Experimental Study Of Droplet Impact On Undercooling Metal Surface

The topography in China is complex and the climate is diverse, so there is an ice covering problem on high voltage transmission line in most parts of China in winter. It caused great potential safety hazard to the transmission network and threated to the safe operation of power grid, even caused much disasters and accidents to result in widespread blackout until the grid collapse and irreparable economic losses. In recent years, more and more power grid accident was caused by the transmission lines icing, which made many scholars paying more attention to research the transmission line ice coating process. However, the mechanism of ice covering on the high voltage transmission line is very complicated, and deicing method used in the practical engineering mostly belongs to the high energy consumption and high cost. In addition, the research on the spontaneous deicing of transmission line is more and more concerned by scholars. In view of the freezing mechanism of droplet on the transmission line, the icing behavior of the droplet is studied.In order to explore the mechanism of the droplet impacting surface and freezing intensively and reveal the behavior of the droplet icing on the metal surface. The dynamic behavior and freezing process of single droplet impacting on low temperature surface were studied. Visualization experimental have studied on the single droplet impacting on metal cylinder surface with different curvature, supercooling degree, infiltration and the change of the inner flow field in the droplet freezing process under different experimental conditions, the motion law and the rule of the heat transfer in the process of movement and freezing after the droplet impact metal surface can be received. In this paper, the main research contents and conclusions are as follows:(1) In this paper, the visual experiment on the droplet impacting on the curved surface of different curvature and condensate depression was carried out at first, the effect of droplet impact velocity, surface supercooling degree and surface curvature on the spreading behavior after single droplet impacting cylindrical surface was investigated. The experimental results showed that the droplet with larger initial impact energy, the spreading of droplet in circumferential and axial direction of cylindrical surface were better. With the increase of cylindrical surface curvature, the spreading of liquid film in circumferential direction was better and the spreading in axial direction was worse. For the droplet spreading process on the cold surface, with increasing the degree of super-cooling, the spread of the droplet was worse, and the droplet oscillation relaxation time was shorter, and the freezing rate of upper liquid film was accelerated. However, the spreading of droplet on the cold surface is better than on the normal temperature surface. By analyzing the influence law of temperature on the droplet surface tension and viscosity, we can conclude that a layer of ice film is formed rapidly in the bottom of the droplet in the spreading process because of lower surface temperature, which changed the liquid-solid interface to make the droplet spreading more easily. In droplet retraction phase, the underlying frozen phenomenon can be clearly observed.(2)Visualization experimental study was carried out on droplet impacting on the surface with different wettability, series of spreading behavior was analyzed, and the dynamic behavior on normal temperature surface was compared with those on low temperature surface. The experimental results show that the spreading and retraction behavior was influenced significantly by the wettability. With the increase of surface apparent contact angle, the spreading of the droplet along the cylindrical circumferential direction was worse and the time of maximum spreading diameter was shorter. The droplet can rebound from the surface when the surface was super-hydrophobic surface. In the process of the retraction, due to the difference of retraction rate between the circumferential liquid film and the axial one, the symmetrical bimodal phenomenon of liquid film occurred on the circumferential liquid film. When the surface was supercooled, the wettability have a significant impact on the oscillation behavior in the retraction process. With apparent contact angle increasing, the droplet oscillation behavior was more severe, and the duration time was longer, and the oscillation cycle was shorter.(3)In order to investigate the internal phase transformation characteristic in the freezing process of droplet, the Particle Image Velocity(PIV) technology was used to observe the phase transition behavior. The visualization research was carried out on the droplet with different velocity impacting upon the horizontal surface. The experiment results show that: in the case of the droplet with no initial velocity impacts the surface, the free surface of the droplet transferred heat and mass with the surrounding environment. And the circumfluence phenomenon occurred obviously inside the droplets. Without any disturbance of the external heat source, the circumfluence intensity in the droplet was steady and persistent. When the temperature of the surface was-10℃, the circumfluence occur the Y type flow inside the droplet before freezing, and it was called marangoni convection. The circumfluence inside the droplet was stronger than the natural evaporation when droplet impact on the surface with initial velocity. The maranoni convection occurred inside the droplet on supercooled surface. With the initial velocity increasing, the area of the droplet spreading on the surface was bigger, the height of hemisphere was smaller. It means that the thermal resistance decrease in the normal direction of the surface, and the droplet internal circulation enhanced, it shorted the last time and promotes the ice growth. Due to the lower hemisphere height, the water thermal resistance of the hydrophilic surface was small, which was more favorable for the freezing process.