Abnormal ice buildup on the wings and tail of an aircraft can disturb airflow to alter the physics of flight and lead to stalls, rolls and high-velocity crashes.

The role of supercooled large droplets (SLD) in this process is a particular concern—so much so that the Federal Aviation Administration (FAA) in 2014 enacted a rule regulating airworthiness standards in ice buildup conditions created by the presence of SLD in mixed-phase and ice crystal icing conditions. Yet despite the known dangers of SLD—droplets whose maximum diameter is greater than 1 millimeter—significant knowledge gaps about the phenomenon have persisted.

The FAA has enacted a rule regulating airworthiness standards in ice buildup conditions created by the presence of supercooled large droplets. Image credit: Pixabay.Now, thanks to the work of aerospace engineers from Shanghai Jiao Tong University, in China, a fuller understanding is emerging. It points to a different icing mechanism at work than previously identified.

The team, led by Professor Hong Liu, PhD, developed and tested a new model of SLD and the ice buildup process known as an “impinging heating model.” It adds a novel and important step lacking in previous efforts: heat generated from the impact thermodynamics.

To study the relationship at impact of water droplet size on the thermodynamics of ice buildup, the team worked in a closed-circuit wind tunnel with a self-designed supercooled droplet generator that can accurately control the droplet size from 20 to 1,500 micrometers (.02 millimeters to 1.5 millimeters), an aluminum plain board and a high-speed visualization system.

"The most critical significance of our model is that it reflects the heat transfer quantity generated from the impact thermodynamics," says Liu.

Test conditions at the Shanghai Icing Wind Tunnel reproduced the meteorological conditions that might be encountered in flight, as well as realistic variable droplet velocities and temperatures. By reproducing the impingement phenomenon in the lab, they observed rapid-freezing characteristics in droplets with diameters of 400, 800 and 1,300 micrometers (.4 millimeters, .8 millimeters and 1.3 millimeters, respectively).

Results of their experimental analysis provide analytical tools for determining the maximum spreading rate and the shrinkage rate of the drop, the supercooled diffusive rate and the freezing time. Combined, this knowledge can be used to more closely characterize meteorological conditions to help pilots safely manage flights that encounter freezing rain and SLD-forming conditions, the researchers say.

Next, the team will continue to validate and refine their impinging heating model through SLD icing simulations. They will also develop fine-grained models of the shrinking rate and attempt to determine the physical mechanism of this phenomenon.