Models.heat.Electronic Enclosure Cooling

7/21/2019 Models.heat.Electronic Enclosure Cooling

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Solved with COMSOL Multiphysics 5.0

1 | F O R C E D C O N V E C T I O N C O O L I N G O F A N E N C L O S U R E W I T H F A N A N D G R I L L E

Fo r c e d Con v e c t i o n Coo l i n g o f a n

En c l o s u r e w i t h F an and G r i l l e

Introduction

This study simulates the thermal behavior of a computer power supply unit (PSU).

Such electronic enclosures typically include cooling devices to avoid electronic

components being damaged by excessively high temperatures. In this model, anextracting fan and a perforated grille generate an air flow in the enclosure to cool

internal heating.

Air extracted from the enclosure is related to the static pressure (the pressure difference

between outside and inside), information that is generally provided by the fan

manufacturers as a curve representing fluid velocity as a function of pressure difference.

As shown in Figure 1, the geometry is rather complicated and requires a fine mesh tosolve. This results in large computational costs in terms of time and memory.

Figure 1: The complete model geometry.


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Model Definition

Figure 1shows the geometry of the PSU. It is composed of a perforated enclosure of14 cm-by-15 cm-by-8.6 cm and is made of aluminum 6063-T83. Inside the

enclosure, only obstacles having a characteristic length of at least 5 mm are

represented.

The bottom of the box represents the printed circuit board (PCB). It has an

anisotropic thermal conductivity of 10, 10, and 0.36 W/(mK) along thex-,y-, and

z-axes, respectively. Its density is 430 kg/m3and its heat capacity at constant pressure

is 1100 J/(kgK). Because the thermal conductivity along thez-axis is relatively low,and that the PCB and the enclosure sides are separated by a thin air layer, it is not

necessary to model the bottom wall, nor take into account the cooling on these sides.

The capacitors are approximated by aluminum components. The heat sink fins and the

enclosure are made of the same aluminum alloy. The inductors are mainly composed

of steel cores and copper coils. The transformers are made of three materials: copper,

steel, and plastic. The transistors are modeled as two-domain components: a core made

of silicon held in a plastic case. The core is in contact with an aluminum heat sink toallow a more efficient heat transfer. Air speed is assumed to be slow enough for the

flow to be considered as laminar.

The simulated PSU consumes a maximum of 230 W. Components have been grouped

and assigned to various heat sources as listed in Table 1. The overall heat loss is 41 W,

which is about 82 % of efficiency.

The inlet air temperature is set at 30 C because it is supposed to come from the

computer case in which air has already cooled other components. The inlet boundary

is configured with a Grilleboundary condition. This pressure must describe head loss

caused by air entry into the enclosure. The head loss coefficient kgrilleis represented

TABLE 1: HEAT SOURCES OF ELECTRONIC COMPONENTS

Components Total power (W)

Transistor cores 25

Large transformer coil 5

Small transformer coils 3

Inductors 2

Large capacitors 2

Medium capacitors 3

Small capacitors 1


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by the following 6thorder polynomial (Ref. 1):

(1)

where is the opening ratio of the grille.

Figure 2: Head loss coefficient as a function of the opening ratio.

The head loss, P(Pa),is given by

where is the density (kg/m3), and U is the velocity magnitude (m/s).

The box, the PCB, the inductor surfaces, and the heat sink fins are configured as highly

conductive layers.

kgrille 120846

422815

609894

465593

199632

4618.5 462.89

+

+ +

=

P kgrilleU

2

2-----------=


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Results and Discussion

The most interesting aspect of this simulation is to locate which components aresubject to overheating. Figure 3clearly shows that the temperature distribution is not

homogeneous.

Figure 3: Temperature and fluid velocity fields.

The maximum temperature is about 73 C and is located at one of the transistor cores.

The components furthest away from the air inlet are subject to the highest

temperature. Although transistor cores are rather hot, Figure 3shows that they are

significantly cooled by the aluminum heat sinks. The printed circuit board has a

significant impact as well by distributing and draining heat off.

On the flow side, air avoids obstacles and tends to go through the upper space of the

enclosure. The maximum velocity is about 1.7 m/s.


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Figure 4shows the head loss created by obstacles encountered by air on its path.

Figure 4: Pressure isosurfaces show the head loss created by components.

As shown in Figure 4, the fluid flow is impacted by obstacles and yields to local head

losses.

