Effects of curvature and inclination on external steam condensation with non-condensable gases under natural convection

Steam-air condensation is a significant heat transfer process within the thermal-hydraulics field. Previous research has predominantly focused on heat transfer tubes with large curvatures, resulting in experimental correlations that are largely limited to circular tubes and often neglect the effects of curvatures and inclinations on external condensation performance of tubes with limited curvatures or even flat plates. This study, conducted on the COAST (condensation and species transport) experimental facility, investigated steam condensation with non-condensable gases (NCGs) outside circular tubes/plates of different curvatures, covering a pressure range of 0.2∼1.6 MPa and an NCG concentration range of 7∼90%. The plate configurations included both upper-condensing plate and lower-condensing plate, with the influence of inclination angles also incorporated. The results show that the condensation heat transfer coefficient ( h c ) increases significantly with greater curvature, and the rate of this increase itself grows. The h c of the 100 mm tube shows an improvement of less than 10% compared to flat plate, whereas the 19 mm tube exhibits an average enhancement of approximately 50% compared to the 100 mm tube. Furthermore, the h c increases notably with decreasing inclination, although the magnitude of this improvement gradually diminishes. For the 19 mm tube, the h c of the horizontal tube is 18∼27% higher than that of the vertical tube. For the 100 mm tube, the improvement is only about 5∼15%. The influence of inclination angles on plate condensation differs from that on tubes. For upper-condensing plate, the h c is weaker in the horizontal orientation than in the vertical orientation. While that of the lower-condensing plate initially enlarges and then diminishes from horizontal to vertical orientation. Moreover, the h c of the lower-condensing plate consistently surpasses that of the upper plate. Finally, an experimental correlation applicable to heat transfer tubes and plates with different curvatures and inclination angles was developed. A comparison with existing correlations demonstrates that the proposed correlation has a predictive accuracy within ±20%.