Investigation on liquid nitrogen and vaporous nitrogen countercurrent flow considering droplet entrainment

To study the droplet entrainment mechanisms in liquid nitrogen (LN2) and vaporous nitrogen (VN2) countercurrent flow, simulation on three fields comprised of the liquid, the vapor, and the droplet phase was implemented for the horizontal segregated flow based on the Eulerian-Eulerian mathematical framework in ANSYS Fluent (R). The droplet entrainment model was adopted to determine the droplet entrainment and deposition source terms, and the position of the generated droplet was also estimated by calculating the velocity gradient in the normal direction of the interface toward the gas core. The model was validated according to the available data of water-air flow in the literature. For further evaluation of the model applied in the cryogenic fluid, the experiment and simulation of LN2-VN2 countercurrent flow were performed in a horizontal pipe of 15 mm diameter and 900 mm long. Three flow patterns, including stratified-wavy flow, churn flow, and pseudo-slug flow, were visually identified. The characteristics of the three flow patterns were obtained through the comparison of simulated contour results and experimental video recording. Furthermore, ligament breakup was found to be the main droplet entrainment mechanism in the churn and pseudo-slug flow. Pressure drop signals of the three flow patterns were also obtained and analyzed.