About once a month on average, an incident is reported in which turbofan jet engines flying at high-altitude lose power. The problem includes power loss, flameout, and even damage to the compressor blades. The pilots report that there is little if any bad weather that might explain the power loss. Although uncommon, this fault is potentially disastrous. The culprit is called ice crystal icing, and NASA scientists are making progress in understanding the problem using a world first test facility that creates an artificial ice cloud similar to that encountered by planes at high altitudes.
Ice crystal icing occurs when a turbofan engine passes through clouds (often invisible) of high-altitude ice crystals. As turbofan engines are extremely hot environments, internal icing poses a real quandary. The ice typically collects on stationary compressor blades or guide vanes, where it can obstruct airflow to the engine, or break off and potentially damage internal compressor blades.
Normally, icing is caused by supercooled water droplets freezing onto cold surfaces. Above an altitude of about 22,000 ft (6.7 km), the temperature is too low for any liquid water to exist. At such heights it was expected that the "dry" ice crystals would simply bounce off cold surfaces, unable to adhere in the absence of liquid water. This turns out not to be the case.
At high altitudes, air that normally would be humid is filled with tiny ice crystals – about 40 microns across. These cannot be seen by eye, and weather radar is quite insensitive to their presence, so the presence of ice crystal strata usually goes unnoticed.
These ice crystals can partially melt and then adhere to warm surfaces internal to the engine through surface tension. Ice forming on the engine inlet, fan or spinner would later dislodge into the fan bypass duct, which would not cause a power loss. Ice crystal icing is actually taking place in the engine core itself, as illustrated in the NASA video below.
There is still much that is not understood about ice crystal icing, partially because the conditions associated with such icing are difficult to reproduce in laboratory testing. Researchers at NASA's Glenn Research Center have now succeeded in performing full scale engine tests with an artificially structured ice cloud similar to that encountered by planes at high altitudes. Their first engine test showed power loss typical of ice crystal icing.
"The big question was, can we simulate the right aspects of the ice crystal cloud that causes the power loss?" says Judy Van Zante, icing cloud specialist at NASA Glenn. "We cannot vary the shape of the ice particles, but our specially-designed spray bars allow us to control the size and total amount of ice in the cloud."
The new capability for studying the interaction of ice crystal clouds with turbofan engines will be used to understand the precise conditions inside an engine which lead to problem icing. The goal is to discover how to detect and warn against ice crystal icing conditions, and how to design turbofans to ameliorate this problem.
Even though there do not appear to have been airplane crashes attributed to ice crystal icing, engine vibrations, power loss, and flameouts are not comforting flight experiences. NASA Glenn's work holds the potential for declawing this particular failure mode.
Source: NASA Glenn Research Center
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