Discretization in GT Power

Objective:

1.Explore the Tutorial ' single cyl DI 'and add case with discretization length 0.1 mm for intake runner and compare with default case with parameter such as

• Torque, BSFC, max cylinder pressure
• simulation time

2.Explore tutorial “Building intake manifold from STL model” .

1.Tutorial ' single cyl DI':

Theory:

Discretization is the processs of converting Physical Volume into No. of Sub volume. And discretization length is the length of each subvolume.

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. The difference in the recommendation for intake and exhaust discretization is the result of differences in the speed of sound due to the temperature.

In engine 'Explicit solver' is used.The relation between the time step and the discretization length is determined by the Courant number.

Δ𝑡/Δ𝑥*(|𝑢 |+𝑐𝑐) ≤ 0.8∗𝑚𝑚

where,

Δt = time step (s)

Δx = minimum discretized element length (m)
u = fluid velocity (m/s)

c = speed of sound (m/s)
M = time step multiplier

Model:

Case Setup:

Two cases has been setup with different Discretization length at intake runner.

Result:

Case 1: discretization length = 40 mm

Case 2: discretization length = 0.1mm

Computational Time:

Fig: Case No. Vs Computational Time

Case 1: discretization length = 40 mm Case 2: discretization length = 0.1 mm

Torque = 96.2 N-m (Brake) Torque = 95.9 N-m (Brake)

BSFC = 238.2 g/kW-h BSFC = 238.9 g/kW-h

Max Cylinder Preasure = 117.11 bar Max Cylinder Preasure = 115.18 bar

Result Discussion:

1. Large Discretization length(40 mm) takes laerge time steps and hence require less computation time. And small Discretization length(0.1 mm) takes small time steps and hence require more computation time.

2. Though small Discretization length require more computation time, result are tend to be more accurate.

2.tutorial “Building intake manifold from STL model”

step 1:

Launch GEM3D and create a new model file by selecting New Document. Once the model file is opened, save it to the GTIHOME\v2019\tutorials\Graphical_Applications\GEM3D\02-IntakeManifold directory with the name "IntMan" by selecting Save As. To import the shell, select Import 3D from the File group in the Home Browse for the plenum.stl file located in the GTIHOME\v2019\examples\Acoustics\Non-Linear_(standard) directory. Make sure the Surface radio button is selected before selecting Next.

Fig: shell imported from CAD model

step 2:

Go to convert tab and select the Separate By Curves , 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. Tolerance of 1.0 should be used for this operation.

Step 3:

The inlet and the ends of the 4 intake runners are closed. We need to remove these caps so the ends are open and available for flow.Select end cap and delete them.

Step 4:

We will separating off the 4 runners from the manifold since the runners can be represented by pipes .Go to convert tab and select 3 Points Cutting Plane and pick 3 points on the underside of the manifold , translate the cutting plane to set at position from which you want to cut the runner ,ensure that only runner are to be cut and not the surface of manifold.to get exact view go to Orientation > Snap to Nearest.After selecting proper cutting plane position click on clip to cut the runner.

Step 5:

To convert the runner into pipes, select the runner pipe and Right click and select Convert Shape to Component .Now select ‘Multiple bend’ and appropriate diameter of pipe. Go to the main tab andclick on init to define Initial State .Open a 'FluidInitialState' template define the Pressure as 1 bar and the Temperature as 300 K. the properties of air from the template choose the "air" 'FluidMixture' object.Repeat the process for remaining runner.

Step 6:

In the conversion process for the runners, information regarding the curved entry from the manifold (fillets) was lost. So we need to add an orifice connection to represent this .Select the runner and right click on it and select add connection allows the selection of which port to add the connection to and what type of connection to use .While giving orifice connection between manifold and runner set discharge coefficient ass parameter to represent smooth flow from manifold to runner.

Simmilarly, define subassembly port at outlet.

Step 7:

Select the entrance of manifold, these section can be converted into two pipes. Go to convert > pipe convert to cutting plane. Cut the pipe from where bend angle changes abruptly .Repeat the process for another one.Now right click on pipe and Convert Shape to Component. And select straight bend.

To add subassembly connection at the entrance of connection, right click on pipe and select add connection > port 1 > 'GEMSubAssExtConn' > port ID > flow direction (inlet)

Step 8:

Now select D shape and go to convert > 3 Points Cutting Plane and select 3 point to to select cutting plane and cut it from manifold. Now right click on pipe and Convert Shape to Component and select flowsplit and check for surface area and volume.

Step 9:

Now the Manifold can be further divided into flowsplits , go to convert > 3 Points Cutting Plane and cut the left side of manifold and to cut right hand side manifold use restore plane option in convert tab.Right click on pipe and Convert Shape to Component and select flowsplit.

Fig: Cutting Left hand side of Manifold

Fig: Cutting Right hand side of Manifold

Step 10:

After cutting pipes , there is gap between two pipe and pipe and manifold to fill this gap select Flow Connection > Flow Connection from the Connections group under the tab Flow.

Step 11:

To discretize the model ,Select the model in the proper orientation and select Export GT from home tab and click on discretize.

Final Discretized model is as follow: