Transient Simulation of flow over a throttle body

Development of fuel injection systems that gives better fuel economy, less emissions and higher performance are central parts when a new engine is designed. This area is where most improvements have been made in the last two or three decades. In fuel management design simulation is one of the most powerful tools. A real engine on the other hand may take several minutes and sometimes hours to reach steady state conditions. When simulating an engine, the various engine components are divided into subsystems, for example, air filter, turbo charger, intercooler, throttle body, cylinders and exhaust system. For each subsystem there are equations or models that are used to describe how a particular subsystem will affect the others. If all subsystems are validated correctly against engine measurements they will behave and perform just as in the real engine. By doing so there are possibilities to get accurate results from simulations of how a real engine would react to a certain fuel management system. The main purpose of this project is to gain a greater understanding of how the air flow is restricted at the throttle plate using Computational Fluid Dynamics (CFD).

Objective:

To simulate Transient flow over a throttle body and post-process the results.

Geometry:

The pre-modelled throttle body was imported into Converge and the geometry was inspected for any surface errors or open edges. After fixing all the errors, the boundaries were flagged and the case was set up.

Case Setup:

The initial and boundary conditions were set to:

Inlet Pressure: 150000 Pa

Outlet Pressure: 100000 Pa

Elbow Wall Temperature: 300K (Law of Wall)

Throttle Wall Temperature: 300K (Law of Wall)

Solver: Transient Flow Solver

Fluid under consideration: Air (O2 + N2)

For meshing Base grid size used is dx= 0.002m, dy= 0.002m, dz= 0.002m. Near the throttle valve grid scaling is done to capture the flow more accurately. It consists of two layers of size 0.00025m. In this simulation the throttle is set to rotate as follows

• From 0 to 2 ms the valve should rotate by 25 degrees.
• From 2 to 4ms it should stay at 25 degrees.
• After which by 8ms the valve should get back to it’s original position

Simulation Run Time Calculation:

The total length of the pipe is 0.2m. From the steady state simulation performed earlier we got the average velocity flow rate equals to 191 m/s. Time = Distance/Speed therfore, flow through time is equal to 0.2/191= 0.00104 sec. Threfore we are simulating it ten times of flow through time i.e. 10 ms

The remaining parameters are set as per the simulation parameters of steady state.

Running the Simulation:

Software Used: Cygwin

Post-processing the Results:

Software Used: ParaView

• Mesh

• Mesh around the throttle

• Velocity Contour

• Velocity Contour of throttle body

• Pressure Contour

• Pressure Contour of throttle body

• Stream line Flow

Plotting the Results:

• Velocity Plot

From the above plot the velocity changes drastically when the valve is changing position in the first 2ms, remains constant when the valve holds its position between 2-4ms and gradually increases as the valve moves back to its initial position and remains constant when the valve has reached its original position.

• Pressure Plot

From the above plot it is observed that initially, when the valve was in open position, there was not much pressure difference in the fluid between the entry and exit points of the throttle valve. As the valve began to rotate, the side of the valve facing the flow experiences maximum pressure of 1.5 bar. This pressure is due to the fact that the total kinetic energy of the fluid is converted into pressure energy when the flow is obstructed.

• Mass Flow Rate Plot

From the above plot the mass flow rate changes drastically when the valve is changing position in the first 2ms, remains constant when the valve holds its position between 2-4ms and gradually increases as the valve moves back to its initial position and remains constant when the valve has reached its original position.

• Total Cell Count Plot

Converge accounts for the change in geometry by generating mesh for each second, resulting in different cell counts when the valve is moving. For phases where the valve is stationary, the cell count remains constant which can be observed between 2-4 ms and 8-10 ms when the valve is not moving.

The Transient Simulation of Flow over throttle body can be visualized here

• The velocity of the flow over the throttle valve was visualised using ParaView.
• The maximum velocity of 310 m/s was recorded when the valve was at 25° from its initial open position where the flow area was obstructed by the valve movement.
• The pressure distribution during the valve movement can be visualized with the help of the below animation recorded using ParaView.
• Initially, when the valve was in open position, there was not much pressure difference in the fluid between the entry and exit points of the throttle valve. As the valve began to rotate, the side of the valve facing the flow experiences maximum pressure of 1.5 bar. This pressure is due to the fact that the total kinetic energy of the fluid is converted into pressure energy when the flow is obstructed.

Conclusion:

By understanding the flow behaviour when the throttle moves, we can calculate how to control the flow input such that the output conditions are favourable. For example, if we require the fluid to have high outlet pressure, we can adjust the throttle movement accordingly. We can also further experiment and study flow behaviour when the throttle is opened at different angles. Transient state simulation of flow over a throttle body helps us to visualize the restriction of flow made by the rotation of the throttle body.