The formation of ice on aircraft is both commonplace and dangerous. An ice layer on wings and control surfaces can change their shape, potentially affecting the pilot’s control of the aircraft. Ice also adds weight. Under certain conditions, usually at fairly low altitudes, so-called icing-clouds can contain droplets of water that remain liquid despite their temperature being below freezing. Upon contact with any solid surface, the droplets instantaneously freeze onto the surface. A thick ice layer can form in just seconds. This can be extremely dangerous for small aircraft, which must avoid such clouds completely. Larger aircraft carry de-icing systems. The main types heat the wing and the engine air intake, via either electric elements or a hot-air system. Although such systems work reliably, they are very energy-hungry, consuming up to 40 % of the total power the aircraft produces. Any reduction in energy consumption would mean significant financial savings for the aircraft operator.
Putting waste heat to use
The EU-funded PIPS project developed a new kind of heating system. Instead of generating heat at great cost, the system transfers waste heat from the engines to the engine air intake. Thus it is highly efficient. The system is initially intended for the air-intake part of medium-sized turboprop aircraft. However, it could eventually be adapted to any aircraft having hot engines near the wings, meaning virtually any aircraft. The patent-pending system is called Capillary Pumped Loop. “This is a closed circuit in which circulates a fluid – methanol – which is always at saturation,” explains project coordinator, Romain Rioboo. “That means it’s always around the point of phase change between liquid and vapour.” The engine supplies heat to the evaporator, which evaporates the liquid. Gaseous methanol circulates to the condenser, where it returns to liquid, releasing heat. The liquid circles back to the evaporator via a capillary wick, which is a passive pump. The physics of the phase change between liquid and gas means that the liquid can absorb and transport a large amount of heat. So the system is very efficient, using engine heat that would otherwise be wasted. This saves having to generate large amounts of heat as in conventional de-icing systems.
Good progress so far
The project has achieved technology readiness level 5. The concept has been demonstrated, but the technology still needs some refinement. Researchers initially planned to use off-the-shelf components. However, problems encountered along the way required design of a completely new system. This was successful but caused delays. “We have yet to change the condenser design to improve heat distribution on the surface of the engine intake,” adds Rioboo, “and test the change before envisaging flight tests.” However, in laboratory and icing wind tunnel testing, the project achieved effective heat transfer in a full-size model engine setup and transported up to 10 kilowatts of heat under icing conditions. This exceeds the requirements for a de-icing system. Further development is uncertain. If that takes place, the outcome will be a cheap and efficient aircraft de-icing solution.
PIPS, aircraft, de-icing, system, heat-exchange, Capillary Pumped Loop, methanol