What makes planes freeze?

Chinese researchers have investigated the process by which ice accumulates on the wings and tail of an aircraft flying during freezing rain. Abnormal ice build-up can disturb airflow to alter the physics of flight and lead to stalls, rolls and high-velocity crashes.

The culprit behind abnormal icing is supercooled large droplets (SLD) — droplets whose maximum diameter is greater than 1000 µm (equal to 1 mm). In November 2014, the Federal Aviation Administration (FAA) enacted a new rule regulating airworthiness standards in ice build-up conditions created by the presence of SLD in mixed phase and ice crystal icing conditions. Yet despite the known dangers of SLD, significant knowledge gaps persist — until now.

Researchers from Shanghai Jiao Tong University developed and tested a new model of SLD and ice build-up process, known as an ‘impinging heating model’. Published in the journal Physics of Fluids, their model adds a novel and important step lacking in previous efforts: heat generated from the impact thermodynamics.

“The thermodynamic effect during the supercooled large droplet impact process has not received sufficient attention,” said Professor Hong Liu, the leader of the research team. “We set out to fill certain knowledge gaps.”

The team worked in the Shanghai Icing Wind Tunnel, using a self-designed supercooled droplet generator that can accurately control the droplet size from 20–1500 µm, an aluminium plain board and a high-speed visualisation system to study the relationship at impact of water droplet size on the thermodynamics of ice build-up.

“The most critical significance of our model is that it reflects the heat transfer quantity generated from the impact thermodynamics,” said Professor Liu. “Nowadays, understanding the mechanism of SLD icing has become a significant goal for researchers concerned with air travel safety, so that is our goal in building and testing the most robust model to date.”

Test conditions of 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, the team observed rapid-freezing characteristics in droplets that had diameters of 400, 800 and 1300 µm.

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. This can be used to more closely characterise meteorological conditions to help pilots safely manage flights that encounter freezing rain and SLD-forming conditions.

“Our results indicate that the drop size is a critical factor influencing the supercooled heat exchange and effective heat transfer duration between the film/substrate interface,” said Professor Liu. “Our detailed experimental results indeed support the safety rationale behind the rule that the FAA recently adopted.”

Next, the team will continue to validate and refine their impinging heating model through SLD icing simulations. Fine-grained models of the shrinking rate must also be developed, as well as determining the physical mechanism of this phenomenon. The team’s hope is that their model will help improve air travel safety, particularly in icy conditions.