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Tutorial: Forced Convection Heat Transfer

Overview

In this tutorial, we demonstrate how to perform a simulation of the forced convection heat transfer for a Raspberry Pi enclosure.

Use the following link in order to copy the project that contains the geometry, in order to follow the tutorial steps:

Import Tutorial Project Into Your Workbench

First Step: Create a New Simulation

As a first step, we will create a new simulation inside the project. In order to do so, select the ‘Raspberry Pi’ element under Geometry, then click the button ‘Create Simulation’ in the pop up dialog.

Create a new simulation for the Raspberry Pi geometry
Create a new simulation for the Raspberry Pi geometry

As it is our goal, select the ‘Convective Heat Transfer’ analysis type in the ‘Create Simulation’ dialog, the the button ‘Create Simulation’.

Select the ‘Convective Heat Transfer’ simulation model
Select the ‘Convective Heat Transfer’ simulation model

Second Step: Create Topological Entities

Topological entities are sets of geometry features (faces in this case) that can be reused for different assignment tasks (such as for boundary conditions). As our model has a total of 95 faces, creating these sets will save us lots of time and effort!

Our first two entity sets are for the flow inlet and outlet faces. In order to create them, first select the desired face, then the ‘+’ icon to the right of the ‘Topological Entity Sets’ element in the right panel. Name the sets accordingly:

Create ’Inlet’ topological entity set
Create ’Inlet’ topological entity set
 Create ‘Outlet’ topological entity set
Create ‘Outlet’ topological entity set

We will also create entity sets for each electronic chip faces and the PCB surface:

 Create ‘Chip 1’ topological entity set
Create ‘Chip 1’ topological entity set
Create ‘Chip 2’ topological entity set
Create ‘Chip 2’ topological entity set
Create ‘Board’ topological entity set
Create ‘Board’ topological entity set

All the remaining faces will be categorized as walls, as they constitute the surfaces of the case and other elements. In order to avoid selecting all faces one by one and risk leaving any face out, we will use the ‘Invert selection’ tool:

  • First, select all the entity sets created until this point, so all their faces are part of the active selection.
  • Then, right click on the graphics area and select the option ‘Invert selection’ from the pop-up menu. You can visualize the selection change.
Use the ‘Invert selection’ tool
Use the ‘Invert selection’ tool

Now we are sure that all remaining faces are selected to create our final entity set for the walls.

Create ‘Walls’ topological entity set
Create ‘Walls’ topological entity set

Third Step: Simulation Setup

Now that we have our entity sets ready, it’s time to set up our simulation parameters. We will specify the gravity field, boundary conditions and computation parameters.

Adding Gravity to the Model

The gravitational field is necessary for the buoyancy effect, so weights in the fluid are correctly computed. Gravity parameters are found in the ‘Model’ element. Setup the gravity vector pointing downwards in the Y direction:

Setup the gravitational field
Setup the gravitational field

Materials

We make use of the Air model in the SimScale’s standard materials library. Click the ‘+’ icon next to the ‘Materials’ element in order to add a new material model, then select ‘Air’ from the library list:

SimScale’s standard materials library
SimScale’s standard materials library

Then assign the fluid domain volume to the newly Air material entry (it is selected by default):

Assign the standard air material to the fluid domain
Assign the standard air material to the fluid domain

Initial Conditions

In this entry we can assign initial conditions to internal variables, if in need to do so. This could help speed up the convergence of the calculation. In our case, default values will do.

Boundary Conditions

We will employ each of the created topological entity sets to apply the boundary conditions as follows. A new boundary condition is added with the ‘+’ icon next to the ‘Boundary conditions’ element. Remember that the topological entities sets are selected from the panel at the right.

Flow Inlet

Flow velocity inlet boundary condition at the ‘Inlet’ face, with a magnitude of 0.1 m/s in the negative Y direction. The inlet air temperature is 20°C.

Inlet velocity boundary condition
Inlet velocity boundary condition

Flow Outlet

Pressure outlet boundary condition at the ‘Outlet’ face, with zero gauge pressure.

Outlet pressure boundary condition
Outlet pressure boundary condition

Thermal Load on Chip 1

No-slip wall boundary condition at the chip 1 faces with a fixed temperature value of 48°C.

Wall boundary condition with fixed temperature at chip 1
Wall boundary condition with fixed temperature at chip 1

Thermal Load on Chip 2

Equal to the chip 1 condition, but this time with a temperature value of 91°C

Wall boundary condition with fixed temperature at chip 2
Wall boundary condition with fixed temperature at chip 2

Thermal Load for Board

For the board we will also use a wall condition with a fixed temperature value, this time of 24°C.

Wall boundary condition with fixed temperature for the board
Wall boundary condition with fixed temperature for the board

Adiabatic Walls

Finally, for the last boundary condition, the remaining walls are modeled as adiabatic walls. For this, we use the zero-gradient temperature option at the wall parameters.

Wall boundary condition with zero-gradient temperature for the board
Wall boundary condition with zero-gradient temperature for the board

Numeric Solution Parameters

Under this element, numerical solvers can be tuned to improve the performance of the calculation, like accelerating convergence or sacrificing precision for quick results. In this case, we will use default values, which are usually suitable to get a valid solution.

Simulation Control

For the simulation control parameters we will use the following values:

Simulation control parameters
Simulation control parameters

Mesh Settings

We leave mesh parameters as default. Notice that the mesh will be computed automatically as part of the simulation run, including automatic boundary layer inflation refinement!

Fourth Step: Run the simulation

In order to start the computation, we need to create a ‘Simulation run’. Select the ‘Simulation runs’ element. On the ‘New run’ dialog, give it a proper name and click the ‘Start’ button to begin the computation.

Start new simulation run
Start new simulation run

The computation will take a little less than an hour to complete, so wait until it is done to continue with the following steps.

Simulation run is finished
Simulation run is finished

Fifth Step: Analyzing the Results

First let’s take a look at the convergence plot by selecting the ‘Convergence plot’ element under the finished simulation run. We can see that the maximum residuals for all fields have fallen below 2e-4, which is a low value, and a good indication that the computation was successful.

Residuals convergence plot
Residuals convergence plot

Now lets visualize the temperature distribution around the chips. For that, select the ‘Solution Fields’ element under the finished simulation run. The posprocessor tool will load. Then:

  1. Select the ‘+’ icon next to the ‘Cutting Planes’ option to create a new cut view.
  2. Make sure the normal is Z.
  3. Use the ‘Point’ slider to move the plane until it cuts both chips.
  4. Select the ‘Temperature’ scalar field to be visualized.
Temperature field on the cutting plane
Temperature field on the cutting plane

In order to get a better visualization, we will change the colorbar range. For that, double click the colorbar and change the ‘Range mode’ to ‘Manual’. Then set the ‘Min value’ to ‘293 °K’ (~20 °C) and the ‘Max value’ to ‘306 °K’ (~33°C).

Improved colorbar scale
Improved colorbar scale

Finally, to visualize the air flow directions, we create a vector plot. This is done by selecting the ‘Cutting Plane 1’ we previously created, and selecting ‘Velocity’ for the vector field.

Final plot with temperature and flow visualization
Final plot with temperature and flow visualization

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