Experimental studies on the dynamics of in-flight and impacting water droplets pertinent to aircraft icing phenomena

摘要

Aircraft icing is widely recognized as a significant hazard to aircraft operations in cold weather. When an aircraft or rotorcraft fly in a cold climate, some of the super-cooled water droplets would impact and freeze on the exposed aircraft surfaces to form ice shapes, which can degrade the aerodynamic performance of an airplane significantly by decreasing lift while increasing drag, and even lead to the aircraft crash. In the present study, a series of experimental investigations were conducted to investigate dynamics and thermodynamics of in-flight and impinging water droplets in order to elucidate the underlying physics of the important microphysical process pertinent to aircraft icing phenomena.A novel lifetime-based molecular tagging thermometry technique (MTT) is developed to achieve simultaneous measurements of droplet size, flying velocity and transient temperature of in-flight water droplets to characterize the dynamic and thermodynamic behaviors of the micro-sized in-flight droplets pertinent to aircraft icing phenomena. By using high-speed imaging and infrared thermal imaging techniques, a comprehensive experimental study was conducted to quantify the unsteady heat transfer and phase changing processes as water droplets impinging onto frozen cold surfaces under different test conditions (i.e., with different Weber numbers, Reynolds numbers, and impact angles of the impinging droplets, different temperature, hydrophobicity and roughness of the test plates) to simulate the scenario of super-cooled water droplets impinging onto the frozen cold wing surfaces. A novel digital image projector (DIP) technique was also developed to achieve time-resolved film thickness measurements to quantify the dynamic impinging process of water droplets (i.e., droplet impact, rebounding, splashing and freezing process). An impact droplet maximum spreading diameter model and a damped harmonic oscillator model was proposed based on precise measurement of the impact droplet 3D shape. A better understanding of the important micro physical processes pertinent to aircraft icing phenomena would lead to better ice accretion models for more accurate prediction of ice formation and accretion on aircraft wings as well as develop more effective and robust anti-/de-icing strategies for safer and more efficient operation of aircraft in cold weather.