Theoretical Modeling of Vapor Condensation in the Presence of Noncondensable Gas on a Horizontal Tube

Double-boundary layer theory was adopted to investigate the distributions of the liquid film, gas film, heat transfer coefficient, and condensate mass fluxes around a horizontal tube for vapor condensation with noncondensable gases like steam–air and steam–CO₂ mixtures under free convection. The investigation considered the effects of the noncondensable gas concentration, surface subcooling temperature, and pressure. The thicknesses of the liquid and gas films increase gradually along the wall from top to bottom, whereas the local heat transfer coefficient and the condensate mass flux decrease. The film thicknesses do not change significantly around the upper part of the tube but increase sharply around the lower part. The liquid film thicknesses, gas film thicknesses, condensate mass fluxes, and heat transfer coefficients of steam–air systems are compared with those of steam–CO₂ systems. The condensate mass flux in the steam–air system is smaller than that of steam–CO₂ system under the condition of the same surface subcooling and gas mass fraction because air has more moles of molecules in the mixture than CO₂ and the steam more easily diffuses through CO₂ than through air. The predicted average condensation heat transfer coefficients agree well with the available experimental data.