Numerical evaluations on enhanced steam–air condensation outside S-type tubes under natural convection conditions

Abstract

Steam-air condensation is extensively utilized in various industrial fields. In pursuit of better efficiency in applications, this study promoted a novel structure of heat transfer enhancing tube named S-type tube. It combines the superior heat transfer performance of short tubes and inclined tubes, demonstrating a high potential for enhanced condensate heat transfer. To evaluate the heat transfer property and obtain the optimized S-type tube structure, three-dimensional CFD methods were used, coupled with the liquid film model in STAR-CCM + software. The effcts of wall sub-cooling, pressure, air mass fraction and downward forced velocity, bend radius, horizontal section length on the steam–air condensation heat transfer characteristics outside S-type tubes were investigated. It was identified that S-type tubes can enhance the steam–air condensation heat transfer capability, and the enhanced heat transfer ability increases with the increase of pressure and the decrease of air mass fraction, and remains basically unchanged with changes in sub-cooling. The condensation heat transfer coefficients outside S-type tubes are 0.9 ∼ 1.71 times that of vertical tube range 0.2 MPa to 0.8 MPa. The main mechanisms affecting the condensation heat transfer are summarized: the scouring effect between tube segments to enhance heat transfer and the air layer pileup effect to inhibit heat transfer. The heat transfer enhancement capacity for the S-type tube is determined by the relative magnitude between the scouring effect and the pileup effect. Both effects increase as bend radius decreases; therefore, selecting an appropriate bend radius is crucial.