Notes About the COMSOL Implementation

To model heat sink fins, enclosure walls and circuit board it is strongly recommended

to use the Thin Layerfeature, which is completely adapted for thin geometries and

significantly reduces the number of degrees of freedom in the model.

COMSOL Multiphysics provides a useful boundary condition for modeling fan

behavior. You just need to provide a few points of a static pressure curve to configure

this boundary condition. These data are, most of the time, provided by fan

manufacturers.

Reference

1. R.D. Blevins,Applied Fluid Dynamics Handbook, Van Nostrand Reinhold, 1984.


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Model Library path: Heat_Transfer_Module/

Power_Electronics_and_Electronic_Cooling/

electronic_enclosure_cooling

Modeling Instructions

The file electronic_enclosure_cooling_geom.mph contains a parameterized

geometry and prepared selections for the model. Start by loading this file.

1 From the Filemenu, choose Open.

2 Browse to the models Model Library folder and double-click the file

electronic_enclosure_cooling_geom.mph .

D E F I N I T I O N S

1 On the Modeltoolbar, click Functionsand choose Global>Analytic.

Define an analytic function to represent the polynomial expression of the head loss

coefficient (Equation 1). This coefficient is a function of the open ratio ORof the

grille.

Ana lytic 1 (an1)

1 In the Settingswindow for Analytic, type k_grillein the Function nametext field.

2 Locate the Definitionsection. In the Expressiontext field, type

12084*OR^6-42281*OR^5+60989*OR^4-46559*OR^3+19963*OR^2-4618.5*OR+

462.89.

3 In the Argumentstext field, type OR.

4 Locate the Unitssection. In the Argumentstext field, type 1.

5 In the Functiontext field, type m^-4.

6 Locate the Plot Parameterssection. In the table, enter the following settings:

7 Click the Plotbutton.

Argument Lower limit Upper limit

OR 0 0.8


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M A T E R I A L S

In this section, you define the materials of the enclosure and its components. The

prepared selections make it more easy to select the appropriate domains andboundaries.

A D D M A T E R I A L

1 On the Modeltoolbar, click Add Materialto open the Add Materialwindow.

2 Go to the Add Materialwindow.

3 In the tree, select Built-In>Air.

4 Click Add to Componentin the window toolbar.

M A T E R I A L S

Air (mat1)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Air (mat1).

2 In the Settingswindow for Material, locate the Geometric Entity Selectionsection.

3 From the Selectionlist, choose Air.



2 In the tree, select Built-In>Acrylic plastic.


M A T E R I A L S

Acr yl ic plast ic (mat2)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Acrylic

plastic (mat2).


3 From the Selectionlist, choose Plastic.



2 In the tree, select Built-In>Aluminum 6063-T83.



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M A T E R I A L S

Aluminum 6063-T83 (mat3)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Aluminum

6063-T83 (mat3).


3 From the Geometric entity levellist, choose Boundary.

4 From the Selectionlist, choose Aluminum Boundaries.

A D D M A T E R I A L1 Go to the Add Materialwindow.

2 In the tree, select Built-In>Steel AISI 4340.


M A T E R I A L S

Steel AISI 4340 (mat4)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Steel AISI

4340 (mat4).


3 From the Selectionlist, choose Steel Parts.



2 In the tree, select Built-In>Aluminum.


M A T E R I A L S

Aluminum (mat5)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Aluminum

(mat5).


3 From the Selectionlist, choose Capacitors.




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2 In the tree, select Built-In>Copper.


M A T E R I A L S

Copper (mat6)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Copper

(mat6).


3 From the Selectionlist, choose Transformer Coils.



2 In the tree, select Built-In>Copper.


M A T E R I A L S

Copper (2) (mat7)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Copper (2)

(mat7).



4From the

Selectionlist, choose

Copper Layers.



2 In the tree, select Built-In>FR4 (Circuit Board).


M A T E R I A L S

FR4 (Circuit Board) (mat8)

1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick FR4 (Circuit

Board) (mat8).




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4 From the Selectionlist, choose Circuit Board.

Modify the thermal conductivity of the FR4 material to model anisotropic

properties of a printed circuit board.

5 Locate the Material Contentssection. In the table, enter the following settings:



2 In the tree, select Built-In>Silicon.


4 On the Modeltoolbar, click Add Materialto close the Add Materialwindow.

M A T E R I A L S

Silicon (mat9)1 In the Model Builderwindow, under Component 1 (comp1)>Materialsclick Silicon

(mat9).


