A condensation heat transfer model with light gas effects in non-condensable gas mixtures

In this study, the condensation heat transfer on a vertical tube in a steam-air‑helium mixture was investigated. The effect of helium on condensation heat transfer was analyzed using a heat and mass transfer analogy model and a new effective diffusivity which was proposed based on the Maxwell-Stefan diffusion model. The condensation heat transfer was increased at a low helium content and decreased at a high helium content, which could be explained by the competition induced by the increasing diffusivity due to helium and reduction in density difference between the bulk conditions and wall. In addition, the vapor content and wall subcooling influenced the effect of helium. An experimental investigation was conducted to analyze the helium effect on condensation heat transfer. The helium content was measured using a gas analyzer, while the vapor content was obtained under the assumption that the gas was saturated. The results of the experiment were consistent with the theoretical analyses, although the helium effect at a low helium content was stronger than that predicted by the theoretical model. An improved helium effect factor was proposed after fitting the theoretical model to the experimental data, and the model predicted the effect of helium reasonably well.