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The diffusion layer model for condensation heat transfer of vapor with noncondensable gases was originally derived on a molar basis and developed from an approximate formulation of mass diffusion, by neglecting the effect of variable vapor–gas mixture molecular weights across the diffusion layer on mass diffusion. This is valid for gases having a molecular weight close to that of the vapor or for low vapor mass transfer rates, but it may cause serious error if a large gradient in the gas concentration exists across the diffusion layer. The analysis herein shows that, from the kinetic theory of gases, Fick’s law of diffusion is more appropriately expressed on a mass basis than on a molar basis. Then a generalized diffusion layer model is derived on a mass basis with an exact formulation of mass diffusion. The generalized model considers the effect of variable mixture molecular weights across the diffusion layer on mass diffusion and fog formation effects on sensible heat. The new model outperforms the one developed by Peterson when comparing with a wide-ranging experimental database. Under certain limiting conditions, the generalized model reduces to the one developed by Peterson.

Reflux condensation in steam generator (SG) is one of the major heat removal mechanisms in a loss of residual heat removal (RHR) system event in mid-loop operation during a pressurized water reactor (PWR) plant outage. In order to evaluate the effectiveness of reflux condensation in SG U-tubes, the condensation heat transfer characteristics in the presence of a non-condensable gas must be clarified. Local temperature data were previously measured for steam-air mixtures under gas-liquid countercurrent flow, in a vertical tube of inner diameter 19.3 mm. In this study, local reflux heat transfer coefficients were calculated by evaluating steam flow rate profile along the tube at low heat fluxes assuming saturated steam conditions and empirical correlations were derived. The correlations are valid over a range 2-9,000 W/m 2 ÁK for 0.1-0.4 MPa, 0.014-0.2 air mass fraction. The axial distributions of the steam-air mixture temperatures calculated using the correlation agreed well with the measured results and validity of evaluation methods and the correlation were verified.

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