3 From the Selectionlist, choose Transistors Silicon Cores.

The next steps define the boundary conditions of the model.

L A M I N A R F L O W ( S P F )

1 In the Model Builderwindow, under Component 1 (comp1)click Laminar Flow (spf).

2 In the Settingswindow for Laminar Flow, locate the Domain Selectionsection.


The thin heat sink fins are represented by interior boundaries in the geometry. An

interior wall condition is used to prevent the fluid from flowing through these

boundaries.

Interior Wall 1

1 On the Physicstoolbar, click Boundariesand choose Interior Wall.

2 In the Settingswindow for Interior Wall, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Fins.

Property Name Value Unit Property group

Thermal conductivity k {10, 10, 0.3} W/(mK) Basic


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Fan 1

1 On the Physicstoolbar, click Boundariesand choose Fan.

Here, the fan condition is set up by loading a data file for the static pressure curve.

2 In the Settingswindow for Fan, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Fan.

4 Locate the Flow Directionsection. From the Flow directionlist, choose Outlet.

COMSOL Multiphysics displays an arrow indicating the orientation of the flow

through the fan. Compare with the figure below.

5 Locate the Parameterssection. From the Static pressure curvelist, choose Static

pressure curve data.

6 Locate the Static Pressure Curve Datasection. Click Load from File.

7 Browse to the models Model Library folder and double-click the fileelectronic_enclosure_cooling_fan_curve.txt .

8 Locate the Static Pressure Curve Interpolationsection. From the Interpolation

function typelist, choose Piecewise cubic.

The exhaust fan previously defined extracts air entering from an opposite grille.

Proceed to create the corresponding boundary condition.


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2 In the Settingswindow for Heat Source, type Large Transformer Coil Heat

Sourcein the Labeltext field.

3 Locate the Domain Selectionsection. From the Selectionlist, choose LargeTransformer Coil.

4 Locate the Heat Sourcesection. Click the Overall heat transfer ratebutton.

5 In thePtottext field, type 5.

Heat Source 3

1 On the Physicstoolbar, click Domainsand choose Heat Source.

2 In the Settingswindow for Heat Source, type Small Transformer Coils HeatSourcein the Labeltext field.



5 Locate the Domain Selectionsection. From the Selectionlist, choose Small

Transformer Coils.

Heat Source 41 On the Physicstoolbar, click Domainsand choose Heat Source.

2 In the Settingswindow for Heat Source, type Inductor Heat Sourcein the Label

text field.

3 Locate the Domain Selectionsection. From the Selectionlist, choose Inductors.


5 In thePtot

text field, type 2.

Heat Source 5


2 In the Settingswindow for Heat Source, type Large Capacitors Heat Sourcein

the Labeltext field.

3 Locate the Domain Selectionsection. From the Selectionlist, choose Large Capacitors.



Heat Source 6


2 In the Settingswindow for Heat Source, type Medium Capacitors Heat Source

in the Labeltext field.


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3 Locate the Domain Selectionsection. From the Selectionlist, choose Medium

Capacitors.



Heat Source 7


2 In the Settingswindow for Heat Source, type Small Capacitors Heat Sourcein

the Labeltext field.

3 Locate the Domain Selectionsection. From the Selectionlist, choose Small Capacitors.



Temperature 1

1 On the Physicstoolbar, click Boundariesand choose Temperature.

2 In the Settingswindow for Temperature, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Grille.

4 Locate the Temperaturesection. In the T0text field, type T0.

Thin Layer 1

1 On the Physicstoolbar, click Boundariesand choose Thin Layer.

To model heat transfer in thin conductive parts of the enclosure, use the Thin Layer

boundary condition on thin domains made of aluminum, copper and FR4.

2 In the Settingswindow for Thin Layer, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Highly Conductive Layers.

4 Locate the Thin Layersection. From the Layer typelist, choose Conductive.

5 In the dstext field, type 2[mm].

Outflow 1

1 On the Physicstoolbar, click Boundariesand choose Outflow.

2 In the Settingswindow for Outflow, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Fan.

M E S H 1

You now configure the meshing part. Because a two-level multigrid solver is planned

to be launched a few steps later, two meshes are created here.


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In the first mesh, you start by discretizing the surfaces of key components. They would

drive the tetrahedral mesh of the whole domain. Boundary layers at walls are added at

the end.

Mapped 1

1 In the Model Builderwindow, under Component 1 (comp1)right-click Mesh 1and

choose More Operations>Mapped.

