Week 4.1: Project - Steady state simulation of flow over a throttle body

Abstract:

In this project, a steady state simulation of the throttle valve is studied using CONVERGE CFD. Pressure based solver with RNG k-epsilon turbulent model is used for simulation. Parameters like mass flow rate, velocity distribution, and pressure variation across the throttle region are observed and analyzed. Embedded scaling is done to refine the mesh at the throttle interface and capture the physics of flow accurately.

 

Aim:

Steady-state simulation over a throttle body.

 

Objective:

1. Set up and run the steady state simulation for flow over the throttle body.
2. Post-process the results and show pressure and velocity contours.
3. Show the mesh (i.e surface with edges)
4. Show the plots for pressure, velocity, mass flow rate, and total cell count.
5. Create an animation in which its throttle movement should be visible.

 

Introduction:

A throttle is a mechanism by which fluid flow is managed by constriction or obstruction.

In an IC 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. For a gasoline engine, the throttle most commonly regulates the amount of air and fuel allowed to enter the engine. Recently, for a GDI  engine, the throttle regulates the amount of air allowed to enter the engine. The throttle of a diesel, when present, regulates the airflow into the engine.

Historically, the throttle pedal or lever acts via a direct mechanical linkage. The butterfly valve of the throttle is operated by means of an arm piece, loaded by a spring. This arm is usually directly linked to the accelerator cable and operates in accordance with the driver, who hits it. The further the pedal is pushed, the wider the throttle valve opens.

Modern engines of both types (gas and diesel) are commonly driven by wire systems where sensors monitor the driver's controls and in response, a computerized system controls the flow of fuel and air. This means that the operator does not have direct control over the flow of fuel and air; the electronic control unit (ECU) can achieve better control in order to reduce emissions, maximize performance and adjust the engine idle to make a cold engine warm up faster or to account for eventual additional engine loads such as running air conditioning compressors in order to avoid engine stalls.

The throttle on a gasoline engine is typically a butterfly valve. In a fuel-injected engine, the throttle valve is placed on the entrance of the intake manifold or housed in the throttle body. In a carbureted engine, it is found in the carburetor. When a throttle is wide open, the intake manifold is usually at ambient atmospheric pressure. When the throttle is partially closed, a vacuum develops as the intake drops below ambient pressure.

 

Geometry:

 

Simulation Case setup:

• We delete the outer covering of the elbow wall, inlet, and outlet wall.

 

• The inlet and outlet sections are open and will generate an open edge error.

 

• To remove the error we provide patching for both inner and outer edges.

 

• After patching

 

• We conduct a diagnosis check to remove all the geometrical errors

 

Boundary Flagging:

 

Case setup:

• Application type: Time based

• Materials

 

• Simulation parameters
• Run parameters

 

• Steady-state monitor

 

• Simulation time parameters

 

• Solver parameters

 

• Boundary Conditions

 

• Regions and initializations

 

• Turbulence modeling

 

• Grid Control

 

• Post variable section

 

• Output files

•  No error in diagnosis

 

Results:

Meshing of the elbow wall region

 

Meshing at the throttle section with embedding effect

 

Mass flow rate:

The mass flow rate is high initially and becomes stable at 0.007kg/s after reaching 10000 cycles. The fluctuation of mass flow is due to variation of inlet static pressure and decrease in velocity.

 

Density variation:

Inlet density variation is caused due to mass flow variation. The inlet density attains stability at 1.45kg/m3 after reaching 10000 cycles. At outlet section density increases and reaches stability at 1.3kg/m3 at 8000 cycles. The fluctuation is caused due to variation of mass flow at the throttle region.

 

Total pressure:

 

Average velocity:

The inlet average velocity variation is very high at starting and then reaches stability at 190m/s after reaching 12000 cycles. The high outlet velocity is due to low is due to low static pressure difference at the throttle region.

 

Procesors used for computation:

The total no of cell count used for running the simulation is around 36000 cells, while in processor 0 no of cells used is 17000 and for processor 1 no of cell count is 18500. the simulation is runned parallely in two processor to reduce the computational time.

 

 Velocity:

The velocity vector and the plane of throttle exposes the maximum velocity of 3.2e2m/s at the inlet section. While at the outlet section the velocity drops to 100m/s. The static pressure increases and the flow becomes seperated. the outlet velocity is low which supports low mass flow rate at the outlet.

 

Pressure:

The inlet pressure is high and encountered at the throttlle front region and the magnitude is about 1.5e5Pa. The air while passing through the corners reduces the total pressure and flow seperation takes place. The pressure reaches around 80000Pa across the throttle corner sectiona and back portion.

 

Discussion:

From the steady state simulation of throttle valve, it is seen that pressure at the inlet is higher than the outlet. The mass flow variation is encountered at the throttle valve front position. The velocity at the inlet position is higher, but some region of high velocity is seen to leak from the corner of the throttle section. At the outlet the flow seeration takes place from the throttle surface and the outlet velocity reeduces down to attain a steady state value . The density variation also a function of mass flow variation and pressure difference. The flow seperation can be seen to take place when pressure tends to drop down.

Conclusions:

1. The steady state simulation is performed to observe the effect of flow seperation across the region of throttle section. The maximum velocity of 3.2e2m/s is observed at the inlet and low velocity of 50m/s at the outlet.
2. The pressure variation is seen to be fluctuating at the inlet and while encountering the valve, the pressure rises up. The outlet pressure becomes low and further detaches the flow seperation.
3. The grid refinement is done at the throttle section to capture accurately the flow behaviour. With refinement the computational cell count is higher and time taken to complete the simulation increases.