Air flow over a printed circuit board is far more complex than one would normally think. It is not simply flow down a square duct. A circuit board contains obstacles which cause recirculation zones (shear layers) on the lea side of these components. These
recirculation zones have the lowest heat transfer rates because hotter air becomes stagnant in these regions. Under some conditions these recirculation zones can become transient, and move around, affecting the heat transfer coefficients from the electronics components to the air flow. It is virtually impossible to predict the heat transfer from the board to the air with empirical convection coefficients. This is where CFD comes in. The CFD computation in ANSYS Iceboard computes the air flow from the fundamental 3D Navier Stokes equations and includes a direct solution of heat transfer. There is no application
of an empirical heat transfer coefficient in the computation of the energy dump in ANSYS Iceboard. This information is all resident in the CFD calculation.
The following animation describes the CFD solution over a simple square block.
An applied heat flux is maintained on the block. The animations show the transient x component of the velocity field showing the buildup of the recirculation zone behind the block. The temperature field animation shows the hot recirculation zone and its affect on the heat transfer. Notice how the hot temperature point creeps around the back side. At some point, the buildup of the recirculation zone slows and eventually the breaks down and becomes transient, evolving into a vortex street. Now imagine the complexity of multiple blocks, with interacting shear layers and heated zones. The application of the CFD solution in ANSYS Iceboard is imperative to an accurate assessment of the board temperatures
