Dropwise condensation (DWC) shows superior condensation heat transfer compared to filmwise condensation (FWC); however, it is limited by the gravity-dependent shedding of condensate. Surface wettability plays a dominant role in controlling condensate-surface interactions throughout nucleation, growth, and departure. In this work, helically bi-philic patterns were fabricated on vertically oriented copper tubes for external condensation, with non-condensable gas (NCG) concentrations in the region of ∼2%. Surfaces with varying ratios (γ) of hydrophobic to hydrophilic region width were synthesised by selectively coating poly-di-methyl-siloxane (PDMS) brushes as a hydrophobic modifier on copper tubes and these were then parametrically optimised. It was shown that PDMS could be grafted in a thermal method from silicone oil, leading to hydrophobicity. The parametric study showed that surfaces with a γ=1.5 gave the best results in condensation, leading to a heat flux enhancement of 67% at a subcooling of 20°C compared to pure DWC. Bi-philic surfaces with the smallest hydrophilic region width tested of 0.8 mm showed higher heat transfer compared to control, PDMS coated and bi-philic surfaces with higher hydrophilic widths, indicating that the hydrophilic regions primarily act only as a drain pathway from the DWC regions. While the helical arrangement creates a longer condensate drainage path in FWC zones, the early onset of droplet departure from DWC regions leads to overall enhanced performance.
Goswami et al. (Wed,) studied this question.