Heat transfer simulation
As processing units become smaller and omnipresent efficient thermal design is a key factor for successful appliances. This example illustrates a heat transfer simulation of a circuit board. The CPU is given a fixed temperature of 75 °C, an arbitrarily chosen value, and full buoyancy is solved for air flowing though with an ambient temperature of 25 °C. Around the CPU an isocontour of 30 °C is visualized. The air flow through the enclosed circuit board system is fixed at 0.1 m/s. Streamlines coloured by velocity illustrate some of the flow features.
To generate this model a CAD model of a basic circuit board was generated, cleaned up, and written as stereolythography (stl) file. The stl file is used for direct meshing using OpenFOAM's snappyHexMesh.
In the video below some more of the dynamics of flow and heat transfer are illustrated. The top image in the video shows the change of termal iso-surfaces around the CPU. For a better view of the CPU and the contour part of the circuit board is made transparent.
In the bottom view in the video streamlines and iso-surfaces of helicity are shown. Helicity is the inner product of velocity and vorticity, showing the "knottedness" or topology of vorticial structures.
The above qualitative data is useful, but typically a quantitative metric of performance is useful in a sensitivity study. In this case cooling of the CPU is a key design feature and one can readily extract CPU surface temperature, near-surface temperature, and heat flux over time. The graph below shows the total wall heat flux on the surface of the CPU in mW. The temperature of the surface of the CPU is held constant at 120 °C in this example. For the first 1.5 s an oscillating inlet condition is used for the velocity field and the compressible solver uses an LES turbulence model. Between 1.5 and 2 s simulated time the inlet condition is changed to a random perturbation of velocity (10% in the streamwise direction and 2% and 3% in the normal directions, respectively). The change in inlet condition results in a clear change in CPU wall heat flux, i.e. cooling efficiency.
