The work aims to explore the dynamic behavior and solidification characteristics of droplets impacting cold curved surfaces, which is of significant importance for improving operation safety and performance in fields such as power transmission, wind energy generation, and aerospace. In the experimental section, an untreated cylindrical surface was prepared, and the surface was cooled to approximately -15 ℃ with a semiconductor thermoelectric cooler. A droplet impacted the cylindrical surface from varying heights, and the dynamic behavior of the droplet was analyzed with a high-speed camera. Two methods, external vibration excitation and surface modification by silicone oil immersion were employed to investigate the effects on the droplet spreading and freezing characteristics. The dynamic behavior of the droplet impacting different surfaces was observed and studied. It was found that as the Weber number of the droplet increased, the liquid film thickness significantly decreased along the cylindrical surface on both sides of the impact location. The distribution of the liquid film thickness gradually grew more uniform, and the spreading range increased. The maximum spreading factor of the droplet, βmax, was found to obey the relationship βmaxµWe0.477, while the time to reach the maximum spreading factor tmaxµWe0.306 was obtained. When the droplet finally froze, the frozen area was large, and the ice thickness was thin. Next, external vibrations with different vibration frequencies and amplitudes were applied to the cold cylindrical surface. The results showed that the vibration caused the liquid film to concentrate mainly in the lower part of the surface, leading to a larger spreading range and thinner liquid film. The final frozen area of the droplet increased, but the ice thickness grew thinner. When the droplet impacting a low-temperature oil-infused cylindrical surface, the lubrication effect of the surface suppressed the spreading. The spreading factor of the droplet was significantly reduced, and a droplet separation phenomenon occurred when the Weber number exceeded a certain critical value. The final ice coverage area decreased and was mainly concentrated in the upper part of the surface, with the ice layer thickness significantly increasing. An energy analysis was conducted, considering the conversion of the droplet's kinetic and vibration energy into the deformation energy of the droplet during the spreading process. A theoretical analytical model was established. The experimental values obtained from repeated experiments were consistent with the theoretical predictions, thus validating the accuracy of the model. The effects of applied vibration excitation and silicone oil modification on energy changes were also analyzed. When vibration was applied, the time for the droplet to reach its maximum spreading factor decreased and the maximum spreading factor increased with the increase of vibration amplitude. The maximum spreading factor (βmax) increased by 12%, and the ice coverage area expanded, though the ice thickness was reduced by 16%. When the droplet impacted the low-temperature oil-immersed cylindrical surface at Weber numbers below the critical value, the final ice thickness increased by 13%, with a smoother variation, while βmax decreased by 5%. However, when the Weber number exceeded the critical value, a droplet separation occurred, and the residual droplet formed a thin, relatively uniform ice layer on the upper part of the surface.
Key words
droplet impacting /
liquid film /
curved surface /
icing /
vibration /
oil-infused surface
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Funding
National Natural Science Foundation of China (51776128)
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