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ME-430 INTRODUCTION TO COMPUTER AIDED DESIGN TRANSIENT and STEADY STATE THERMAL ANALYSIS OF

HEATSINK Pro/ENGINEER and Pro/MECHANICA Wildfire 2.0

Dr. Herli Surjanhata

Background A steady state thermal analysis calculates effects of constant thermal loads on a model and is used to determine temperatures, heat flow rates, and the heat fluxes in a part. A steady state analysis is commonly used as a precursor to a transient thermal analysis to determine the initial conditions.

A transient thermal analysis is used to determine the temperature, heat storage and other thermal quantities in a model due to a time varying load. Because the applied load may be time dependent, the solution is time dependent. All other considerations such as the type of the thermal load and the modes of heat transfer are the same as in a steady state analysis. Transient analysis is probably the most common form of thermal analysis.

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Problem Statement An engineer must attach copper heat sink to a CPU. The heat sink will be subjected to a time varying 50 Watt load and a constant convection of 100W/mC. The time required to reach steady state will be determined. A fringe plot of the temperature distribution after 1000 seconds will be made.

Create the Heat Sink Part

Create a new part called heat_sink.prt

Edit -> Setup -> Units Select Meter Kilogram Second (MKS) in the Units Manager dialog box. Click on Set button.

Choose Interpret dimensions OK -> Close to close the Units Manager dialog box.

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Create a block 0.062 m width X 0.08 m depth x 0.04 m height. Extrude both sides.

Make a cut with the dimensions shown below.

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Pattern the cut with increment of 0.004 m and total of 15 cuts.

Create a cut to accommodate the fan at the top of the heat sink.

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Create another cut to facilitate the installation clamp. The dimensions of the cut are shown below.

The final part is shown in the following figure.

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Transfer the Model to Pro/MECHANICA

From Applications pull-down menu select

Mechanica

Make sure the Units are correct, then click the Continue button

For Model Type, choose Thermal.

Click the OK button.

Create a Surface Region On the top of CPU, the size of area in direct contact with the heat sink when the heat sink is installed is 0.012 m x 0.012 m. This contact area is situated in the middle of the bottom surface of the heat sink. To apply the heat source in this contact area, a surface region should be created.

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From Insert pull-down menu, select

Surface Region

Or click on the Create a Simulation Surface Region button on the right toolbar.

Sketch -> Done

Pick the bottom surface of the heat sink as sketching plane.

Select Right under SKET VIEW, and pick RIGHT datum plane as a horizontal or vertical reference for sketching.

Make sure to include FRONT datum plane as additional reference.

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Create two centerlines to ensure the symmetry, then create a square with 0.012 as the sides.

Click .

When prompted with Select surface or surfaces to be split, Select surface to add, pick anywhere on the surface (bottom surface of the heat sink).

OK -> Done -> OK to complete the surface region creation.

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Assign Material to the Part

From Properties pull-down menu, select Materials.

Or click the Define Material button from the right toolbar.

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Assign CU as the material of the heat sink.

Assign -> Part

Pick the heat sink.

Click the OK button.

Click the Close button.

Generate the Elements Using AutoGEM The model can be autogemmed during the run, or before the run, if materials have been assigned.

Select AutoGEM -> Create

Or click the Create p-mesh for Geometric Element Modeling button.

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Close.

File -> Save Mesh.

Close.

Assign Boundary Conditions Convection Condition Create a convection condition on all surfaces except the bottom.

The temperature inside the computer is assumed to be a constant 30 degrees C. Although MKS units are defined, degrees C can be interchanged with degrees K if this is done consistently for all inputs and outputs. Prescribed temperature boundary conditions are generally not used in transient thermal analyses. They may conflict with the initial model temperature.

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From Insert pull-down menu, select

Convection Condition

Choose Surface.

Or click - New Surface Convection Condition.

Bndry Conds -> New -> Conv Cond -> Surface

Click on button under Surface(s).

Pick all surfaces except the square area (surface region) located in the middle of BOTTOM SURFACE.

Enter 100 for Convection Coefficient h, and 30 for Bulk Temperature.

OK

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Tip: For easy surface selection choose Box Select then drag a bounding box. Hold CTRL and select the surface region (square area) located in the middle bottom surface to deselect it.

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Place Heat Loads Place a time varying heat load of 50 watts on the base surface.

From the Insert pull-down menu, choose

Heat Load.

Select Surface

Or click on New Heat Load on Surface icon in the toolbar.

Pick the square surface region on the bottom surface of the heat sink.

Enter 50 for Q, and click the Time Dependent radio button as the heat load in the problem varies with time.

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Select the Function button on the Heat Load definition form.

The Function Definition dialog box appears.

Under Description, enter the following text:

CPU heats up and reaches maximum output of 50 W at 150 seconds.

Under Type change to Table.

Click on Add Row button.

Fill in the dialog box as shown below.

Click OK.

