Results
The computation for this model takes about 30 seconds and about 3 GB of memory. The longer computational time is due to the extra degrees on freedom corresponding to the velocity, pressure, and temperature in the air domain.
Add a predefined plot as a result template showing temperature and velocity field and hide some walls from the display to visualize the temperature in the heat sink and on the channel walls together with the flow structure.
Temperature and flow in the channel
1
In the Home toolbar, click  Result Templates.
2
Go to the Result Templates window and select Study 1/Solution 1 (sol1) > Nonisothermal Flow 1 > Temperature and Fluid Flow (nitf1).
3
Click Add Result Template  and close the Result Templates window.
Filter 1
1
In the Temperature and Fluid Flow (nitf1)  3D Plot Group, expand the Fluid Flow  and select Filter 1 .
2
Locate the Element Selection section and, in the associated text field, type spf.U>1.2*nitf1.Uave.
Geometry 1
1
In the Model Builder window, under Component 1 (comp1) click Geometry 1 .
2
In the Graphics toolbar, click the Click and Hide button . Then, click the Select Boundaries button .
3
In the Graphics window, select the boundaries corresponding to the inlet, the outlet, the upper boundary of the channel and the left boundary.
4
In the Model Builder window, select the Temperature and Fluid Flow (nitf1) node to display the temperature field as well as the fluid flow.
Duplicate this plot and keep only temperature subnodes to focus on the temperature distribution.
Temperature of the channel walls and heat sink
1
In the Model Builder window, right-click Temperature and Fluid Flow (nitf1) and choose Duplicate .
2
In the Settings window for 3D Plot Group, type Temperature of the channel walls and heat sink in the Label text field.
3
Expand Temperature and Fluid Flow (nitf1), right-click Fluid Flow  and choose Delete . Click Delete to confirm.
4
Select the Temperature and Fluid Flow (nitf1) node. In the Settings window for 3D Plot Group, locate the Color Legend section and clear the Show units checkbox.
Adding Surface-to-Surface Radiation Effects
Now modify the model to include surface-to-surface radiation effects. First, add the surface-to-surface radiation physics interface to the model. Then, study the effects of surface-to-surface radiation between the heat sink and the channel walls.
Surface-to-Surface Radiation (rad)
In the Home toolbar, click the Add Physics button .
Add Physics
1
In the Add Physics window select Heat Transfer > Radiation > Surface-to-Surface Radiation (rad). 
2
Click Add to Component 1 .
3
Click Add Physics button  again to close the Add Physics panel.
Surface-to-Surface Radiation (rad)
1
In the Model Builder window, under Component 1 (comp1), click Surface-to-Surface Radiation (rad) .
Specify that the radiation interface is active only on the heat sink and channel walls.
2
In the Settings window for Surface-to-Surface Radiation, locate the Boundary Selection section.
3
From the Selection list, choose Exterior boundaries without heat sink base (Heat Sink - Straight Fins 1).
4
To add the channel walls, click the Paste Selection button and type 1 3 4 44 (note that boundaries 1 and 4 have been hidden previously and are not visible in the graphics window).The interface selection now contains boundaries 1, 3, 4, 6, 7, 9, 11–16, 22, 24–29, 31–35, 37, and 39–44.By default the radiation direction is controlled by the opacity of the domains. The solid parts are automatically defined as opaque while the fluid parts are transparent. You can change this setting by using the Opacity feature in the Surface-to-surface Radiation interface. When the Diffuse Surface boundary condition defines the Emitted radiation direction as opacity controlled (the default setting), the selected boundaries should be located between an opaque and a transparent domain. The exterior is defined as transparent by default. Change the default setting to make the exterior opaque and have the radiation direction automatically defined on the channel walls.
Opacity 1
1
In the Physics toolbar click Domains  and choose Opacity .
2
In the Settings window for Opacity, locate the Domain Selection section.
3
From the Selection list, choose All voids which corresponds to the geometry exterior.