Effect of light gas on condensation heat transfer of steam–air mixture and gas stratification

•The condensation heat transfer coefficient of a steam–air–helium mixture is measured.•The condition for helium stratification is identified.•Models of effective diffusion coefficient to predict condensation heat transfer coefficient are evaluated. Condensation has been considered for various industrial applications. However, the characteristics of condensation heat transfer have not been fully elucidated. In this study, we identify the characteristics of condensation heat transfer deterioration due to light gas. We measure the condensation heat transfer coefficient by varying the mass fraction of noncondensable gas and the mole fraction of helium to noncondensable gas. We discuss parameters that can affect the condensation heat transfer coefficient, i.e., the gas density, diffusion coefficient, and mole fraction of helium. Gradual deterioration of condensation heat transfer coefficient by increasing mole fraction of helium to noncondensable gas is observed. Furthermore, conditions for helium stratification under condensation are identified. Models for predicting condensation heat transfer are evaluated by varying an effective diffusion coefficient model based on experimental data.