Simulation and experimental investigation on kinetic and thermodynamic characteristics of liquid nitrogen droplets impacting superheated wall

Abstract

Liquid nitrogen droplets impacting superheated wall is an essential phenomenon in cryogenic phase-change spray cooling. In this study, the Volume of Fluid (VOF) method was used to develop a numerical model to investigate the kinetic and thermodynamic characteristics of liquid nitrogen droplets impacting superheated wall. The experiment for the impact of liquid nitrogen droplets was conducted to validate the simulation. The findings indicate that liquid nitrogen droplets impacting superheated wall exhibit three boiling regimes: contact boiling, transition boiling, and film boiling. During contact boiling, as We increases, droplets undergo three modes sequentially: spreading, forming fingering-like structures, and fragmentation. During transition boiling and film boiling, as We increases, droplets exhibit spreading, splashing, and fragmentation. Increasing the wall temperature leads to the formation of vapor pockets and vapor film, which results in deteriorated heat transfer at the solid–liquid contact area, and meanwhile reducing the extent of droplet spreading. Increasing We promotes droplet spreading, reducing vapor pockets and vapor film thickness, increasing wetting area, and postpones the onset of heat transfer deterioration. Increasing the wall temperature and We both lead to a higher decreasing rate of liquid volume of droplet which indicates an intensified vaporization rate. The larger the contact angle of the wall, the less the droplet spreads, and the lower the heat transfer between the droplets and the wall.