Transient simulation of flow over a throttle body

Most of the spark-ignited Internal Combustion Engines use the stoichiometric air-fuel ratio to control the emission level. For controlling engine power, a throttle valve is used. The objective of the throttle valve is to control the air subsequently air-fuel ratio at different loading conditions.

In this project, unsteady one-dimensional CFD simulation has been done using Converge CFD. In this simulation, the non-reacting flow (only air) has been simulated.

Geometry Description :

Geometry is simply curved (90 degree) pipe and throttle is placed at the junction as shown above.

Surface Preparation:

Since in this case only gas flow simulation has been done, so solid part is unnecessary (We are not solving Conjugate Heat Transfer). Solid parts have been removed from geometry and open edges have been flagged.

Meshing Details:

Equal size mesh has been generated i.e. dx = dy = dz = 0.002 m

Fixed embedding refinement has been done to capture the flow near the valve.

Fixed embedding refines the grid at specified locations and times. Fixed embedding is used to refine the grid at specific locations in the domain where a finer resolution is critical to the accuracy of the solution.

To include fixed embedding in a simulation, set embedded_flag = 1 in inputs.in and include an embedded.in file.

For each fixed embedding, the embedding scale is specified that indicates how much CONVERGE will refine the grid in that location. The scaling is done according to

`dx _(embedded ) = dx_(base) /2^(e sc)`

where e sc is embedding scale.

Time Step Determination :

Since in this simulation throttle body is moving, therefore transient solver is used.

Flow-through time is determined using

`t = L/u`

where L is length scale of the geometry and u is average velocity. The average velocity is captured using steady-state simulation. The total time of simulation can be chosen 4 times higher than the flow-through time.

Boundary Condition :

Inlet Boundary Condition 150000.0 Pa Pressure

Outlet Boundary Condition 100000.0 Pa Pressure

Pin > Pout, So Flow will take place from inlet to out as favorable pressure gradient.

Elbow_Wall: Stationary wall and Law of Wall Boundary Condition applied.

Throttle: Rotating Wall Boundary. Finding the rotation center arc normal method is used. Rotation Rate is given as such

• at t = 0 sec, the angle is 0 degree.
• at t = 2 ms, the angle is 25 degree.

Post Processing:

Velocity Contour is shown as below

Boundary_Id 3 represents inlet velocity while Boundary Id-2 shows outlet boundary.

We can observe inlet velocity is negative because of normal vector making 180 degrees from the inlet while outlet velocity is positive because normal makes zero degrees with outlet.

Inlet and outlet velocity is converged 2.5 ms.

Paraview is used for visualizing the velocity contour.

The above diagram shows the streamline.

Pressure Contour is shown as below

Boundary_Id 3 represents inlet while Boundary Id-2 shows the outlet boundary.

Static Pressure is shown for inlet while total pressure variation is shown at the outlet. Because of the wake zone forming at the downstream of throttle (zero velocity or recirculation zone), the total pressure is increasing which represents the stagnation loss because of the recirculation zone.

Pressure contour can be seen as below

Here we can see downstream of the throttle low-pressure zone.

Mass Flow rate

Boundary_Id 3 represents inlet while Boundary Id-2 shows the outlet boundary.

We can observe here magnitude of inlet and outlet is the same which satisfies the continuity equation.

Cell counts

Simulation is done using MPI parallel processing and 4 processors are used. The no of cell counts and total number of cells can be seen from the above diagram.