Flow and heat transfer characteristics of ambient air condensation on a horizontal cryogenic tube

• A mathematical model of superheated vapor condensation on a horizontal tube is built.
• The effect of superheating on condensation is obvious for air while weak for water.
• Buoyance plays a significant role on the superheated air condensation.

Ambient air condensation on a cryogenic horizontal tube is investigated using a newly built mathematical model, in which the liquid film and the vapor boundary layer are coupled together with a major emphasis on the effect of buoyancy. Based on the model, the heat transfer coefficients and the film thickness as well as the interfacial shear are obtained under different conditions to investigate the effects on the flow and heat transfer characteristics of the superheating between vapor and film, the buoyancy in the boundary layer and the subcooling between wall and film. In addition to the flow and heat transfer characteristics of air, the other four different vapors, i.e. H2O, R134a, methane (CH4), argon (Ar), are also discussed. The results show that the superheating has a more significant contribution to the increase of heat transfer coefficient for air comparing to the other vapors, e.g. in the cases of superheating ΔT∞ = 100 and 200 K the mean heat transfer coefficient increases by 10.3% and 24.3% for air, while it increases by only 1.8% and 3.9% for H2O. In contrast to superheating, the subcooling has a negative effect on the increase of heat transfer coefficient. Noteworthy, the buoyancy plays a non-negligible role on the flow and heat transfer characteristics of superheated air condensation. The analytical results are of great importance in the design and improvement of ambient-heated cryogenic vaporizer (AHCV).