CFD Pro 14.5 WS06 Electronics Cooling CFX

7/16/2019 CFD Pro 14.5 WS06 Electronics Cooling CFX

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© 2013 ANSYS, Inc. December 12, 2013 1 Release 14.5

14. 5 Release

Introduction to ANSYS

CFD Professional

Workshop 06

Electronics Cooling




• This workshop models the heat dissipation from a hot electronics

component fitted to a printed circuit board (PCB) via a finnedheat sink. The PCB is fitted into a casing, which is open at the top

and bottom.

Introduction

Introduction Setup Solving Postprocessing Summary




• 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 on to 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

(workshop_input_files\WS_08_Electronics

Cooling ) and click Open

Loading Mesh (Workbench)





• In the tree expand Case Options, double-click

General and ensure that Automatic DefaultDomains is switched on and Automatic

Default Interfaces is active

• Set the Interface Method to One Per Domain

Pair

• Click OK

Options





• First add a domain for the fluid region.

– Right-click on Flow Analysis 1 and insert a new domain

named Fluid

– Set the Location to Fluid

– Set the Material to Air at 25 C

– Click the Fluid Models tab

– Set the heat transfer option to Thermal Energy and

turbulence option to k-Epsilon

– Click OK

Fluid Domains





• In the tree right-click on Materials and select

Insert > Material . Name it ComponentMat

• Define the material as a Pure Substance in the

Material Group called CHT Solids

•Enable Thermodynamic State and select Solid – It can then 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]

• Click OK

Creating MaterialsCFX contains a library of many materials, but for this case we will create user materials

for the component and Printed Circuit Board (PCB).



http://slidepdf.com/reader/full/cfd-pro-145-ws06-electronics-cooling-cfx 7/20© 2013 ANSYS, Inc. December 12, 2013 7 Release 14.5

• Repeat the material creation steps on the previous page to

make a new solid material, named PCBMat

– Density = 1250 [kg m^-3]

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

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

Creating Materials




• 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 the above steps to create a solid domain called Component located at IC using the Material ComponentMat

• Create a further solid domain called PCB located at PCB using the Material

PCBMat

Solid Domains

You may see some red physics warnings indicating an update to conditions is required.

These messages will go away when the Wall boundaries are created later on and can beignored for now.

When all 4 domains are created the Default Domain will automatically be removed from

the tree. Separate interfaces between each domain will have been automatically

created, rather than combined into a single interface




• Insert > Subdomain > in the domain,

Component

– Set Name to Chip

• Set Location to IC, so that the subdomain

occupies the whole of the domain called

Component

• Go to the Sources tab and check the

Sources box

• Click the Energy box

• Set the Option to Total Source and enter a

Total Source of 75 [kg m^2 s^-3]

• Click OK

Energy SourceThe 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




• Right-click on the domain called Fluid 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

• Click OK

• In the PCB domain rename PCB Default to PCBwalls and check that Heat

Transfer is set to Adiabatic

Boundary ConditionsFor 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 be transferred

between all of the solid and fluid domains because interfaces have been automatically created.





• In the Fluid domain insert a new boundary called Inlet

• On the Basic Settings tab change the Boundary Type to Opening and set the

Location to Open1

• 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, entering a temperature of 45 [C]

• Click OK

Boundary Conditions

The Opening Pressure and Opening Temperature options set Total values when flow is

entering the domain and Static values when flow is leaving.

The use of Total values for inflow in effect represents the case in which the flow outside

the domain is accelerated from rest before entry.


The flow of air cooling the component will be driven by forced convection. A fan will draw air

through the upper boundary at a rate of 5 m3 min-1. This will be implemented as a mass flow outlet

using an expression. Flow into the domain will be through an opening boundary on the lower face.The steps below explain how to set the inflow boundary.




• In the Fluid domain rename the boundary, Fluid Default, to Fan and open it for

editing

• On the Basic Settings tab change the Boundary Type to Outlet

• In the Boundary Details tab set the Mass and Momentum option to Mass FlowRate

• For the mass flow rate provide the expression 5 [m^3 min^-1]*1.185 [kg m^-3].

The density of Air at 25 C is 1.185 kg m-3.

• Click OK

Boundary Conditions


For the outlet boundary representing the fan, we need to convert the known volumetric flow rate

(5 m3 min-1) into a mass flow.




• From the tree right-click Solver Control and

select Edit

• Increase the Max. Iterations to 500

• Change the Fluid Timescale Control set to

Physical Timescale and set the value to 1 [s]

• Leave Solid Timescale set to Auto Timescale

– Note that solid regions will use a much larger

timescale than fluid regions because only the

energy equation is being calculated within the

solid

• Reduce the Residual Target to 1e-6

• Click OK

Solver Control





We are interested in the temperature

reached in the Component and so will createa monitor to track the maximum value

• From the tree right-click Output Control and

select Edit

•

Select the Monitor tab and Check theMonitor Objects box

• Click the New icon and name the monitor

MaxTempComponent

• Set ‘Option’ to ‘Expression’ and enter the

expressionmaxVal(Temperature)@Component

• Click OK

Output Control





• Select File > Close CFX-Pre and Save the Project

•

In the Project Schematic , right-click on Solution andselect Update

• While the solver is running right-click on Solution

again and select Display Monitors to check on

progress

•

It takes about 100 iterations for the monitorMaxTempComponent to level out. To save time,

stop the run by going to the Project Schematic ,

right-clicking on the Solution cell and selecting

Interrupt Update.

• To view a completed run, drag and drop into the

Project Schematic the file called CFX_001.res fromthe folder “results”. Then right-click on the Solution

cell of the new system (B) and select Display

Monitors

Solving the Simulation





• In the Project Schematic double-click on the Results cell

B3, or right-click and choose Edit, to open thecompleted results in CFD-Post

Opening CFD-Post


• The results for a simulation

with a higher ventilation rate

of 7.5 m3 min-1 are also

available in the “results”folder. In CFD-Post we can

compare the two runs.

• Load the second results file by

selecting File > Load Results...

and select CFX_002.res. WhenCFD-Post is run from

Workbench, the option Keep

current cases loaded is active




• Click on to synchronise the cameras

• Create a YZ plane (Location > Plane). Name it

Centre, set X to 0 [m] and colour it according to

Temperature

• Create a contour plot (Insert > Contour ) . Use

the solid sides of the fluid-solid interfaces as

the location (use the ‘…’ icon and Ctrl key to

select multiple locations from both

configurations. Refer to the regions in the

Outline Tree to check which side is which). Setthe Variable to Temperature and Range to

Global

TemperatureTemperature is a key variable for any electronics cooling application. So we will display it in

several locations, such as within the flow, on the surfaces of the solid region and by extracting the

maximum temperature within the component. When these plots are created they appear in theUser Locations and Plots section of the tree.





• To create a plot of the differences

between the two results, we willswitch on Case Comparison

• Doubles click on Case Comparison in

the Outline Tree and in the details

check the box by Case Comparison

Active

• The plots are shown on the next

slide. CFX_001, the lower flow rate,

is on the left-hand side.

Temperature





Temperature





• Select the Calculators tab and double click

on Function Calculator ( ). Set theoptions to:

– Function: maxVal

– Location: Component

– Case: All Cases

–Variable: Temperature

• Click Calculate

Temperature


In the run set up in this workshop, the maximum

temperature of the Component was monitored. We

will now calculate this value for the two simulations

along with the difference between them

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CFD Pro 14.5 WS06 Electronics Cooling CFX

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