Putting an aircraft in the cooler

Research in focus November 2013

Picture the scene: brilliant blue skies and the sun beating down. Just the thing for a beach holiday, you might say, but not when you’re sitting in a plane waiting to take off. As temperatures inside the cabin continue to rise – despite the air conditioning system’s best efforts – passengers are left hoping for a speedy go-ahead from the control tower. But it’s not just delayed takeoffs in hot countries that are prompting the aerospace community to tackle the problem of unwanted heat aboard aircraft. More and more it’s about the trend toward the all-electric aircraft, which presents new challenges to manufacturers and researchers alike. In modern airliners, electronics already govern a variety of functional and control units, from the engines to radio communication, while fly-by-wire, a technology that converts movements of flight controls into electronic signals and transmits them via wires, is the state of the art in flight control systems. Not only that, but there are plans to replace current compressed-air and hydraulic systems, given that these call for compressors, pipes branching out in all directions and a not insubstantial amount of hydraulic fluid. This all adds to a jet’s weight, increasing its fuel consumption.

To save weight in aircraft, it is important to fit planes with lighter electronics systems, since lower weight means reduced emissions of pollutants such as carbon dioxide (CO 2) and nitrous oxides (NO x) as well as improved fuel consumption. Launching less weight into the air also means less energy consumption. The flipside is that electronics generate heat – you need only think of a running computer or a cell phone on charge. That is why the Ground Thermal Test Bench was set up at the Fraunhofer Institute for Building Physics IBP in Holzkirchen. This unique test facility allows researchers to investigate the problem of heat distribution aboard aircraft – and how to channel away unwanted heat.
“The facility was developed as part of the EU Clean Sky project and allows us to simulate the conditions on the interior and exterior of the aircraft just as they would be if the aircraft was actually in flight or on the ground,” explains Markus Siede, researcher from the Vehicle Climate Control Systems working group at Fraunhofer IBP. “This means we can examine, compare and optimize a whole range of avionics systems.” Researchers can make use of the Ground Thermal Test Bench to test new developments step by step – from simulation calculations on the computer to experiments in small spaces known as simulation boxes and, ultimately, testing under real conditions using actual sections of aircraft fuselage and with test subjects. Conducting testing in this way offers a number of advantages, as it cuts down on the number of actual test flights required while bringing down costs and protecting the environment.
Test bench design
The Ground Thermal Test Bench at Fraunhofer IBP’s flight test facility comprises a cutting-edge cooling system, heat exchangers, several simulation chambers, an aircraft fuselage divided into three sections – cockpit, cabin and rear – and an aircraft calorimeter (ACC). The ACC is used to simulate the most extreme conditions such as rapid decompression and thermal shock (a rapid change of temperature in a material that causes mechanical tension between the outer and inner parts of the material as heat to or from its surface is conveyed more quickly than to its interior). The aircraft fuselage, meanwhile, gives researchers the opportunity to study individual test configurations in detail.
The ACC comes into play once computer simulations and thermal models have been completed. At this point the task is to validate parameters such as airflow patterns, thermal comfort, energy efficiency, exhaust emissions and temperature changes under real conditions. The simulation chamber gives researchers the opportunity to test modular measurements in a small space based on extreme changes in temperature and pressure.
Fraunhofer researchers use the fuselage to simulate the environment in the cabin and how this relates to the climate on the exterior of the aircraft. Here, original avionics components can be substituted by faithfully reconstructed dummy parts that share the same thermal properties as the actual components. This affords extra flexibility, as the heat emissions and geometry of these “equipment simulators” can be manipulated at will. There are a whole range of important questions to consider in the process: What heat sources are there on the interior? How is the temperature influenced by the passengers and the on-board electronics? Where exactly does heat build up? Is there a possibility that equipment will overheat and how does this affect its operation? The motivation behind the testing is to find solutions that will enable researchers to channel heat away and direct where it goes. A basic solution of the sort applied in server rooms in office buildings, where simply adding ventilation holes can reduce the temperature, is clearly not an option in aircraft. The problem is that an aircraft is exposed to huge changes in pressure during flight, meaning that simply exchanging air with the outside is far from straightforward. Creating openings in the exterior of the aircraft would also lead to turbulence, increasing the aircraft’s air resistance – and its fuel consumption. One interesting idea could be to make use of the fuel tanks as heat/cooling reservoirs.
In principle, the facility also presents the opportunity to carry out experiments with test volunteers, as the three-part fuselage can be fitted with seats. “Of course, to test extreme scenarios – for instance, how damage to the fuselage in flight would affect passengers – we will be using the DressMAN we developed ourselves in-house. These dummies could also be used to investigate worst-case scenarios that are far too dangerous for human test subjects,” explains Siede.
Temperatures high and low
To reach the core of the Ground Thermal Test Bench you have to go down to the cellar where there is a high-performance air treatment unit that can generate temperatures of up to minus 70 degrees Celsius. This was quite a challenge for the employees who designed the unit, given that conventional air treatment units cannot get lower than minus 50 degrees Celsius at standard pressure. This special unit, though, is capable of replicating extreme conditions that cool the fabric of the aircraft down as far as minus 55 degrees Celsius. In real terms, that is equivalent to a long-haul flight in the northern hemisphere at 10,000 meters altitude.
A second, significantly smaller air treatment unit ensures that temperature and humidity in the cabin can be controlled with precision. Here, the range of temperatures is between three and 70 degrees Celsius. This allows researchers to simulate factors such as the high exterior temperatures before takeoff in a desert country and to identify solutions that can direct the heat involved away from the cabin in an energy-efficient way.
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