2 In the Settingswindow for Mapped, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Wire Group Surface.

4 Click to expand the Advanced settingssection. Locate the Advanced Settingssection.Select the Adjust evenly distributed edge meshcheck box.

Size 1

1 Right-click Component 1 (comp1)>Mesh 1>Mapped 1and choose Size.

2 In the Settingswindow for Size, locate the Element Sizesection.

3 From the Predefinedlist, choose Finer.

4 Click the Build Selectedbutton.

Mapped 2

1 In the Model Builderwindow, right-click Mesh 1and choose More

Operations>Mapped.

2 In the Settingswindow for Mapped, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Small Wire Surface.

4 Locate the Advanced Settingssection. Select the Adjust evenly distributed edge meshcheck box.

Size 1

1 Right-click Component 1 (comp1)>Mesh 1>Mapped 2and choose Size.


3 From the Calibrate forlist, choose Fluid dynamics.

4 From the Predefinedlist, choose Extra fine.


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Convert 1

1 In the Model Builderwindow, right-click Mesh 1and choose More

Operations>Convert.

2 Right-click Convert 1and choose Build Selected.

This conversion divides the quadrilateral mesh obtained into triangular elements.

This is necessary to make it compatible with the tetrahedral elements created a few

steps later.

Free Triangular 1

1 Right-click Mesh 1and choose More Operations>Free Triangular.

2 In the Settingswindow for Free Triangular, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Heat Exchange Surface.

Size 1

1 Right-click Component 1 (comp1)>Mesh 1>Free Triangular 1and choose Size.




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Free Triangular 2

1 In the Model Builderwindow, right-click Mesh 1and choose More Operations>Free

Triangular.

2 In the Settingswindow for Free Triangular, locate the Boundary Selectionsection.

3 From the Selectionlist, choose Curved Area.

Size 1

1 Right-click Component 1 (comp1)>Mesh 1>Free Triangular 2and choose Size.



4 From the Predefinedlist, choose Coarser.


6 In the Model Builderwindow, right-click Mesh 1and choose Free Tetrahedral.

Size

1 In the Model Builderwindow, under Component 1 (comp1)>Mesh 1click Size.


3 From the Predefinedlist, choose Coarse.

Free Tetrahedral 1

In the Model Builderwindow, under Component 1 (comp1)>Mesh 1right-click Free

Tetrahedral 1and choose Build Selected.

Boundary Layers 1

1 Right-click Mesh 1and choose Boundary Layers.

2 In the Settingswindow for Boundary Layers, locate the Domain Selectionsection.

3 From the Geometric entity levellist, choose Domain.


5 Click to expand the Corner settingssection. Locate the Corner Settingssection. From

the Handling of sharp edgeslist, choose Trimming.

Boundary Layer Properties

1 In the Model Builderwindow, under Component 1 (comp1)>Mesh 1>Boundary Layers

1click Boundary Layer Properties.

2 In the Settingswindow for Boundary Layer Properties, locate the Boundary Selection

section.


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3 From the Selectionlist, choose Walls.

4 Locate the Boundary Layer Propertiessection. In the Number of boundary layerstext

field, type 2.

5 In the Thickness adjustment factortext field, type 5.

6 Click the Build Allbutton.

The second mesh is a refinement of the first one. Hence, duplicate Mesh 1to work

on each element of the mesh sequence.

7 In the Model Builderwindow, right-click Mesh 1and choose Duplicate.

C O M P O N E N T 1 ( C O M P 1 )

In the Model Builderwindow, expand the Component 1 (comp1)>Meshesnode.

M E S H 2

Size

1 In the Model Builderwindow, expand the Component 1 (comp1)>Meshes>Mesh 2

node, then click Size.


3 From the Predefinedlist, choose Normal.


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Size 1

1 In the Model Builderwindow, expand the Component 1 (comp1)>Meshes>Mesh

2>Mapped 1node, then click Size 1.2 In the Settingswindow for Size, locate the Element Sizesection.

3 Click the Custombutton.

4 Locate the Element Size Parameterssection. Select the Maximum element sizecheck

box.

5 In the associated text field, type 0.4.

6Select the

Minimum element sizecheck box.


Size 1


2>Mapped 2node, then click Size 1.




box.


Size 1

1 In the Model Builderwindow, expand the Component 1 (comp1)>Meshes>Mesh 2>Free

Triangular 1node, then click Size 1.




box.


