CFD analysis of steam condensation with air in the tubes bundle channel under natural convection conditions

Abstract

The condensation heat transfer characteristics of steam with the non-condensable gases (NCGs) in the tubes bundle channel are quite different from those in the simple geometry structure, such as the single tube and flat plate. In the present work, to evaluate the thermal characteristics of steam condensation with NCGs in the vertical tubes bundle channel, the numerical model of steam condensation was established based on the CFD code ANSYS FLUENT, which combined the gas component transport equations and the diffusion balance model. And several conditions with different thermal parameters were simulated, the simulation results are in good agreement with the experimental data. The condensation heat transfer in a hexagonal 7-tubes bundle channel with various tube pitches (from 1.5d to 5d) is investigated. The results indicate that the distribution of typical thermal parameters, such as velocity, air mass fraction, temperature and density, is quite different from that in the single tube due to the cross and mixture in each condensation tube boundary layer. As the condensation length increases, the air mass fraction rises, the gas flow rate decreases and the sub-cooling decreases, resulting in a lower natural convection intensity. When the tube pitch is less than 4d, the condensation heat transfer capacity in the tubes bundle channel is weakened obviously, and the heat transfer coefficient with the tube pitch 1.5d is only about 32% of that in a single tube. The air mass fraction and pressure have a significant positive effect on the heat transfer coefficient, and the wall sub-cooling has a significant negative effect on that.