As a first step, we will create a new simulation inside the project. In order to do so, select the ‘Raspberry Pi’ element under Geometry, then click the button ‘Create Simulation’ in the pop up dialog.
As it is our goal, select the ‘Convective Heat Transfer’ analysis type in the ‘Create Simulation’ dialog, the the button ‘Create Simulation’.
Second Step: Create Topological Entities
Topological entities are sets of geometry features (faces in this case) that can be reused for different assignment tasks (such as for boundary conditions). As our model has a total of 95 faces, creating these sets will save us lots of time and effort!
Our first two entity sets are for the flow inlet and outlet faces. In order to create them, first select the desired face, then the ‘+’ icon to the right of the ‘Topological Entity Sets’ element in the right panel. Name the sets accordingly:
We will also create entity sets for each electronic chip faces and the PCB surface:
All the remaining faces will be categorized as walls, as they constitute the surfaces of the case and other elements. In order to avoid selecting all faces one by one and risk leaving any face out, we will use the ‘Invert selection’ tool:
First, select all the entity sets created until this point, so all their faces are part of the active selection.
Then, right click on the graphics area and select the option ‘Invert selection’ from the pop-up menu. You can visualize the selection change.
Now we are sure that all remaining faces are selected to create our final entity set for the walls.
Third Step: Simulation Setup
Now that we have our entity sets ready, it’s time to set up our simulation parameters. We will specify the gravity field, boundary conditions and computation parameters.
Adding Gravity to the Model
The gravitational field is necessary for the buoyancy effect, so weights in the fluid are correctly computed. Gravity parameters are found in the ‘Model’ element. Setup the gravity vector pointing downwards in the Y direction:
We make use of the Air model in the SimScale’s standard materials library. Click the ‘+’ icon next to the ‘Materials’ element in order to add a new material model, then select ‘Air’ from the library list:
Then assign the fluid domain volume to the newly Air material entry (it is selected by default):
In this entry we can assign initial conditions to internal variables, if in need to do so. This could help speed up the convergence of the calculation. In our case, default values will do.
We will employ each of the created topological entity sets to apply the boundary conditions as follows. A new boundary condition is added with the ‘+’ icon next to the ‘Boundary conditions’ element. Remember that the topological entities sets are selected from the panel at the right.
Flow velocity inlet boundary condition at the ‘Inlet’ face, with a magnitude of 0.1 m/s in the negative Y direction. The inlet air temperature is 20°C.
Pressure outlet boundary condition at the ‘Outlet’ face, with zero gauge pressure.
Thermal Load on Chip 1
No-slip wall boundary condition at the chip 1 faces with a fixed temperature value of 48°C.
Thermal Load on Chip 2
Equal to the chip 1 condition, but this time with a temperature value of 91°C
Thermal Load for Board
For the board we will also use a wall condition with a fixed temperature value, this time of 24°C.
Finally, for the last boundary condition, the remaining walls are modeled as adiabatic walls. For this, we use the zero-gradient temperature option at the wall parameters.
Numeric Solution Parameters
Under this element, numerical solvers can be tuned to improve the performance of the calculation, like accelerating convergence or sacrificing precision for quick results. In this case, we will use default values, which are usually suitable to get a valid solution.
For the simulation control parameters we will use the following values:
We leave mesh parameters as default. Notice that the mesh will be computed automatically as part of the simulation run, including automatic boundary layer inflation refinement!
Fourth Step: Run the simulation
In order to start the computation, we need to create a ‘Simulation run’. Select the ‘Simulation runs’ element. On the ‘New run’ dialog, give it a proper name and click the ‘Start’ button to begin the computation.
The computation will take a little less than an hour to complete, so wait until it is done to continue with the following steps.
Fifth Step: Analyzing the Results
First let’s take a look at the convergence plot by selecting the ‘Convergence plot’ element under the finished simulation run. We can see that the maximum residuals for all fields have fallen below 2e-4, which is a low value, and a good indication that the computation was successful.
Now lets visualize the temperature distribution around the chips. For that, select the ‘Solution Fields’ element under the finished simulation run. The posprocessor tool will load. Then:
Select the ‘+’ icon next to the ‘Cutting Planes’ option to create a new cut view.
Make sure the normal is Z.
Use the ‘Point’ slider to move the plane until it cuts both chips.
Select the ‘Temperature’ scalar field to be visualized.
In order to get a better visualization, we will change the colorbar range. For that, double click the colorbar and change the ‘Range mode’ to ‘Manual’. Then set the ‘Min value’ to ‘293 °K’ (~20 °C) and the ‘Max value’ to ‘306 °K’ (~33°C).
Finally, to visualize the air flow directions, we create a vector plot. This is done by selecting the ‘Cutting Plane 1’ we previously created, and selecting ‘Velocity’ for the vector field.
Last updated: March 23rd, 2020
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