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.