Transient simulation of flow over a throttle body   (Converge CFD, Skill Lync, Medium)

Aim:

To simulate a transient flow over throttle body using Converge CFD

Introduction:

A throttle body is a device that controls the amount of air fuel mixture entering the combustion chamber of IC engine. The motion is controlled by accelerator pedal of driver. When accelerator pedal is pressed, the throttle valve opens and more air fuel mixture enters in the chamber causing more heat release and more speed of engine. In this report, a transient simulation of flow over throttle body is done. Following was the geometry used for simulation.

Lx = 0.106 m and Ly = 0.119 m

When simulations were done using this geometry, there was backflow towards inlet at the start due to closed valve. It created oscillations in data. To reduce this effect, the inlet was extended by 0.15 m.

Following are the case setup details:

Mesh:
Element size: (x, y, z) --- (0.002, 0.002, 0.002) m

Fixed embedding on throttle body --- Scale = 2 and Layers = 3

Boundary conditions:

Inlet: Constant pressure inlet at 150000 Pa

Outlet: Pressure outlet at 100000 Pa

Region initial pressure: 100000 Pa

Throttle valve motion:

00 ms to 02 ms --- Closed position at 0 Deg

02 ms to 04 ms --- Valve opening till 60 Deg

04 ms to 06 ms --- Valve stays at 60 Deg

06 ms to 10 ms --- Valve closes till 0 Deg

10 ms to 12 ms --- Closed position at 0 Deg

Solver: Transient Pressure based PISO solver with 'step' flux limiter

Time settings:

Start time: 0 ms

End time: 12 ms

Start time step = 1e-7 sec

Minimum time step = 1e-7 sec

Maximum time step = 0.0005 sec = 0.5 ms

Max Convective CFL = 2

3D output files saved at every 0.5 ms

Turbulence model: RNG k-e model with standard wall functions.

Results:
1) Mesh:

2) Pressure Contours:

The Pressure stays low at the output till the valve opens and it reaches a max value below 110000 Pa. It smoothly reduces till 100000 Pa after valve starts closing. In the video, we can see the pressure contours changing. The pressue behind the throttle body is low suggesting the presence of wake region. Since the mesh was coarse the flow features and wake regions were not captured smoothly.

3) Velocity plots and contour:

The velocity at inlet can be seen having a backflow at the start, till the valve opens. Then, the air flows through the pipe. At outlet, some fluid is escaping at the start. This is because of flow through sides of throttle body. In real life, the throttle body is sealed with rubber gasket. Here, in this simulation, the throttle body is not sealed, so there is a flow escape. When the valve starts closing, both velocities start reducing to a low value.

The vectors show the fluid moving over throttle body and moving inside the gaps between wall and body.

We can see changing contours of velocity in this video.

4) Streamlines

We can see flow getting restricted when valve is closed and flowing when it is open. Also we can see the escaping flow.

5) Mass flow rate:

6) Cells and Wall clock time:

The cell increased in number when the valve was open and it reduced when it came back to original position.

The cpu took more time when the mesh was moving.

Conclusion:

The transient simulation of flow over throttle body is done using Converge CFD.