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HEAT TRANSFER IN AEROSPACE SYSTEMS

“ Could Thermal Management For Aerospace Application Is Required ? ”

A capsule coming from space with blunt end of the heat shield
A capsule coming from space with blunt end of heat shield

Heat Transfer In General

Heat is form of energy which is transferred when there is a difference in a temperature between two or more bodies. In some cases transfer of heat energy can be determined by simply applying the laws of thermodynamic and fluid mechanics. Heat can be transferred by three difference means namely: conduction, convection, and radiation. In general, Heat transfer is the important parameter specially for aerospace industry. Numerous improvement techniques, for example, extended surfaces such as the fins are used in heat exchangers to enhance the surface area for heat transfer or the heat transfer coefficient. The aim of improvement techniques could be reduced the size for heat exchangers for a given function to increase the capacity of an existing heat exchangers, or to reduce the approach temperature difference. Implementation of heat transferred enhancement techniques in aerospace heat exchangers might help to reduce fuel consumption and the size of heat exchangers.

Increasing power reqirements and heat dissipation demands in space exploration missions are gradually posing critical challenges to the worldwide space agencies. Various approaches to the heat transfer enhancement have been implemented to increase the efficiency of power utilization and reduce overall weight and volume. Unlike heat transfer in the terrestrial environment, the effects of the variable gravity field, large temperature difference, and corpuscular radiation must be endured in the space systems. Heat transfer in the space environment and its application in aerospace engineering are of great significance in various fields such as electronic cooling, air craft cooling, cryogenic system for astronomical instruments and so on.

Application of heat exhangers in thermal management

The trend to pack current and future aerospace and
military platforms with power-hungry, heat-generating
electronic components and systems drives the need for
efficient, effective, compact, and lightweight thermal
management systems. Shrinking electronics packaging and
high-density integration of power electronics to enable
more power and functionality in a small unit, coupled with
the extremes of military and aerospace environments,
constantly place ever-increasing demands on precise and smart thermal management solutions to maintain junction
temperatures below levels that degrade performance. The
temperature of the components can be maintained at a
safe level by air cooling for low-heat-flux components. As
the heat flux increases, the limits of air cooling technology
are being approached, i.e., forced air heat sinks have
become significantly larger, more expensive, and more
complex. Liquid cooling is promising to provide the
needed level of thermal performance, with an increase in
energy efficiency compared to traditional air cooling. The
liquid must pass through the heat sources to carry away
the heat, resulting in a temperature increase in the liquid.

Then the liquid passes through a liquid-to-air heat
exchanger, such as a plate-fin heat exchanger (PFHE), to
transfer the stored heat to the air. An important
component for liquid cooling is the cold plate. The cold
plate must be able to dissipate the waste heat efficiently
within a relatively small unit. Fig.5 shows examples of cold
plates, namely, the tube liquid cold plate and the
powdered metal cold plate [11], with the former suitable
for low heat fluxes and the latter for high heat fluxes. Cold
plates with embedded microchannels are promising to
dissipate high heat fluxes. In many cases, heat pipes are
embedded in cold plates to increase the effective thermal
conductivity of the cold plates. Basically the cold plates
should be designed by conforming to the heat-generating
components. Besides, the cold plates should be optimized
to augment the thermal performance while maintaining a
relatively low pressure drop penalty by properly
designing, e.g., the liquid flow passages and manifold
distributions inside the cold plates.

Thermal Management And Its Techniques

Thermal Management is the ability to control the temperature of a system by means of Technology based on Thermodynamics and Heat Transfer.

The phrase Thermal Management is therefore describing all possible means and processes like heat transfer, conduction, convection, condensation and radiation, etc. to increase or decrease the temperature and/or the temperature distribution of a specified system.While there are many different cooling products for electronics, three types tend to rise to the top due to their efficiency and cost effectiveness: heat sinks, fans, and Peltier modules. They can all be used separately but for maximum effectiveness, they often benefit from being integrated together.

Heat Sinks

Heat Sinks

A heat sink is one of the most basic, yet commonly used thermal management systems in embedded systems. A basic heat sink does not require any electricity whatsoever and can be used on any kind of device. The design of the heat sink gives it the ability to transfer heat from a higher temperature device, such as an embedded system, to a lower temperature medium. Generally, for most embedded systems, this medium is air. Most heat sinks are created from aluminum alloy because they have some of the highest thermal conductivity values.Also, while they make a significant difference, they are not as effective by themselves as other technologies, which is why they are often paired with fans to move the dissipated heat more effectively out of the application. To learn more about heat sinks, read our “How to select the heat sinks” blog.

Heat Pipes

Heat pipes

Heat pipes can be used to move heat over distances ranges from a few inches (>50mm) to greater than 3 feet (> 1 meter). In a heat pipe, heat from a heat source enters the evaporator end of the heat pipe, causing the working fluid to change phase from liquid to vapor. The vapor travels through the vapor space within the heat pipe to the other end, the condenser end, where a heat sink or other secondary heat dissipation device removes the heat energy. The release of heat in the condenser end causes the vapor to condense back to liquid which is absorbed into a capillary wick structure. The capillary wick structures incorporated into the internal walls of a heat pipe allow the liquid condensate inside the heat pipe to return from the condenser section of the heat pipe to the evaporator section via capillary action.

The heat-moving efficiency of this thermal solution is determined by factors such as wick, working fluid, diameter, length, bending, flattening and orientation.

The four common, commercially produced heat pipe wick structures are grooves in the internal tube wall, wire or screen mesh, sintered powder metal and fiber/spring. Different wicks have varying capillary limits (the capillary pumping rate at which the working fluid travels from condenser to evaporator).

Synthetic jet air cooling

Synthetic jet air cooling is very similar to conductive cooling except a little more advanced. Instead of relying on the hot air rising out of the system, the warmer air is sucked out of the system while the cooler air is jetted into the system. This helps to cool the system much more efficiently than the conductive cooling system. Also, the synthetic jet air cooling system is low maintenance as well, which makes it an excellent option for thermal management for many embedded systems.

Recent Thermal Management Project

Recently, the development of modern vehicles has brought about aggressive integration and miniaturization of on-board electrical and electronic devices. It will lead to exponential growth in both the overall waste heat and heat flux to be dissipated to maintain the devices within a safe temperature range. However, both the total heat sinks aboard and the cooling capacity of currently utilized thermal control strategy are severely limited, which threatens the lifetime of the on-board equipment and even the entire flight system and shrink the vehicle’s flight time and range. Facing these thermal challenges, the USA proposed the program of “INVENT” to maximize utilities of the available heat sinks and enhance the cooling ability of thermal control strategies. Following the efforts done by the USA researchers, scientists in China fought their ways to develop thermal management technologies for Chinese advanced energy-optimized airplanes and spacecraft. Subsequently, active thermal management technologies in China including fuel thermal management system, environment control system, non-fuel liquid cooling strategy are reviewed. At last, space thermal control technologies used in Chinese Space Station and Chang’e-3 and to be used in Chang’e-5 are introduced. Key issues to be solved are also identified, which could facilitate the development of aerospace thermal control techniques across the world.

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Nishant Chaulagai

Nishant Chaulagai

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