Intake Manifold Discretization and the Effect of Discretization Length on Simulation time and Key Performance Parameters by using GEM-3D and GT-ISE

OBJECTIVE:

• Set discretization length of 0.1mm and compare it with the case of discretization length of 40mm
• Compare Parameters like Torque, BSFC, Maximum Cylinder Pressure, Simulation Time
• Explore the tutorial "Building Intake Manifold from STL model" i.e. conversion of 3D Model into 1D GT Model

Discretization:

Discretization is the technique used to divide the computational domain into small volumes of small lengths of dx where solution quantities like pressure, temperature, mass fractions etc are calculated. The 1D Navier Stokes equations are solved with the conservation of continuity, momentum and energy occur.

Discretization length affects the time step when the explicit flow solver is used. The less the discretization length, the more time is consumed as the solvable no. of equations increases due to increase in no. of cells.

In case Engine cycle simulation where explicit solution methods are used:

- A discretization length of approximately 0.4 times the cylinder bore diameter is recommended for the intake system.

- and 0.55 times the bore is recommended for the exhaust system

- this difference is because of the sound velocity at exhaust its temperature is high, and the velocity decreases at high temperatures

- the discretization length should decrease for any simulations that will require high-frequency analysis such as acoustics or engine performance above 6000 rpm

Case1: 1CylDI-final.gtm

Below model is a simple single cylinder 2 valve Diesel injector with intake and exhaust runner.

End Environment

It is a turbocharged engine with 2.4 absolute pressure and 350K temperature.

Below are specifications for the intake runner with 120mm as length and 40mm as the diameter. The discretization length is kept as variable as highlighted below.

The speed is kept constant with 2 different discretization length as 40mm and 0.1mm as seen in the image.

Simulations Results

For case1: Discretization Length (40mm) Final RLT Interval = 3.86sec

For case2: Discretization Length (0.1mm) Final RLT Interval = 9745.08sec

For discretization length 0.1mm, Final RLT Interval is 2523 times more.

For case1: Discretization Length (40mm) Brake Torque = 96.2 N-m

For case2: Discretization Length (0.1mm) Brake Torque = 95.9 N-m

For discretization length 0.1mm, Brake Torque is 0.311% less.

For case1: Discretization Length (40mm) BSFC = 238.2 g/kW-hr

For case2: Discretization Length (0.1mm) BSFC = 238.9 g/kW-hr

For discretization length 0.1mm, BSFC is 0.293% more.

For the Pressure plot below, we observe with 0.1mm its 115 bar, but with 40mm its 117 bar. For discretization length 0.1mm, the maximum pressure is 1.7% less.

CASE2:

• Explore the tutorial "Building Intake Manifold from STL model" i.e. conversion of 3D Model into 1D GT Model

GEM-3D helps the 3D geometry to convert into 1D model.

Step1: Getting Started

Select GEM-3D and create a new model by selecting file/new Document or new from the Home Tab and clicking on the finish making sure that the document selection on the left is set to GEM D(.gem).

Step2: Importing a shell from an STL file

The first step is to import the intake manifold from an STL file that is provided with the GT-SUITE installation. To import the shell, select Import 3D from the File group in the Home tab. This will launch the Import 3D Wizard, shown below.

Browse for the plenum.stl file located in the GTIHOMEv20XXexamplesAcousticsNonLinear_(standard) directory. Select Next and then make sure the Surface radio button is selected before selecting Next for the second window. Since STL files do not contain unit information, it is very important to select the correct unit when importing STL files using the surface method. Since our example STL file was created using mm as the unit, make sure the unit is set to mm, then click Finish.

Step3: Separating Sections by curve

Inlets and the end of the 4intake runners are closed and we need to remove these caps to allow the flow charge to enter into the runners.

To do this we will use Sperate by curves options. This will separate the geometry into parts based on changes in the surface of the object. This process will also separate the intake runners into two different sections near the area change at the top of the runners. Ensure the part is selected and choose Separate By Curves from the Shape Operations group in the Convert tab.

The tolerance controls how sensitive this operation is to change in surface area of the part and the tolerance of 1.0 is recommended.

Now that the end sections of the runners and plenum are separated from the main shape, we can delete them from the model.

To do this select the 5 end sections (remember to hold down Ctrl to select multiple shapes) and press the delete key. This will open a confirmation dialogue verifying that you want the mesh shapes deleted.

Click Yes and the model should look like the image below, where the inner surfaces are now partly visible through the open ends where the caps were removed.

Step 4: Use the cutting plane

Now that the end caps have been removed we can continue separating the shapes into sections that can be represented by pipes and flow splits.

