Week 4: Project - Transient simulation of flow over a throttle body

Objective: The aim of this project is to study the Transient-state analysis of Flow over the Throttle body and examine the parameters like velocity, pressure, and mass flow rate at both inlet and outlet by using the Converge Studio software.

Introduction: Throttling Valve

Throttling valves are a type of valve that can be used to start, stop, and regulate the flow of fluid through a pump. Throttling valves are used to regulate the flow, temperature, or pressure of the service. These valves can move to any position within the stroke of the valve and hold that position, including the full-open and full-closed positions. Therefore they can act as on-off valves also.

Throttling valves are equipped with actuators or actuation systems, which provide greater thrust and positioning capability, as well as automatic control. Few throttle valve designs are also provided with a hand-operated manual handwheel but accuracy is a concern there.

• Some of the applications are :

In an internal combustion engine, the throttle is a means of controlling an engine's power by regulating the amount of fuel or air entering the engine. In a motor vehicle, the control used by the driver to regulate power is sometimes called the throttle, accelerator, or gas pedal.

Steam locomotives normally have the throttle or regulator in a characteristic steam dome at the top of the boiler (although not all boilers feature these). The additional height afforded by the dome helps to avoid any liquid (e.g. from bubbles on the surface of the boiler water) being drawn into the throttle valve, which could damage it, or lead to priming.

Geometry:

• The geometry we have imported is an elbow pipe geometry, placing the throttle valve a the bend of the pipe.
• The throttle valve inside the elbow pipe is fixed to the walls of the elbow, and the rotation movement of the throttle valve has maintained by a pin that is attached to it. 

• We can see the throttle valve which is inside the elbow by hiding a quarter part of the elbow by choosing a perfect axis.
• The throttle valve looks like:

• We can see the cylinder in the above figure, it is the pin that attaches with the throttle body and maintains the movement of the valve.

Flow-through time: It is an amount of time that tells us how long the flow takes place from inlet to outlet in one instance.

In this case, we consider the characteristic length of the elbow is 0.2m

The average velocity from the study-state is 100 m/s.

So, the flow-through time is:

`t=(l_c/v)`

`t=(0.2/100)=0.002`

So, let us say we want to simulate maybe 5 times. So the total simulation time would be 0.01.

Case Setup:

Application Type - Time-Based

Materials - Air (Mixture of O2 & N2)

Simulation Parameters:

• Run parameters: Solver - Transient
• Simulation time parameters:  Start time - 0

                                                   End time - 0.01

                                                   Initial time step - 1e-9

                                                   Minimum time step - 1e-9

                                                   Maximum time step - 1

Boundary Conditions:

1) Inlet:

• Boundary type: INFLOW
• Pressure: 150000 Pa
• Fluid: Air (O2 - 23%, N2 - 77%)

2) Outlet:

• Boundary type: OUTFLOW
• Pressure: 100000 Pa
• Fluid: Air (O2 - 23%, N2 - 77%)

 3) Throttle Wall:

• Boundary type: WALL
• Wall motion type: Rotating
• Surface movement type: Moving

For the rotation of the throttle valve, we need to give any three co-ordinates of the pin attached to the valve.

Then we need to enter those co-ordinates in the respected places called Rotate centre and Rotate about.

For the valve rotation, we need to give the inputs in the form of angles with respect to the end-time. The graph looks like:

• From the above graph, we can say that when the valve is going to open and how much degree it rotates.
• The maximum angle we have used is 25 degrees. At 25 degrees the valve has been opened for 0.002 seconds and after another 0.002 seconds, the valve starts to close and maintains 18 degrees. Finally, the valve gets closed at 0.008 seconds.

 4) Elbow Wall:

• Boundary type: WALL
• Surface movement: Fixed

Physical Model:

• Turbulence Model: RNG K-Epsilon

Grid Control:

• Base grid: 0.002 (dx,dy,dz)

Fixed Embedding:

• Entity type: Boundary
• Scale:3
• Layers: 2

Output/Post-processing:

• Output files: 

1. Time Interval for writing 3D files: 0.001 sec
2. Max. Number of restart files saved: 3

Meshing:

CONVERGE automatically generates an optimized Cartesian mesh at runtime and dynamically refines the mesh throughout the simulation using AMR. All of this remains the same in CONVERGE 3.0, and you can still use meshes exactly as they were in all previous versions of CONVERGE.

