Now that we have a heat sink geometry perfectly aligned with the CPUs, we can move to performing
the thermal analysis of the model.
Go to the Simulation tab.
Let’s first set the materials of our components.
Click on the Materials tool and select the heat sink.
The predefined material is Structural Steel, we can open the dropdown menu and then select
Copper.
Hide the heat sink and select the CPUs.
For these components we will use Silicone instead.
In order to better understand the effect of the heat sink on the CPUs temperature, let’s
start with a thermal analysis of the CPUs only.
In the model tree click on the circle icon
for the heat sink to exclude it from the simulation.
Now let's specify boundary conditions for this heat transfer analysis.
For any type of analysis, appropriate boundary conditions need to be specified which are
then used for solving equations in the background and obtain the final results.
In this thermal analysis problem, we would be determining the temperature of the CPUs
based on the heat generated inside the CPUs.
So, the boundary condition that we would be specifying is heat generation.
Click on Solid Thermal in the ribbon menu.
Draw a box to select the three CPUs and set up a heat volumetric source of 200 Watts.
This value would be divided among the CPUs based on their volume.
Once this condition is set a natural convection boundary condition appears by default and
is applied to the solids faces.
This defines the convection of heat from the hotter CPUs to their colder surroundings.
In our case we will assume that the bottom faces of the CPUs are actually insulated,
meaning that they do not exchange heat with the surroundings.
Select the three faces and set the condition to insulated, clicking on the left most option
in the thermal selections HUD menu.
This automatically updates the convection boundary condition.
Now only the sides and top faces exchange heat with the surrounding fluid.
In our case we can assume that the CPUs will be exposed to air at a temperature of 20 degrees
Celsius and that only natural convection occurs.
Hence, we can keep the default convection coefficient and reduce the temperature from
22 to 20 degrees Celsius.
We are ready to start our simulation.
Save the model first.
Now click on the start simulation button near the bottom right corner of the screen.
Wow, the simulation is already done!
This is indicated by border of the SID or the Simulation Information Display which turned
from yellow color to green color.
Also, the chips now show different colors.
These colors represent the different temperatures of the chips surfaces.
The temperatures corresponding to these colors can be seen in the Legend.
The red color indicates higher temperatures while the blue color indicates lower temperatures
in the model.
Look at the temperature of the chips!
They are hotter than the Sun!
Why is that?
Well, in reality, the material would first melt before reaching those temperatures and
the computer would fail.
In our simulation we are not using any complex model to keep track of that.
With this simple simulation we can understand that the CPUs alone would burn out without
a system that could let us limit their temperatures.
Now click again on the same button to stop the simulation.
Go to the model tree and make the heat sink visible and include it to the simulation by
simply clicking on the two icons in red.
If you hover the mouse on the convection boundary condition, you can see that by default it
is been applied to all the heat sink faces.
As done for the CPUs we would need to set the bottom face to an insulated condition.
Hide the results from the previous simulation to visualize only the geometry.
Click on contours on the results arc to deactivate them.
Next click on Solid Thermal to set up the boundary condition, rotate the model and select
the bottom face and set it to insulated.
Then press the green checkmark to confirm the selection.
In the Physics menu now you can see a new item set up by default, the bonded contacts.
This would let Discovery account for the heat transfer due to conduction between the CPUs
and the heat sink at their contact points.
Start again the simulation.
The final results will quickly appear.
Pause again the simulation.
Click on the show chart button right under the Solve button.
This shows a relevant plot for the simulation we are analyzing.
In this case it reports the value of maximum temperature in the model.
As you can see both from the plot and the legend, the temperature drastically reduced
thanks to the heat sink.
The temperature in the model is not uniform.
This is because the different heat that each chip is releasing and also because of their
different shapes and positions.
For a better visualization of the location of the maximum temperatures you can hover
the mouse on the Contours icon in the results arc and let the options appear.
Then change the Surface display priority to Highest value.
In this way you can see through the model and identify the regions of high temperatures.
Analyze the model and then save it to prepare for the next session.