Enter the time, and value as shown on the figure on the left.

The value in the right hand column is set to after 150 seconds. This value of 1 is a multiplier for the Q load of 50 Watts.

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A graph of the function can be seen by selecting Review.

Click the Graph button.

Create a Transient Thermal Analysis Create a transient thermal analysis with Single-Pass Adaptive convergence method. For some runs, the Quick Check convergence method may be used to get a quick reading of the model. Specify 22 C for uniform initial temperature. Typically the estimated variation is left set to Auto. In this case the user estimates the heat sink will start at 22 C and reach a maximum of about 52 C so a value is input. This preliminary knowledge is most useful when Automatic output intervals are used. Tip: Optionally, a temperature gradient from a previously completed steady state analysis can be used by setting Initial Temperature Distribution to MEC/T instead of Uniform.

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Analysis -> Mechanica Analyses/Studies

Or click on Run a Design Study in the top toolbar.

From File pull down menu, select

New Transient Thermal

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Under Name, type in Transient_Analysis.

Under Description, type in the following:

Determine the temperature of the CPU heat sink during the first 1000 seconds after the computer is booted.

Click on BndyCondSet1 to highlight it. Both BndyCondSet1 and ThermLoadSet1 are highlighted.

Enter 22 for the initial Temperature.

Keep the rests default.

Click on Convergence tab, and make sure Single-Pass Adaptive method is selected.

Click on Output tab.

Under Output Intervals, select User-defined Output Intervals.

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Change the Plotting Grid to 4.

Change the Number of Master Intervals to 40.

Click on User Defined Steps

button.

Click on to unchecked all.

Drag the scroll toolbar to the last interval number 40.

Enter 1000 seconds for number 40.

Check on Full results at row number 40.

Click on

OK.

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Run the Transient Thermal Analysis

Run the analysis by clicking

.

Click Yes.

Click to check the analysis status.

When the run is completed without error.

Close the window.

Close the Analyses and Design Studies dialog box.

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Review the Results

Click on icon.

Enter Window Name as temp for temperature distribution.

Enter Title Temperature Distribution of Heat Sink

Under Design Study and Analysis, make sure that Transient_Analysis is selected.

Note that Transient_Analysis is the name of analysis previously given.

Click Display Options.

Click this icon.

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Make sure you fill in the following form (window) as shown below:

Check on Continuous Tone.

Click the OK and Show button.

Format -> Result Window

Enter the setting as shown on the left.

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Click on .

Enter the Name of max_dyn_temp.

Type in the Title of Graph of Max Dynamic Temp.

Under Display type, select Graph.

Select Measure in the Quantity box.

Click on .

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Select max_dyn_temperature.

Click on OK.

Click on OK in the Result Window Definition dialog box.

View -> Display. Or click on in the toolbar.

Select max_dyn_temp.

Click on OK.

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File -> Exit Results.

Click on Yes button.

Type in File Name: transient for the results to be saved.

OK.

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Click the Close button.

Save the model.

Steady State Thermal Analysis

Expand the Loads/Constraints in the Model Tree until you see HeatLoad1.

Left-click HeatLoad1, then right-click it.

Select Edit Definition.

Uncheck Time Dependent option, and click OK -> Done.

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Create a Steady State Thermal Analysis

From Analysis pull-down menu, select Mechanica Analyses/Studies

From File pull down menu, select

New Steady State Thermal

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Name the analysis as Steady_State.

Convergence Method: Multi-Pass Adaptive.

Set the Maximum Polynomial Order to 9.

Convergence: Local Temperatures and Local Energy Norms 5 %.

OK.

With the Name Steady_State highlighted, run the analysis by

clicking .

Click Yes.

Click to check the analysis status.

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When the run is completed without error.

Close the window.

Close the Analyses and Design Studies dialog box.

Review the results.

Click .

Create a Temperature Fringe Plot

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Enter Window Name as temp_steady for temperature distribution.

Enter Title Steady State Temperature Fringe Plot of Heat Sink

Make sure you fill in the following form (window) as shown below:

Click Display Options.

Check on Continuous Tone.

Check on Show Element Edges.

Un-check Show Loads and Show Constraints.

Click the OK and Show button.

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Create a Capping Surface Display

Turn off Show Element Edges, and display the heat sink in default orientation.

Insert -> Cutting/Capping Surfs

Under Type, select Capping Surface.

OK.

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To delete Capping Surface display, select

Edit -> Delete Capping Surface.

Create a Maximum Temperature Convergence Graph.

Click on to copy the definition. Fill in the Result Window Definition dialog box as shown below.

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Click on OK button. Display the maximum temperature convergence graph by selecting View -> Display, and select conv.

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Click on to copy the definition. Fill in the Result Window Definition dialog box as shown below.

Click on OK button. Display the maximum temperature convergence graph by selecting View -> Display, and select flux.

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Save the all result windows -> and also Save the model before exiting Pro/ENGINEER.