Aerospace & Defense Technology - August 2021 - 6

Avionics Cooling
For example, if the heat generated by
avionic circuitry does not exceed the
thermal resistance (°C/W) of a passive
heat sink finned cold plate, then this
would be the preferred method due to
its relatively low cost. Adding forced-air
flow if required increases the cost and
complexity somewhat.
Note: One distinct disadvantage of
passive cold plates is they can only cool
and cannot heat in circuits that are required
to be kept warm when their
power output does not generate enough
thermal energy to keep them within
this temperature range.
Typical Aluminum Heat Sink
Fixed vs. Thermostatic Liquid Control
In liquid-cooled cold plate designs,
coolant is circulated in one of two ways
- fixed flow or thermostatic flow. With
traditional fixed flow through a liquid
cooling design, coolant continuously
moves between the cold plate and the
heat exchanger, regardless of the
coolant's actual temperature. This decreases
cooling efficiency and increases
coolant usage.
To remedy this design limitation,
thermostatic valves are used to direct
coolant flow either to the heat exchanger
or back through the system,
solely based on coolant temperature.
This ensures efficient usage of coolant,
facilitates stable and uniform electronic
device temperatures, and reduces overall
system wear, extending the life of
system components.
Which Cooling Technique Is Best?
This question is best answered by
analyzing the application and is most
aptly driven by the severity of heat
loss required and space restrictions -
if any.
Heat Exchanger
Pump
Cold
Flow
In
Flow Back to Pump
When Cold
When the thermal resistance of a passive
heat sink with forced-air flow is exceeded
or space limitations prohibit the
use of the larger finned plates, the liquid
cooled cold plate becomes the next
best choice. Compact and efficient, liquid
cooling (or heating) is ideal for designs
with space constraints and high
thermal output circuitry, making them
a very good fit for many aircraft applications.
With the addition of a diverting
valve into the cooling loop, the coolant
sensed from the cold plates outlets will
either be recirculated back to the plates
if colder than desired or diverted to the
heat exchanger when the preferred operating
temperature is reached.
To help optimize the size of the recirculating
pump the addition of self-operating
flow restrictor valves can be added to
the outlet of each cold plate. These valves
serve to reduce the flow to an individual
plate when cold, then open fully when
operating temperature is reached. This
way the pump does not have to supply all
plates with 100% flow continuously -
only when each individual plate's output
temp reaches the desired temperature.
In Conclusion
Flow to H/X
When Hot
M/D in
Diverter
Mode
Hot Out
Cold Plates x4
Example of a Liquid Cooling System
6
Intro
Cov
www.aerodefensetech.com
ToC
+
-
For less complex systems, forced air
and cold plates may satisfy basic thermal
management needs. However, as
UAV designs become more complex and
compact requiring cooling or possibly
heating, design engineers are likely to
continue turning to liquid cooling to
solve their thermal management needs.
This article was written by Glenn Quinty,
Senior Design Engineer, ThermOmegaTech,
Inc. (Warminster, PA). For more information,
visit http://info.hotims.com/79416-500.
Aerospace & Defense Technology, August 2021
A
µ
È
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Aerospace & Defense Technology - August 2021

Table of Contents for the Digital Edition of Aerospace & Defense Technology - August 2021

Aerospace & Defense Technology - August 2021 - Intro
Aerospace & Defense Technology - August 2021 - Sponsor
Aerospace & Defense Technology - August 2021 - Cov1
Aerospace & Defense Technology - August 2021 - Cov2
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Aerospace & Defense Technology - August 2021 - Cov3
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