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Aircraft Liquid Cooling

Figure 1: Liquid-cooled chassis for electronics cooling Figure 1: Liquid-cooled chassis
for electronics cooling

Aircraft engineers today are charged with creating a More Electric Aircraft (MEA) with greater processing capabilities, while also minimizing the aircraft's weight and power consumption. With MEA, hydraulic and pneumatic systems are being replaced with electric systems. Combined with the addition of more high-end embedded computing systems, the need for liquid cooling on jets, helicopters, unmanned aerial vehicles, and other types of military and commercial aircraft is increasing. By moving from air cooling to liquid cooling, engineers can eliminate thermal restrictions that might otherwise force them to compromise on system performance. Engineers can also reduce weight and power consumption and increase meantime between equipment failures by lowering operating temperatures. Liquid-cooled chassis and cold plates provide very effective cooling for high-power modules and high-power density components, while heat exchangers provide cooling of engine or hydraulic fluids or dispose of the heat absorbed by the chassis or cold plates.

Liquid-Cooled Chassis

Liquid-cooled chassis, also known as Liquid Flow-Through (LFT) electronics chassis, are board level heat-absorbing technologies. (See Figure 1.) The chassis consist of aluminum cold plates that have corrugated aluminum fin vacuum-brazed into the sidewalls. The heat is moved via conduction from the board to the sidewalls to the fluid. The internal fin creates additional surface area for heat transfer and creates turbulence in the fluid to optimize performance.

Figure 2: Comparison of the densities and thermal conductivities of aluminum and copperFigure 2: Comparison of the densities and
thermal conductivities of aluminum and copper

Although copper has better thermal conductivity, aluminum is usually preferred in aircraft liquid cooling applications due to its lighter weight. Aluminum has approximately 50% of the thermal conductivity of copper, but only about 30% of the density. (See Figure 2.) By using aluminum, you can obtain the best performance-to-weight ratio.

With aircraft liquid cooling, ruggedization of liquid cooling components and systems is also essential. Liquid-cooled chassis are designed to be able to handle the shock, vibration, and acceleration requirements of MIL-STD-810F, RTCA/DO-160D or other military standards/specifications that are imposed. Vacuum-brazing ensures that chassis will be leak free and have the structural integrity to withstand the environmental extremes, including temperatures of -40°C to 120°C.

Most liquid-cooled chassis will use an ethylene glycol and water solution (EGW), oil, polyalphaolefin (PAO), or other dielectric fluids such as Fluorinert™ as the coolant. Ethylene glycol can provide freeze protection as well as corrosion protection that water alone cannot, while dielectric fluids can help protect sensitive electronics in the case of exposure to the coolant. The use of PAO as a cooling medium is popular for aerospace and military applications due to its dielectric properties and wide operating temperature range.

The VMEbus International Trade Association (VITA) Standards Organization is developing standard VITA 48.3 Ruggedized Enhanced Design Implementation (REDI), the first open standard for liquid-cooled commercial off-the-shelf (COTS) chassis. These standards will specify heights of secondary-side components, distances between cards, allowances for module covers or cold plates on each side, and more. Liquid-cooled chassis provide an upgrade path from conduction cooling to liquid cooling with minimal disruptions to the platform and without the need to replace the chassis at each technology refresh.

Cold Plates

Figure 3: Vacuum-brazed cold plates for aircraft Figure 3: Vacuum-brazed cold plates
for aircraft

Cold plates are the foundation of liquid-cooled chassis and are utilized for cooling phased array radars, electronic countermeasures pods, environmental control units, and more. Cold plate technologies include tubed, flat tube, and performance-fin. Vacuum-brazed performance-fin is the cold plate technology most often found on aircraft. (See Figure 3.) Performance-fin cold plates consist of two plates metallurgically bonded together with internal fin, the most common of which are vacuum-brazed.

