Convective dropwise condensation heat transfer in mini-channels with biphilic surface

Abstract

For dropwise condensation (DWC) in confined channels, droplets slide on surface and may spoil the nano-pillars, thus the nano-pillars structure is not suitable. Here, we provide a robust biphilic surface with polymer film as hydrophobic wettability. Mesh screen is sintered on copper surface to form welding junction array. Dipping the sintered package in a Teflon solution followed by baking technique forms a hydrophobic coating film. Hydrophilic dots are exposed by separating mesh screen from copper substrate. Condensation experiment was performed in a mini-channel with dimensions of 30.0 × 3.13 × 0.97 mm3 and pure water–vapor as the working fluid. Mass flux covered a range of 10.5–63.8 kg/m2 s. It is shown that condensation heat transfer coefficients on biphilic surface were 5.54 times of those on polished copper surface, and 1.89 times of those on hydrophobic polymer layer surface, maximally. Pressure drops are almost identical in condensers with biphilic surface and hydrophobic surface. For DWC on biphilic surface, growing droplet on hydrophilic dot attracts and merges neighboring droplets to generate directional-droplets-coalescence, explained by the Gibbs energy analysis. Higher droplet departure frequency was observed on biphilic surface than hydrophobic surface. Compared with hydrophobic surface, droplet on biphilic surface has different contact angle, covers several hydrophilic dots and reduces droplet thermal resistance. The above effects explain the heat transfer enhancement mechanism when biphilic surface is present.