Electronics Cooling with Natural Convection and Radiation Part A

Goals

In this workshop, you will model the heat dissipation from a hot electronics component fitted to a printed circuit board (PCB) via a finned heat sink.

The PCB is fitted into an enclosure which is open at the top and bottom.

Initially only the heat transfer via convection and conduction will be calculated. The effect of thermal radiation will then be included as a later stage.

Mesh Import (Workbench)

This workshop can be done either inside or outside of ANSYS Workbench.
If working outside of Workbench, you should skip this page.

Open a new Workbench session and select a new FLUENT session from Component Systems

Use Save As to save the session.

Import the the mesh file.

– Right-click on the Setup cell.

– Change Files of Type to Fluent
Mesh File

– Select the mesh file heatsink.msh

– Click Open.

Launch FLUENT using the default options.

Mesh Import (Stand-alone)

Start a 3D FLUENT session from the icon or from the Windows Start menu

Select either

– File ? Read ? Mesh from the top
menu

– Open File icon from toolbar

Open the file heatsink.msh

Check the grid to verify that there are no errors in the mesh.

View the model:

Display the mesh and color the faces by ID:

– Select Graphics and Animations

– Highlight Mesh, then Setup just below

Set Faces to on, and Edges to Feature

Deselect all currently selected faces

Select Surface Types Wall, Pressure Outlet and Velocity Inlet (note effect on Surfaces list)

Select Colors and Color by ID.

Display

– Select the Lights button, and turn on headlight.

– Make the outer walls transparent

Use Scene button

Select wall_left, wall_right and wall_top

Select Display and set transparency to roughly 50

Apply and close Display Property panel

Apply and close Scene Description panel

– Redisplay the image (Use Setup and Display buttons as above)

Model setup

1. Display the mesh and adjust the display settings.

a) Highlight Mesh and click Setup.

i. Select Feature and Edges. Set Edge Type to Feature.

ii. Deselect all currently selected faces

iii. Select Surface Types Wall, Pressure Outlet and Velocity Inlet (note effect on Surfaces list)

iv. Select Colors and Color by ID.

v. Click Display

1. Change temperature units to °C Define ? Units

1. Select Temperature as a Quantity
2. Select c as the temperature units.
3. Close the panel.

3. Enable the energy equation.

a.Select the Models tree item

b.Double-click on Energy and enable the equation.

Comments on Model setup

General

– It is good practice to display the grid after import to check for any boundary zone misassignment and that you have opened the correct model.

– Workbench uses SI units (meters, kg etc) but if importing a mesh from another source check the scale and dimensions are correct.

– Check mesh is used to confirm the mesh is suitable for use in a CFD simulation.

– Report Quality is a backup to the quality tools available within the meshing application.

By default the energy equation is not solved to reduce CPU load because many problems are isothermal. In this case, temperature must be calculated so the energy equation needs to be enabled.

The onset of turbulence is specified by the Reynolds Number (pipe flow) or Rayleigh Number (natural convection). Calculating these numbers using boundary conditions indicates that the flow will be laminar.

Material properties

The air density needs to change with temperature (but not pressure)

– Select Materials à Air à Create/Edit

– Change density to incompressible
ideal gas

– All other properties remain unchanged

– Click Change / Create then close the fluid materials window.

Define two additional solid materials (for the board and the heat sink).

– Select Materials à Solid à Create/Edit

– Click the FLUENT Database button.

– Change Type to Solid

– Select Copper

– Copy then close the database window

Modify the copper material to produce two different materials.

– The PCB is made of material Fr-4.

Change Name to fr-4

Delete the chemical formula

All other properties remain unchanged

Click Change/Create.

Click No when prompted to overwrite copper.

– Selecting No will create a new material Fr-4, but copper remains in the material list.

– Selecting Yes will overwrite the copper material for the current case only.

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