Aerospace & Defense Technology - August 2021 - 5

design
engineers
have several
cooling options
at their disposal including
various styles of
heat sinks, forced air systems
and fans, heat pipes, and
others. While the method implemented
often depends on design restrictions
such as space and thermal load,
each method has its own inherent advantages
and disadvantages that must
be considered.
Natural & Forced Air Convection
Natural and forced air convection systems
were the original cooling method
for early UAV's and are often the least
costly option available.
Air provides thermal relief simply by
flowing through the system either freely
through vents in a natural convection
design or propelled via fans in forced
convection systems. To facilitate heat
transfer, heat sinks are often integrated
with the heat producing device.
Despite the benefits of simple design
and the abundance of coolant available
in the Earth's atmosphere, aircooled
systems are limited in their
thermal management capabilities. Air
can only remove so much heat, therefore
these systems' cooling capabilities
typically cannot compensate for the
amount of heat generated by modern
UAV electronics. In addition, using unfiltered
air may pose additional complications
in applications where maintaining
isolation from the external
environment is required.
Heat Sinks - Passive / Radiant
Radiant heat sinks, which are essentially
metal cold plates with cooling
fins, are efficient at removing heat due
to their increased surface area exposed
to the secondary cooling system, typically
forced air.
In common use, heat sinks feature a
metal object brought into contact with
an electronic component's hot surface.
In most cases, a thin thermal interface
Aerospace & Defense Technology, August 2021
Intro
Cov
In days gone by, engines like this vintage biplane engine could be air cooled.
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ToC
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material (TIM) such as thermal transfer
paste mediates between the two surfaces
to maximize the thermal transfer rate.
The thermal resistance from junction to
case of the semiconductor device is usually
stated in units of °C/W. For example,
a heatsink rated at 10°C/W will get
10°C hotter than the surrounding air
when it dissipates 1 Watt of heat. Thus,
a heatsink with a low °C/W value is
more efficient than a heatsink with a
high °C/W value.
Due to heat sink size demands to accommodate
both fins and a forced air
system, designs implementing this
cooling method must often be larger in
scale.
Liquid
In liquid cooling system designs,
coolant runs through the cold plate,
removing heat and releasing it through
a heat exchanger. Using this method,
the cold plates are kept at a fairly even
temperature, avoiding temperature
spikes and allowing for effective thermal
transfer.
This also applies to systems exposed
to temperatures approaching cryogenic
in which a heating fluid rather than a
cooling fluid passes through the plates
to keep the electronic system within an
optimal operating temperature range.
Cold Plate
In this arrangement, the heat source
is cooled under a thick plate (cold plate)
instead of being cooled in direct contact
with the cooling fluid. The thick plate
can significantly improve the heat
transfer between the heat source and
the cooling fluid by way of conducting
the heat current in an optimal manner.
A TIM is still used at the interfaces to
maximize the energy transfer. The most
attractive advantage of this method is
that no extra heat transfer surface area
is required as with the fins (extended
surfaces) used in passive heat sinks.
To accommodate the increasing thermal
relief demands of modern electronics,
design engineers have turned to liquid-cooled
cold plates. Cold plates use a
metal plate to remove heat from power
electronics. Electronic devices are
mounted onto the metal, facilitating
heat transfer to a cooling fluid that runs
through the cold plate. While typically
a simple and compact cooling method,
cold plates must also be paired with an
additional cooling method to remove
heat from the cooling fluid. This reliance
on a secondary cooling method
may complicate designs and severely
limit the cold plate's thermal management
capabilities in complex or enclosed
systems.
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