Effect of mixed hydrophilic and hydrophobic surface coatings on droplets freezing and subsequent frost growth during air forced convection channel flows

The formation of frost and ice decreases the energy efficiency of refrigeration systems and of air-source heat pump systems. This paper presents new experimental data of frost nucleation and frost growth on cold flat plates operating in frosting conditions with air forced convective flow. The plates surfaces had different wettability: from an uncoated aluminum surface with fine finish roughness and contact angle of 75°, to hydrophobic (θ ≈ 110–116°) and hydrophilic (θ ≈ 19–29°) coatings. Three biphilic coatings with regions of hydrophilic coatings adjacent to areas with hydrophobic coatings were investigated. These coatings manage water droplets movements, reduce freezing temperatures, and delay icing and onset of frost nucleation. A new experimental technique was developed in order to mimic actual field type operating conditions of convective channel flow experienced by fin structures of heat exchangers of typical air-source heat pump systems. Hydrophobic surface and one biphilic surface had early phase changeovers and high thresholds of the frost thickness before switching to the frost growth phase due to the presence of large droplets on the surfaces before they froze into ice beads. Fine-finished aluminum surface and hydrophilic coating had delayed phase changeovers and low frost thicknesses with respect to the other surfaces.