Electronic Cooling with Thermo-Fluid Simulation

This simulation tutorial will guide you through:

• Choosing a fluid material and changing its coefficients
• Activating thermo-fluid physics
• Setting up flow and thermal boundary conditions
• Setting up fluid and thermal environment settings
• Meshing fluid Volume
• Simulate and post-process CFD results

Note

For internal flow problems, a CAD program is needed to extract fluid volume from its corresponding solid model. An example on how to do so using Onshape can be found here. In this tutorial, this step is skipped.

Import the CAD file

1. In OnScale Solve, from the Projects tab of the dashboard, create a new project.
2. In the Tool Bar, click (+) and then Library. Under OnScale Library, Select Enclosure.
3. Select Meters as the length unit.

Assign a Fluid material to the geometry

1. In the Model Tree, select the part Enclosure.
2. Using the Material dropdown in the properties panel, search for Air and assign to the selected part. ( Note: Fluid Analysis will only work with “Fluids” material)
3. Click on the pencil icon to edit the material properties
   
4. Edit the material properties to make sure they are setup as following:

• Mass Density: 1.225 kg/m3
• Specific Heat: 1000 J/(kg*K)
• Thermal Conductivity: 0.02514 W/(m*K)
• Thermal expansion: 0.00369 1/K
• Dynamic Viscosity: 1.8375e-5 Pa.S

5. Click on Done

Setup Fluid boundary conditions

1. Select the  Physics
2. In the Physics Tree, toggle off  Mechanical Analysis and toggle on Fluid Physics
3. Click on the arrow at the left of Enclosure to display the surfaces in the CAD model.
   
4. In the physics toolbar, under the Fluid Physics icon , select -> Flow and assign it to the face named “Inlet” under part Enclosure. Enter 1 m/s in the properties panel and click Done.
   
5. In the toolbar, under the Fluid Physics icon , select -> Pressure and assign it to face named “Outlet” under part Enclosure. Click Done.
   
6. Select Fluid Environment settings under Fluid Physics and in the property panel, enter the following values:

• Duration:0.05
• Characteristic Length:0.05
• Contraction Area Ratio:2

Note: The Contraction Area Ratio influences the size of the time step used to calculate the transient fluid results, so increasing it will also increase the accuracy of the results obtained, but at the same time it will increase the simulation as well. For thermo-fluid, a Contraction Area Ratio superior to 1 is recommended.

Setup Thermal boundary conditions

1. Select the  Physics
2. In the Physics Tree, toggle off  Mechanical Analysis and toggle on Thermal Physics
   
3. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “chip_1” under part Enclosure. Enter 100 °C in the properties panel and click Done.
   
4. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “chip_2” under part Enclosure. Enter 80 °C in the properties panel and click Done.
   
5. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “chip_3” under part Enclosure. Enter 110 °C in the properties panel and click Done.
   
6. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “chip_4” under part Enclosure. Enter 100 °C in the properties panel and click Done.
   
7. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “chip_5” under part Enclosure. Enter 90 °C in the properties panel and click Done.
   
8. In the physics toolbar, under the Thermal Physics icon , select -> Heat Flux and assign it to the faces named “Heat Sink” under part Enclosure. Enter 60 W/m2 in the properties panel and click Done.
   
9. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “condensators” under part Enclosure. Enter 110 °C in the properties panel and click Done.
   
10. In the physics toolbar, under the Thermal Physics icon , select -> Temperature and assign it to the faces named “Inlet” under part Enclosure. Enter 20 °C in the properties panel and click Done.
    
    Important: A thermal boundary condition (Temperature or Heat Flux) must be applied to the Inlet or it will be considered as a floating boundary condition and set automatically as an outlet for heat simulation. Inlets must be assigned with a Flow and a Thermal Boundary Condition.
11. Select Thermal Environmentt settings under Thermal Physics and in the property panel, enter the following values:

• Ambient Temperature: 20 °C
  

Note: Ambient Temperature represents the initial temperature of the fluid when the simulation starts.

Run a simulation

1. Select the simulator
2. Click on the Global Mesh once the meshing is completed (It will appear in orange)
3. In the right panel, toggle on Advanced Mesh Settings to have some indication about the mesh size.
4. Click on Coarsen to generate a coarser mesh size.
   

Note: This mesh size is calculated based on the Characteristic Length defined in the fluid settings, thus it is important to choose a Characteristic Length that represents well your geometry.

Run a simulation

1. Select the simulator
2. Click on the Launcher to run the simulation.
3. Once the meshing and estimation is complete, click on the gear icon to display the Core/Time Adjustment Panel.
4. Drag the blue dot to the right to increase the number of Core used to solve this problem and fasten the simulation.
   
5. Select Launch to run the simulation study.
   

Note 1: CFD Simulation can take a lot of time to obtain results that are reaching the Steady State, use the Core/Time Adjustment Panel wisely to accelerate your simulation and get results faster.

Note 2: CFD Simulation in OnScale Solve uses the Lattice Boltzmann Method (LBM) to compute the results. This methods takes a bit longer than traditional RANS-Based CFD solvers, but it is more accurate as it simulates the turbulence rather than approximating it with a model which might lead to taking many disputable hypotheses.

Analyze the results

1. Once the simulation has been completed, select Fluid Study 1 in the tree and click on Load Results to open the results in the Results tab.
2. In the tree, click Global Sensor.
3. In the properties panel on the right, select an Output Dataset of interest. Then, expand Slices, and create a Slice.
4. In the Legend panel, Choose Rainbow to change the color Palette.
5. Change the Bounds in the Legend and bring the max to around 1.60 m/s
   
6. Expand Timesteps, and click the play button to advance through the time frames.

Note: When the mesh is coarse, some values higher than expected can appear into the corners of the model, those are just numerical artifacts present in very small areas that can be ignored. Changing the min/max bounds of the legends will help you to get a more realistic contour.

Velocity Magnitude Results

Temperature Results

Note: You can also change the bounds of the legend to set the Maximum temperature as you wish. When mesh is coarse, simulation might provide temperature results higher than expected.

Conclusion

Mesh Size is pretty coarse for this model, but it is enough for a first simulation. Once you verified that your simulation runs and results are looking close to what you expect, you can decrease the mesh size, increase the simulation time and increase the contraction area ratio and re-calculate to get more accurate simulation results.