CADFamily.com: Electronics Cooling with Natural Convection and Radiation

Goals

This workshop models 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 a casing, which is open at the top and bottom.

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

Loading Mesh (Workbench)

Open a new Workbench project and save it as HeatSink.wbpj
Look in the Component Systems section of the toolbox and drag a CFX system onto the Project Schematic
Double-click Setup to start CFX-Pre

In CFX-Pre, right-click Mesh and select Import Mesh > ANSYS Meshing
Select HeatSink.cmdb and click Open

Options

In the tree expand Case Options, double-click General and ensure that Automatic Default Domains is switched on and Automatic Default Interfaces is active.
Set the Interface Method to One Per Domain Pair. Click OK.

Create Fluid Domains

First add a domain for the fluid region. The effects of buoyancy must be included, as the flow is driven by natural convection. The buoyancy reference density represents the density at the ambient conditions.

Right-click on Flow Analysis 1 and insert a new domain named Fluid
Open the details for Fluid and set the Location to Fluid
Set the Material to Air Ideal Gas
Switch the Buoyancy option to Buoyant and set the directional components to (0, -g, 0)

– Click on the expression button to enter –g

Set the Reference Density to 1.1093 [kg m^-3]
Click the Fluid Models tab
Set Heat Transfer to Thermal Energy and Turbulence to None (Laminar)
Click OK

Creating Materials

CFX contains a library of many materials, but for this case we will create user materials for the component and Printed Circuit Board (PCB).

In the tree right-click on Materials and select Insert > Material. Name it ComponentMat
Define the material as a Pure Substance in the CHT Solids Material Group
Enable Thermodynamic State and select Solid

– This must be set to allow it to be used in a solid domain

Click the Material Properties tab and set Density to 1120 [kg m^-3]
Select Specific Heat Capacity and set it to 1400 [J kg^-1 K^-1]
Expand Transport Properties and set Thermal Conductivity to 10 [W m^-1 K^-1]
Select OK

Repeat steps 1-7 to create PCBMat using

– Density = 1250 [kg m^-3]

– Specific Heat Capacity = 1300 [J kg^-1 K^-1]

– Thermal Conductivity = 0.35 [W m^-1 K^-1]

Create Solid Domains

This case contains three different solid parts that use different materials. Each part will be created as a different domain.

Insert a new domain called HeatSink
Set the Location to HeatSink
Set the Domain Type to Solid Domain with the Material set to Aluminium
Click OK to create the domain

– Note that an interface between the two domains is automatically created

Repeat steps 1-4 to create a solid domain called Component located at IC using the Material ComponentMat, and a further solid domain called PCB located at PCB using PCBMat

Adding Energy Source

The component is generating 75 [W] of heat which must be added to the simulation. To add this energy source in CFX, a subdomain must be created.

1.In the tree right-click on the Component domain and select Insert > Subdomain, using the name Chip

2.Set the Location to IC so the subdomain occupies the whole of the Component domain

3.Switch to the Sources tab and check the Sources box and the Energy box

4.Set the Option to Total Source, enter75 [kg m^2 s^-3] then click OK

Boundary Conditions

For this case all of the heat will be extracted by the air passing over the heat exchanger so all solid walls will be defined using adiabatic settings. Within the simulation heat can pass between all of the solid and fluid domains because interfaces have been automatically created.

To allow air to enter or leave the simulation domain, the top and bottom face of the fluid domain are defined as openings.

Right-click on the Fluid domain and insert a new boundary called Walls and set the Boundary Type to Wall
Set the Location to Wall
Switch to the Boundary Details tab and check that Heat Transfer is set to Adiabatic then click OK
In the PCB domain rename PCB Default to PCBwalls and check that Heat Transfer is set to Adiabatic

In the Fluid domain rename Fluid Default to Openings and check that the Location is set to be the two ends of the fluid domain
In the Basic Settings tab change the Boundary Type to Opening
In the Boundary Details tab set the Mass and Momentum option to Opening Pres. and Dirn with a relative pressure of 0 [Pa]
Set Heat Transfer to Opening Temperature at 45 [C]