Transient simulation of flow over a throttle body using CONVERGE CFD

Abstract:

In this project, a transient state flow analysis of a throttle valve is done. The throttle movement for a few seconds is given, to allow the movement of air and observe the velocity and pressure across the throttle region. Turbulent modelling using RNG k-epsilon model is chosen to model the flow behaviour.

Aim:

Transient state simulation over a throttle body

Objective:

1. Setup and run transient state simulation for flow over a 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. Also show the calculations on how you calculated an end time for the simulation.
6. Create an animation in which it throttle movement should be visible.

Introduction:

A throttle is the 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 air flow 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 drive by wire systems where sensors monitor the driver 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 vaccuum develops as the intake drops below ambient pressure.

Transient state:

A transient state is defined as the state where variables or process parameters change over time and attain a steady state at a later time. In other words, the time duration taken to reach steady state is called transient time.

Case setup:

1. Open the converge studio

Drag and drop the stl geometry file of throttle valve

2. Geometrical clean up and creation:

To ensure better flow simulation, we delete the outer covering of the elbow wall>> select triangle entity selection>> choose angle mode>> select the outer elbow wall>> select delete

Follow the similar procedure for the inlet section as given in step 3.

Follow the similar procedure for the outlet section as given in step 3.

The inlet and outlet section are open and will generate open edge error.

To remove the error we provide patching>> select edge tab>> select open edge mode>> go to repair section>> select the inlet and outlet edge>> select patch and the apply. The openings will get closed.

for outlet

Go to option>> select bounding box. The dimension of the box will be highlighted>> change the scale to 0.001 in order to convert to meters.

3. Diagnosis check up for identifying any internal error in geometry
4. 

3. Select application option>>select time based option.
4. 

Go to materials>> select gas simulation >> select default setting

Go to global transport parameters >> select default settings>>select ok.

5. End time calculation:

The dimension in x and y direction is roughly 0.1m.

Length of the pipe should be approximately 0.1*2=0.2m

The inlet velocity = 20m/s from steady state case

Flow through time= characteristic length/time to flow= 0.2/200=0.001sec

We can simulate the flow about 10 times the interval

So we round off to 10millisecond

Keeping the cfl limit for convective limit to 0.8

6. Solver parameter

Go to simulation parameters>> select run parameters>> choose transient solver with full hydrodynamic case

solver: pressure based, Navier stokes equation

7. Region and initialization

Grouping the boundaries to form volumetric regions. We define the region name as region0 with air as input material

8. Boundary conditions

Elbow wall:

Boundary type: wall

Assign region0 under region name

No slip boundary condition

Throttle:

Boundary type: wall

Wall motion type: rotating

Surface movement: moving

Assign region0 under regions

Now we will measure the arc normal and arc center to provide the location of rotation

Go to geometry>> measure>>direction>> arc normal>> select vertex entity-any>> select three vertices>> then apply

Rotation of valve:

Check use file method. The required rotation is given as per the requirement

From 0 to 2ms, the valve rotate by 25degree

From 2 to 4ms, it remains at that position

From 8ms, the valve returns to its origin

Inlet:

Boundary type- inflow

Pressure: 1.5e5Pa

Temperature:300K

Species: air

Outlet:

Boundary type- inflow

Pressure: 1e5Pa

Temperature:300K

Species: air

9. select turbulent model as RNG kepsilon

10. Grid control

Base grid: give the base grid size as 2mm

Enable the fixed embedding

Add entity type- boundary

Boundary Id- throttle

Mode- permanent

Output/post processing

Use default settings

Output files:

Time to write 3D output files=simulation running time/no of output file=

T=0.01/100=0.0001

Export all the input files

go to file option>> select export>> select export input>> choose the directory>> click ok

Simulation run:

1. Export all the case set up from the converge studio
2. Go to Cygwin>> go to desired path or directory
3. Type the command mpiexec.exe -n 2 converge-intelmpi.exe to run the simulation.

Post processing solution

Meshing of the elbow wall region

Meshing movement at the throttle section with embedding effect

Mass flow rate:

From this plot we can see that the mass flow is fluctuating initially when the throttle turns to rotate. The law of mass conservation is maintained at the inlet and outlet section. From 0 to 0.002sec, the mass flow reaches the maximum. This is due to initial movement of throttle. From 0.002 to 0.004sec the throttle fluctuation decreases and throttle remains stationary. From 0.008 sec the mass flow converges and remains at steady state.

Cell count:

The total cell count is not constant, because the mesh is changing continuously as the throttle changes. It is seen that from 0 to 0.002 sec the cell count changes due to rotation of valve. From 0.002 to 0.004sec the cell count remains constant. From 0.008 the cell count reduces and remains constant.

Average velocity:

Velocity flow is a function of mass flow rate. The graphical behaviour is similar to mass flow variation. From 0 to 0.002ms, the velocity fluctuation is high at the throttle rotation of 25 degree. From 0.002 to 0.004sec the throttle fluctuation reduces and from 0.008 the velocity converges and reaches stable position.

Static pressure:

The inlet static pressure fluctuates because of velocity variation. The throttle rotates from 0 to 0.002sec and the pressure fluctuates a bit initially and reaches a maximum of 1.5e3. from 0.002 to 0.004 sec, the pressure fluctuation reduces and the pressure keeps reducing until the throttle starts coming to its origin. From 0.008sec the throttle comes to its position and the pressure remains steady.

Velocity contour:

The velocity magnitude records maximum of 2.1e2m/s. During the initial phase air strikes the throttle with high velocity. During rotation of the valve air passes through sides and above the region. The pressure gradient reduces, while at the outlet, the flow separation takes place. The velocity at outlet is found lowest 7.4m/s. the streamline also predicts the fluctuation taking place and the rise in velocity magnitude while the rotation takes place.

Animation

Pressure contour:

During the initial phase the pressure at front of the throttle is maximum. This indicates that static pressure is also high as 1.5e5Pa. from 0 to 0.002, the pressure fluctuation is seen at the throttle. From 0.002 to 0.004, the pressure magnitude starts to reduce down. From 0.008 the pressure flow separates out at outlet and remains within a certain limit.

Animation

Discussion:

The transient simulation of throttle shows velocity fluctuation during the opening of the valve. The streamline fluctuation around the throttle is seen at the rotating period. The mass flow rate is also seen to fluctuates and obeys the conservation principle by getting converged at the outlet. From 0.002 to 0.004 the pressure fluctuation is seen to reduce down as most of the air passes through the outlet. Density fluctuation also remains stable. The mesh movement with fixed embedding motion is adaptive to capture the flow properly.The pressure also remains stability when the valve comes to its original position. The flow seperation during the movement of the throttle valve is seen clearly which predicts the physics of flow behaviour across the throttle.

Conclusions:

1. Dynamic mesh movement across the throttle movement is captutred appropriately to simulate the flow across it.
2. The mass flow, average velocity and pressure gets converged after attaining a steady state within a time interval domain.
3. With finer mesh at the throttle section and transient approach increases the computation time.
4. the cell count also fluctuates with the mesh refinement pattern.
5. The velocity distribution and pressure zones are captured accurately during the rotation of the valve.