• The Base grid is given as 0.002 in all the dx, dy, dz directions.

• The fixed embedding method has been used to refine the mesh at the throttle area because the flow gets resisted in that environment only.
• We can observe in the above figure, the throttle area has been given more number of elements because of the embedding method.

Mesh Animation: https://youtu.be/lJyVJvH3y7w

Movement of Throttle:

• Using the opacity technique in the Paraview, we can see the moment of throttle valve inside the elbow pipe. The moment looks like:

Velocity:

• The contour represents the throttle is at the angle of 0 degrees open, means the throttle valve is fully closed and the flow is passing over it.
• The max. velocity we observed here is nearly 260 m/s and t5he min. velocity is recorded as 2.3 m/s over the throttle valve and behind the throttle valve.

• Here the above contour represents the valve opened at 25 degrees angle.
• In the contour, the max. velocity obtained at the narrow passage between the elbow wall and the throttle valve because of the reduction of the cross-sectional area.
• It happens because the cross-section area gets reduced at that place.
• The max. velocity recorded inside the elbow is 260 m/s.
• The least velocity recorded in the elbow is at two places: one is on the throttle valve, and the other region is behind the throttle valve as it separates the flow. Due to flow separation, the wake or negative pressure zones were created.
• The min. velocity recorded is 2.4 m/s.

• The sudden shock visible in the above graph is because at that place the flow hits the throttle valve ad gets separated hence lost its velocity.
• The outlet velocity is more than the inlet velocity. This is the phenomenon we have to study and analyze in these kinds of simulations.

Velocity Animation: https://youtu.be/0i_O8xvsoKo

Velocity streamlines and traces: https://youtu.be/c54XOXAU4X0

• In this video animation, we can see how the velocity gets reduced when the throttle valve gets opened and the flow increased at the narrow passage in the left side animation.
• In the right side animation, we can study the velocity vectors. In this, we can see how the vectors get distracted when the valve gets opened and after some distance how they come into alignment and continued with the flow.

Pressure:

• The contour represents the throttle is at the angle of 0 degrees open, means the throttle valve is fully closed and the flow is passing over it.
• he max. the pressure was obtained at the left side of the throttle valve because flow hits there and reversed while passing through that region. We call it as the flow gets staggered.
• The lower pressure was obtained at the outlet of the elbow and also at the narrow passage between the elbow wall and throttle valve.

• The above contour represents the valve is opened at 25 degrees of angle. When the throttle moved with some angle, the flow gets altered and separated.
• As the valve moves, the max. pressure on the throttle valve has increased suddenly.

• From the time 0 sec to 0.002 sec, the flow is irregular because at this time only the throttle gets a moment and separate the flow.
• At the inlet the max. the pressure is recorded at the end of simulation as 1.4 bar and the min pressure is 1 bar.

Pressure Animation: https://youtu.be/M5d4t0WVcEk

Mass Flow Rate:

• From the time 0 sec to 0.002 sec, the flow is irregular because at this time only the throttle gets a moment and separate the flow.
• At the time from 0.002 to 0.004, the throttle valve angle is constant with 25 degrees and the graph is smooth in that time period.
• After the time period of 0.004 seconds, the flow has slight distraction because the valve is getting to close (means the valve starts to move from 25 degrees to 18 degrees.).
• From 0.008 to 0.01 seconds, at the final stage of the simulation, we have no movement in the throttle valve. So the curve is straight forward and has no distraction.

Total Cell-Count:

• To speed up the simulation we have included the 4 processors into the simulation instead of running on 1 processor.
• Here while running the transient case, there will be no constant number of cell-count. Because the throttle valve is moving throughout the simulation. So that mesh count of the throttle valve is continuously changed.
• As we discussed above, from 0 to 0.002 seconds the throttle is moving and cells are also increasing. But from 0.002 to 0.004 seconds the cell count is constant because at that time period there is no throttle movement.
• A total of 40000 cells has been created while running the transient simulation of flow over a throttle valve.

Conclusion:

• The flow over a throttle body simulation has been performed and analyzed in the transient condition.
• The pressure is increased on the throttle valve while it is run at the angle of 25 degrees.
• As we applied the fixed embedding to the mesh, it captures more number of cells and data while the throttle valve is moving.
• The velocity at the narrow passage between the elbow and the throttle valve is high because of reduction in the cross-section area. At this particular narrow-passage, the pressure gets decreased.