Modern unmanned aerial vehicles (UAVs) and military aircraft carry advanced electronics and equipment critical to their successful operation. All electronic devices and circuitry generate excess heat and thus require thermal management to improve reliability and prevent premature failure. The amount of heat output is generally equal to the power input so long as there are no other energy interactions, so selecting an appropriate thermal relief system is often obvious and limited, depending on the amount of heat that must be removed.

Innovation is carrying airborne technologies farther and higher than ever before, and avionic cooling practices have had to evolve to keep up.

Cooling Options

To accommodate the immense heat generated by modern UAV electronics, 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, air-cooled 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 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.

In days gone by, engines like this vintage biplane engine could be air cooled.

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.

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.

Example of a Liquid Cooling System

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.

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.

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

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 here .


Aerospace & Defense Technology Magazine

This article first appeared in the August, 2021 issue of Aerospace & Defense Technology Magazine.

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