Dynamic characteristics of droplet impact on heated surfaces near Leidenfrost temperature

Spray cooling features with high heat flux and good temperature control ability, which has been widely used in industrial environments such as electronic devices and nuclear power plants. In this study, the transient process of droplet-wall collision under film and transition boiling conditions has been investigated experimentally via the high-speed shadowgraph imaging and state-of-the-art image processing techniques. Based on intensive test conditions, the impact morphology, number, size, trajectory, ejecting angle and velocity of secondary droplets are presented and analyzed comprehensively. In experiments, distilled water was the test fluid, with initial droplets at 3 +/- 0.03 mm. Droplet collision velocity was calculated based on the center distance in two precollision frames. The Weber number changes from 15.4 to 104.2, while the wall temperature varies from 307.6 degrees C to 511.8 degrees C. The observed collision phenomena can be divided into four types, including rebound, rebound with secondary atomization, breakup with secondary atomization, and breakup. An Improved Cascade Gaussian Fitting method (ICGF), which performs well in the dense and mutually occlusive areas, is adopted for particle identification. The statistical results show that the secondary droplet diameter mainly distributes in the range of 0.02 mm to 0.2 mm under different Weber numbers. Under similar wall temperature conditions, the number of secondary droplets and the average diameter of secondary droplets increase with Weber number. Moreover, the separation mass ratio increased from 0.18 % to 2.29 %. Specifically, it is found that the amount and size of secondary droplets decrease significantly when the boiling mode transfers from transition to film. Based on the time-resolved shadowgraph images, a four-frame optimal estimation particle tracking algorithm is used to obtain the two-dimensional trajectory of the secondary droplets. The measured velocity distributes in the range of 0.3 m/s-4.0 m/s, mainly concentrating in the range of 0.5 m/s-2.0 m/s. The number and size variation with time is also analyzed, which indicates most of the secondary droplets are generated due to the strong instability in the transition boiling states.