The chassis dissipates almost 200 W and operates on military aircraft reaching altitudes up to 50,000 ft. Flotherm from Flomerics Inc., Marlborough, Mass., provided the simulation software.

Hybricon engineer Michael Palis says the ATR uses forced-air cooling, so fan performance, heat-sink design, and the amount of airflow going through the enclosure are critical. Problems arise because electronics are difficult to cool at high altitudes, where air has less density than at sea level. Airflow through the device must rise by a large factor just to maintain the same cooling level.

Among other items, the chassis contained a fan, nine PCBs, two power supplies, and two cold walls for mounting various components. Cards in the unit conduct heat to their edges through traces. PCB fasteners provide connections between the PCB and the conductioncard case. The fans draw ambient air across twin heat sinks. The critical requirement: The chassis must maintain PCB temperatures at 75°C or less at high altitudes.

“We created a Flotherm model using simple cuboid elements to represent most components,” Palis says. “We modeled heat dissipation as a 70% transfer from the PCB to the card cage, and a 30% transfer through the fastener. We used planar sources to simulate heat loadings, and did not consider convection in the card cage.”

The fan curve came from the manufacturer’s Web site, but Palis modified it to account for varying air density at different altitudes. He selected a fan from the Flotherm library with performance close to the actual fan, tweaked parameters, and dropped the modified fan on the cooling model.

“We also optimized the number of fins on the heat sink,” says Palis. “We set up the software to vary parameters over a range, as well as monitor points on the edges of each PCB to track the performance of each design. The software ran each iteration and simulated flow velocity and temperatures in the enclosure.”

Palis swapped in fans until the simulation showed temperatures well below 75°C. The real ATR chassis, based on Flotherm results, performed almost exactly as predicted by the simulation.