Implementation of a generalized diffusion layer model for condensation into MELCOR

Condensation of co-current steam–noncondensable gas mixtures in vertical tubes is an important, yet difficult to model, component of many passive nuclear reactor cooling systems. The stagnant film model, which is used by the severe accident code MELCOR, gains its name by assuming that the gas–vapor film formed along the condensation surface is stagnant. Liao developed a generalized diffusion layer model that removes limitations of the stagnant film model and considers additional phenomena to improve predictive capabilities for condensation heat transfer with noncondensable gases. Similarities between the formulations of the stagnant film model and generalized diffusion layer model allow for the generalized diffusion layer model to be implemented into MELCOR. Input decks representing experimental facilities that produced co-current condensation data have been created to analyze and validate the generalized diffusion layer model implemented in MELCOR. The experimental data span a wide range of noncondensable gas mass fractions and include condensation mass transfer both on a vertical flat plate and in vertical tubes. MELCOR predictions of the condensation mass flux are seen to improve when using the generalized diffusion layer model instead of the stagnant film model.