Numerical analysis of nitrogen condensation heat transfer enhancement with liquid film fluctuation at cryogenic temperature

Film condensation is the main condensation mode for cryogenic fluids with extremely low surface tension. Deep understanding of how fluctuating liquid film affects heat transfer is of great significance to the design and optimization of cryogenic condensers. In this study, a numerical model of nitrogen condensation on a vertical plate is established which adopts an accurate condensation mass transfer model, and the simulation results match well with the experimental data. The transient flow behaviors like thickness, temperature field, velocity field of liquid nitrogen film and various heat transfer coefficients are analyzed. Results show that the condensed liquid nitrogen film undergoes a flow pattern evolution from laminar flow to sinusoidal-like wave flow and then solitary wave flow. Compared to water condensation, nitrogen condensation has a more intense fluctuating flow pattern due to the lower latent heat. Large local heat transfer coefficient could be found at both the trough and peak areas of the film waves, while the lowest value occurs at the front of the wave peak. The analysis shows that the liquid film in the laminar flow zone between the two peaks is thinned to below Nusselt's theoretical value, and the convection effect at the peak is enhanced due to a sharp increase in the wave velocity. Finally, the time domain characteristics of the local heat transfer coefficients reveal that the synergistic effect of intermittent thinning of film thickness and enhanced convection in solitary waves improves the heat transfer coefficient up to 30% compared with the Nusselt's theoretical value for laminar flow. (C) 2019 Elsevier Ltd. All rights reserved.