processing:

• The output files are converted to a format in which paraview can understand using cygwin or converge studio.
• These converted files can be used for visualising contours, animations etc. using Paraview which is the post-processing software.

Post-Processing Results:

Geometry Mesh:

A base grid of 1e-3 was provided along with fixed embedding near the throttle valve for the scale of 3 with 2 embed layers. Since the geometry is 3D we will be looking into different parts of the geometry. The mesh element of the elbow pipe can be seen below:

The mesh element at the inlet can be seen below:

The mesh element at the outlet can be seen below:

The mesh element at the throttle valve can be seen below:

We will be creating a slice along Z-normal direction in order to have a clearer look of the mesh.

In the figure below we can see the mesh around the throttle valve.

The total cell count of the geometry is as follows:

The total number of cells for a mesh size of 1e-3 along with fixed embedding is around 157000.

Mass Flow Rate Plot:

From the above plot we can say that the solution is getting converged around 10000 cycles.

The velocity contour is as shown below:

The velocity contour of the slice created from the above geometry can be seen below:

Glyph Plot coloring with respect to Velocity:

This plot helps us in visualizing the direction of flow.

Stream Tracer coloring with respect to Velocity:

Usually a streamline path is traced out by maseless particles as these particles moves along the flow.

Throttle Velocity Profile:

From the plots of glyph and stream tracer, we can see that the velocity is maximum at the regions where flow is restricted (i.e. on either sides of the throttle valve). The velocity increases due to sudden decrease of the area near the throttle valve. In this case study, the position of the valve is fully open and hence we can see that the flow through the elbow is free. Also from the stream treacer plot we can see that the velocity is low on the surface of the throttle as there is formation of boundary layer. As the valve is in open condition there isn\'t much variations in the stream lines. The average velocity can be seen in the plot below:

Velocity Animation:

The pressure contour is as shown below:

The pressure contour of the slice created from the above geometry can be seen below:

Throttle Pressure Profile:

From the above contours we can see that the highest pressure is near the upper portion of the throttle valve where the air is making an initial contact with the valve. The fluid comes from the inlet and creates an impact leading to increase in the pressure on the frontal circumference and on the other side the pressure is minimum as it is a wake region where the flow doesn\'t take place.

Pressure Plot:

From the above graph we can see that the total pressure at the outlet increases because of the narrow area near the throttle valve which increases the velocity which can be seen in the velocity contours.

Pressure Animation:

Conclusions:

The flow around the throttle valve was successfully simulated and can be seen in the above animations and contours. The valve in this case is kept stationary due to which the flow isn\'t restricted. The velocity is maximum is at the regions where flows is restricted. The high pressure is observed near the upper portion of the throttle valve where the fluid is making an initial contact with the valve.