Investigation of Dynamic Characteristics of Liquid Nitrogen Droplet Impact on Solid Surface

Liquid nitrogen spray cooling technology exhibits excellent heat transfer efficiency and environmental protection performance. The promotion of this technology plays an important role in improving the sustainable development of the refrigeration industry. In order to clarify its complex microscale behavior, the coupled Level Set-VOF method was adopted to study the dynamic characteristics of liquid nitrogen droplet impact on solid surface in this paper. The spreading behaviors under various factors (initial velocity, initial diameter, wall temperature, and We number) were systematically analyzed. The results show that the spreading behaviors of liquid nitrogen droplet share the same process with the normal medium, which are rebound, retraction, and splashing. For the droplet with smaller velocity and diameter, Rebound is the common phenomenon due to the smaller kinetic energy. With the increase of droplet diameter (0.2 mm to 0.5 mm) and velocity (0.1 m/s to 5 m/s), the spreading factor increases rapidly and the spreading behaviors evolve into retraction and splashing. The increase of wall temperature accelerates the droplets spreading, and the spreading factor increases accordingly. For the liquid nitrogen droplets hit the wall, the dynamic behaviors of rebound (We < 0.2), retraction (0.2 < We < 4.9), and splashing (We > 4.9) will occur with the droplet weber number increased, which are consistent with the common medium. However, due to liquid nitrogen having lower viscosity and surface tension, the conditions of morphological transformations are different from the common media. The maximum spreading diameter has a power correlation with We, the power index of We is 0.306 for liquid nitrogen, lager than common medium (0.25). The reasons are: (1) the better wettability of liquid nitrogen, and (2) the vapor generated by the violent phase change ejects along the axial direction. The article will provide a certain theoretical basis for liquid nitrogen spray cooling technology, and can also enrich the flow dynamics of cryogenic fluids.

Three-dimensional numerical investigation on micro-meter droplet impact and penetration into the porous media with different velocities

Droplets impacting and penetration into porous media is commonly seen in natural and engineering processes, in which the kinetics and capillary effect are of great importance to the lateral spreading and vertical penetration. In this study, a three-dimensional numerical simulation method was proposed to study the micro-meter droplet impact and penetration into the porous media. It is found that both the lateral spread and vertical penetration occur on the millisecond timescale and larger velocity will enhance the lateral spreading but have little influence on the penetration time and depth. The direct numerical method proposed in this study can be applied to predict the actual spreading and penetration status in the droplet-powder system and further insight into the droplet-powder interaction.

Liquid Droplet Impact on a Sonically Excited Thin Membrane

Impacting droplet characteristics on hydrophobic surfaces can be altered by introducing surface oscillations. Impacting water droplet contact duration, spreading, retraction, and rebounding behaviors are examined at various sonic excitation frequencies...