We will start by separating off the 4 runners from the manifold since the runners can be represented by pipes. To do this we will make use of the cutting plane operations available in the Cutting Planes group in the Convert tab. To enable the cutting plane, select 3 Points Cutting Plane from this group. A cutting plane can then be created by clicking on 3 points on the model to define a plane.

The current position of the cutting plane looks like it will also cut through the manifold, which is not desired, since we want to cut off the runners without cutting the manifold. Therefore, let's translate the cutting plane away from the manifold enough to cut just the runners

Once the cutting plane is in place, we need to select the mesh shape that we want to cut. This is done by selecting the mesh shape in the graphical window. When selected, the shape will turn magenta in colour. Once the mesh shape is selected, we need to clip the runners. To do this, select Clip Shape from the Shape Operations group in the tab Convert, or select press the Clip button in the Plane Control window. GEM3D will separate the mesh shape along the cutting plane and create the new mesh shapes, as shown in the image below.

Step5: Converting Sections into Flow Components

Now its time to convert all of the pipes into the flow components. We do this by right-clicking on the first section of the first runner (blue coloured bent section in the image above) and select Convert Shape to Component. This will open the convert mesh window, shown in the image below. Then we do it for all the pipes as seen below.

This window appears once we right-click and select Convert Shape to Component. The port numbering suggests the flow of the air.

After doing the NEXT, For the geometry, Multiple Bends is selected and Linear interpolation between ends is been selected for Cross Section Variation. End measurement Location is selected and user-defined Diameter of 51.6mm is selected. The same diameter is kept at port2.

Then we do it for all the pipes and we convert all the pipes into 8 different components as seen below.

Step6: Adding a custom connection

In the conversion process for the runners, information regarding the curved entry from the manifold (fillets) was lost. This curved entry would represent a better flow transition than a default orifice connection, so we need to manually add an orifice connection to represent this. To add a custom orifice to a flow component right-click on the first section of runner 1 (blue component in the image above, "run1-1") and choose Add Connection. This will open the port connection window, shown in the image below, that allows the selection of which port to add the connection to and what type of connection to use.

We want to select port 1 (the port at the manifold) and the orifice connection ('OrificeConn'). Click OK and the connection will be added and the appropriate template window will open.

Step7: Using Parameters

Give the parameter a value of 0.95, which should represent a nice transition since an abrupt area change would normally result in a discharge coefficient of approximately 0.8. Click OK to Case Setup to save the value and return to the connection window.

Repeat the above procedure for the rest of the runners using connection names of "ori-run2", "ori-run3", and "ori-run4". When doing this, it is not necessary to go to Case Setup each time since the value was already defined the first time. You can use the value selector to select the parameter for the subsequent orifice connections since it already exists, thus avoiding potential typing mistakes.

Step8: Adding a Subassembly connection to Impose Connection Direction

The next thing we want to do is specify the end (boundary) connections, meaning the connections that do not connect to anything. In our model the ends of each runner need an end connection specified. This is done with a special subassembly connection. To add the subassembly connection right-click on the end section of the first runner component ("run1-2") and choose Add Connection. This will open the port connection window, shown in the image below.

Step9: Making more difficult cuts

Step10: Dividing pipe section with Area Changes

Step11: Determining Orientation during Conversion

Here, we need to convert the entire section into flow split, Right-click on it and select Convert Shape to Component. Again, all open ports will be identified.

Step12: Adding the Flow Connections

The remaining gaps between the pipe and plenum need to be closed and hence we again use extruded connections from connection option is taskbar. And as we can see in the below image the gaps are closed a rendered transparent pipe connecting plenum and pipes.

Step13: Discretizing the model

To discretize a model, select Export GT Model from either the Home tab or the toolbar. This will open the discretization window that contains the available discretization options. Let's use a pipe discretization length of 40 mm and make sure the output file name is "IntMan.gtsub".

Once the model is in this orientation, click the Discretize button and GEM3D will create a GT-SUITE model file (.gtsub). You will get a message saying the discretization completed successfully. Clicking the More Info button will provide more details about the discretization operation (this is also stored as a .msg file with the same name as the model with _GEM3D added). Press the Open in GTise button to open the newly created model file in GT-ISE. The model file map will look like the image below.

CONCLUSION

1. Discretization Length affected on simulation time and Engine performance:

1. For discretization length of 0.1mm, Final RLT Interval is 2523 times more

2. For discretization length of 0.1mm, Brake Torque is 0.311% less.

3. For discretization length of 0.1mm, BSFC is 0.293% more

4. For discretization length of 0.1mm, the maximum In-cylinder pressure is 1.7% less

2. Discretized the intake manifold by the use of GEM-3D software.

3. Practically converted 3D model to the 1D model

4. Learned discretization aspect in GT-Power