6 Select the Minimum element sizecheck box.


8 Select the Maximum element growth ratecheck box.


10 Select the Curvature factorcheck box.

11 In the associated text field, type 1.


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12 Select the Resolution of narrow regionscheck box.

13 In the associated text field, type 1.

Size 1

1 In the Model Builderwindow, expand the Component 1 (comp1)>Meshes>Mesh 2>Free

Triangular 2node, then click Size 1.


3 From the Predefinedlist, choose Normal.

Boundary Layer Properties


2>Boundary Layers 1node, then click Boundary Layer Properties.

2 In the Settingswindow for Boundary Layer Properties, locate the Boundary Layer

Propertiessection.

3 In the Number of boundary layerstext field, type 3.

4 Click the Build Allbutton.

The building process may take some time due to the high mesh resolution.


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S T U D Y 1

Step 1: Stationary

1 In the Model Builderwindow, expand the Study 1node, then click Step 1: Stationary.

2 In the Settingswindow for Stationary, click to expand the Mesh selectionsection.

3 Locate the Mesh Selectionsection. In the table, enter the following settings:

Solution 11 On the Studytoolbar, click Show Default Solver.

By default, a Multigrid solver is generated. The automatic method for additional

meshes may not be adapted for this particular geometry. In the next steps, create

two multigrid levels with Mesh 1and Mesh 2to control the element distributions.

2 In the Model Builderwindows toolbar, click the Showbutton and select Advanced

Study Optionsin the menu.

Step 1: Stationary

1 Right-click Step 1: Stationaryand choose Multigrid Level.

2 In the Settingswindow for Multigrid Level, locate the Mesh Selectionsection.

3 In the table, enter the following settings:

4 In the Model Builderwindow, right-click Step 1: Stationaryand choose Multigrid

Level.

Now, modify the Multigrid solver to account for your custom multigrid levels.

Solution 1

1 In the Model Builderwindow, expand the Study 1>Solver Configurations>Solution

1>Stationary Solver 1node.2 Right-click Study 1>Solver Configurations>Solution 1>Stationary Solver 1and choose

Fully Coupled.

3 In the Settingswindow for Fully Coupled, locate the Generalsection.

4 From the Linear solverlist, choose Iterative 1.

Geometry Mesh

Geometry 1 mesh2

Geometry Mesh

Geometry 1 mesh1


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5 Click to expand the Method and terminationsection. Locate the Method and

Terminationsection. From the Nonlinear methodlist, choose Automatic (Newton).

6 In the Maximum number of iterationstext field, type 150.

7 In the Model Builderwindow, expand the Study 1>Solver Configurations>Solution

1>Stationary Solver 1>Iterative 1node, then click Multigrid 1.

8 In the Settingswindow for Multigrid, locate the Generalsection.

9 From the Hierarchy generation methodlist, choose Manual.

You are now ready to launch the simulation. The computation may take a few hours

to complete.

10 On the Studytoolbar, click Compute.

R E S U L T S

Velocity (spf)

The first default plot group shows the air velocity profile through a slice plot.

Pressure (spf)

The second default plot group shows the pressure field plot of Figure 4.

Temperature (ht)

The third default plot shows the temperature field. To reproduce the plot in Figure 3

of the temperature and the air velocity, proceed as follows.

1 In the Model Builderwindow, expand the Temperature (ht)node, then click Surface 1.

2 In the Settingswindow for Surface, locate the Expressionsection.

3 From the Unitlist, choose degC.

4 In the Model Builderwindow, right-click Temperature (ht)and choose Streamline.

5 In the Settingswindow for Streamline, click Replace Expressionin the upper-right

corner of the Expressionsection. From the menu, choose Component 1>Laminar

Flow>u,v,w - Velocity field.

6 In the Settingswindow for Streamline, locate the Selectionsection.

7 From the Selectionlist, choose Grille.

8 Locate the Coloring and Stylesection. From the Line typelist, choose Tube.

9 Right-click Results>Temperature (ht)>Streamline 1and choose Color Expression.

10 In the Settingswindow for Color Expression, click Replace Expressionin the

upper-right corner of the Expressionsection. From the menu, choose Component

1>Laminar Flow>spf.U - Velocity magnitude.

S l d i h COMSOL M l i h i 5 0


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11 On the 3D plot grouptoolbar, click Plot.

Solved with COMSOL Multiphysics 5 0


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Models.heat.Electronic Enclosure Cooling