With aluminum vacuum-brazed cold plates there is a great deal of flexibility in the design, making them ideal for cooling embedded systems within the aircraft. Cold plates can provide contact cooling for numerous high heat load components of different shapes and heights within very tight spaces. They can be manufactured ultra thin and can be machined, drilled, and/or tapped and still be flight worthy. There is a variety of internal fin that can be used to precisely match the cold plate's fluid path to the thermal requirements of the application. Cold plates and liquid-cooled chassis are designed with bosses for mounting, and dual-sided mounting is an option for cold plates. The cold plate fluid is usually cooled via a heat exchanger.

Heat Exchangers

Figure 4: Liquid-to-Liquid Plate-Fin Heat Exchanger Figure 4: Liquid-to-Liquid Plate-Fin Heat Exchanger

Plate-fin heat exchangers and flat tube heat exchangers are two types of heat exchangers commonly found on aircraft. Heat exchangers are used to cool auxiliary power units, hydraulics, gearboxes, and more. They are engineered for high performance with poor heat transfer fluids such as oils and ethylene glycol solutions. Aluminum plate-fin heat exchangers consist of finned passages separated by flat plates and have a unique internal configuration to maximize heat transfer. (See Figures 4 & 5.) They offer the best performance-to-weight ratio and can be used for air-to-air, air-to-liquid, or liquid-to-liquid cooling. Aluminum flat tube heat exchangers consist of a number of flat tubes with multiple extended surface channels within each tube. Fins are vacuum-brazed between the flat tubes and form the passages for the second fluid. These flat tubes provide a lower cost alternative to plate-fin designs. Both types of heat exchangers are vacuum-brazed for ruggedization.

Plate-Fin Heat ExchangerFigure 5: Plate-fin oil cooler heat exchanger
used on a helicopter application

Heat exchangers are often used for cooling hydraulic oil, engine oil, and EGW with RAM air via a RAM air intake system or with a fan. RAM air may also be replaced with an ethylene glycol solution or PAO as the heat sink. For example, PAO is sometimes used in intercooler applications to cool gaseous nitrogen (GN2) for inerting fuel tanks to reduce fire hazards. One way that engineers can save space and minimize weight is to route fluids currently in use on the aircraft through the liquid cooling loop. For example, fuel may be used as the heat sink to cool oil since fuel is a readily available fluid, eliminating the need to have an additional fluid on board. Another way to save space is to create a heat exchanger with multiple cooling circuits to cool several fluids at once. For example, a 4-circuit heat exchanger may be used to simultaneously cool air, oil, and EGW via RAM air or a fan.

Figure 6: Comparison of air densities at various altitudesFigure 6: Comparison of air densities
at various altitudes

Although temperature is always a consideration with liquid cooling, when heat exchangers are used at high altitudes there are the additional considerations of air density and pressure. The heat exchanger's fan must be carefully selected to ensure that it will provide sufficient airflow based on the ambient air pressure. At high altitudes, the density of air is significantly lower. (See Figure 6.) It takes more airflow to remove the same amount of heat since the same volume of air has fewer air molecules for heat absorption. For example, at sea level and standard ambient temperature and pressure (70°F and 14.696 psia), the density of dry air is 0.075 lb/ft3 or 1.19 kg/m3. At an altitude of 25,000 feet, the density is only 0.549 kg/m3 - less than half the density of air at sea level. In order to achieve more mass airflow at higher altitudes, a higher volumetric airflow is needed and a larger fan may be required. (See ComairRotron's application note, "Solving High Altitude Cooling Problems".) It's also important to note that the addition of water vapor reduces the density of air, as does higher temperatures. However, the impact of humidity on density is less than the impact of high altitudes. In addition, at high altitudes the air is generally significantly colder than at sea level. Assuming that RAM air is being used, the cold air will provide for better heat transfer, helping to offset the affects of operating with lower air density.

Liquid cooling can provide significantly better performance than air cooling alone, may be quieter than air cooling, and can be insensitive to altitude. By moving from air cooling to liquid cooling, engineers can optimize system performance. They can also reduce weight and power consumption by eliminating the need for large fans or the need for wide spacing of components. Liquid-cooled chassis, cold plates, and heat exchangers provide total thermal solutions for cooling aircraft fluids and electronics.