Numerical investigation of convective condensation with the presence of non-condensable gases in a vertical tube

Steam condensation is degraded when non-condensable gases are present. Convective condensation of steam-air mixture and steam-helium mixture in vertical tubes were simulated using the CFD code FLUENT. The condensation process was modeled by defining source terms for the mass, momentum, species and energy conservation equations. Several cases with various steam mass fractions were simulated, the results matched well with the experimental data. Detailed velocity field and species distribution were investigated. The radial velocity was clearly represented, and the suction effect was modeled, which needs to be accounted for when using the heat and mass transfer analogy theory. The Nusselt and Sherwood numbers predicted by CFD models were compared with the classical correlations, and the suction effects were analyzed. The suction effect is proportional to steam mass fraction, while the suction factor is little affected by the Reynolds number. For forced convection flow in this work, the buoyant force can be neglected, so the larger diffusion coefficient of steam-helium mixture would improve the steam condensation compared to steam-air mixture. The condensation mass fluxes of steam-helium mixture and steam-air mixture are almost the same at relatively high steam inlet molar fraction (>= 90%). (C) 2015 Elsevier B.V. All rights reserved.