An Experimental Investigation on Unsteady Heat Transfer and Transient Icing Process upon Impingement of Water Droplets

Abstract

This study presents an experimental investigation of a single droplet with different impingement velocity impinge on the hydrophilic and superhydrophobic substrates under normal and icing temperature by using high-speed image and infrared image techniques. The aim is to better understand the unsteady heat transfer and transient icing process of the aircraft icing caused by the supercooled large droplets, which has been recently identified as a severe hazard in aviation. The Reynolds number and Weber number of the impingement droplet ranged from 3708 to 6109 and from 117 to 319, respectively, while the temperature of the impingement substrate ranged from -5˚C to 5˚C. Droplet impingement, spreading, receding, and rebound phenomenon was recorded by a high-speed imaging system, while the surface temperature variation upon the impingement droplet on the substrate was recorded by an infrared imaging system. The time needed for the impingement droplet to be static on the superhydrophobic surface was much shorter than that on hydrophilic surface, while the time needed for cooling the impingement droplet on the superhydrophobic surface was much longer. The temperature variation on the surface of the impingement droplet was gradually on superhydrophobic substrate, while that on the hydrophilic substrate under icing temperature had obvious fluctuation. For the hydrophilic substrate, the temperature had little influence to the maximum spreading diameter of the impingement droplet, while the final receding diameter of the impingement increased with the decreasing of the substrate temperature, moreover, the water temperature decreasing speed increased while the icing process was faster. Lower droplet impingement velocity would lead to a slower cooing process of water and a slower icing process, while the temperature fluctuation at the surface center of the impingement droplet decreased for a shorter water receding time.