A unified correlation for steam condensation rates in the presence of air-helium mixtures under naturally driven flows

Steam condensation in the presence of multiple noncondensable gases is critical in many nuclear plant safety applications, not the least of which are hypothetical severe accidents where H2 is present in addition to the containment air inventory. Experimentalists have used helium as a surrogate for H2, and have proposed a number of correlations and/or databases to estimate steam condensation rates in the presence of binary air-helium mixtures under free convection regimes typical of reactor flows. These studies are purely empirical, and hence do not allow to draw clear dependencies of the heat transfer rate on critical thermal-hydraulic parameters. In this study, we do away with the particular forms of the correlations, and go back to the original experimental data, consolidate them in a single database, and propose a unified correlation that is compatible with the heat and mass transfer analogy. This best-estimate correlation for steam-air-helium mixtures, based on four different investigations and 180 data points, covers ranges of conditions expected in nuclear severe accidents. The consolidated raw data gather around a curve with a standard deviation of 20%, which is within typical experimental error bands. We show in addition that the correlation can directly be used to estimate steam condensation rates in the presence of binary hydrogen-air mixtures.