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.

Entropy analysis in spray cooling for dosing water injection

Spraying water in air improves air-cooling capacity, which then relies on the evaporation of water. Even for small drop sizes, literature reports that the evaporation remains limited inside the spray and below saturation limits. In this article, we describe the evolution of thermodynamic quantities in a mixture of air and evaporating liquid water. A complete and coherent formulation is used to express enthalpy, entropy and chemical potential. At constant enthalpy, we show that the chemical equilibrium corresponds to an intermediate state in which droplet evaporation is not complete and entropy is maximum under certain conditions. Results are compared with some experimental values measured in a wind tunnel downstream of a spray. The calculated values are consistent with observations. Cooling efficiency is discussed for the various parameters, which are the amount of water, air temperature and ambient humidity. Then, the numerical approach is inverted in order to forecast the amount of water needed to reach a target cooling temperature. This numerical approach is used to set water flow depending on inlet flow conditions and cooling objectives.