Effective reduction of non-condensable gas effects on condensation heat transfer: Surface modification and steam jet injection. (25th June 2020) - Ji, Dae-Yun
Highlights: The effects of non-condensable gas(NCG) on condensation heat transfer were studied. Super-hydrophobic surface modification and steam jet injection method were compared. Attached filmwise condensation was observed on a super-hydrophobic tube. The steam jet injection method yielded a steady heat transfer enhancement ratio. Abstract: Various methods have been proposed to reduce the performance degradation effects of non-condensable gases (NCG) on condensation heat transfer. Among the papers that can reduce the negative effects of NCG, the paper about comparing passive and active methods is insufficient. In this study, to overcome the performance degradation effects of NCG, super-hydrophobic surface modification (passive method) and the steam jet injection method (active method) were applied to horizontal aluminum tubes, which were evaluated experimentally. The test variables were the initial pressure and total pressure for adjusting the NCG mass fraction. The overall and condensation heat transfer coefficients were derived according to the NCG mass faction. In surface modification experiments, super-hydrophobic tubes exhibited better overall heat transfer coefficients up to three times that of filmwise condensation in bare tubes. By reducing the NCG mass fraction from a high level to a low level, the condensation phenomena of the super-hydrophobic tubes transformed in the order of dropwise condensation, flooded condensation, and attached filmwise condensation, resulting in the degradation of heat flux efficiency. In particular, attached filmwise condensation resulted in worse condensation performance than normal filmwise condensation, so no performance improvements can be expected based on surface modification under these circumstances. In contrast, the steam jet injection method provided a noteworthy heat transfer enhancement ratio of 1.18–1.77 times for a broad range of NCG mass fractions.