Early stage condensation frosting characteristics on plain and nano Al2O3-epoxy mixture-coated brass

Early stage condensation frosting on plain and nano Al₂O₃-epoxy mixture-coated brass were experimentally investigated. The contact angles of water on the plain and coated brass were 95.8° ± 80.8° and 135.5° ± 80.5° respectively. Surface temperature of both samples ranged from −7.4 °C ± 1.6 °C to −21.3 °C ± 0.3 °C. Dropwise condensation, crystal growth and early frost layer growth were observed. Effects of surface temperature and hydrophobicity on nucleation, crystal growth and crystal shape were examined. It is found that the critical nucleation size of the condensed droplet varies with surface hydrophobicity at different surface temperatures. The crystallization is remarkably postponed using the present coating. The crystal height does not increase until t = 1924 s ± 180 s at T_surf = −7.4 °C ± 1.6 °C, t = 422 s ± 57 s at T_surf = −15.0 °C ± 1.2 °C, and t = 73 s ± 12 s at T_surf = −21.3 °C ± 0.3 °C on the coated brass, compared with t = 39 s ± 5.1 s, t = 30 s ± 2.4 s and t = 14 s ± 3.3 s respectively on the plain brass. Besides, three types of crystal shapes are captured, including the column clumping, stacked stellar plates and dendrites. A stronger dendritical pattern is built at a lower surface temperature. However, no obvious dependency of the crystal shape on the surface hydrophobicity is detected. Moreover, the crystal height growth rate is quantitatively analyzed. The maximum growth rate of crystal height, which corresponds to the moment when the sensible heat removal rate equals to the latent heat release rate, increases with decreasing surface temperature, and is achieved sooner at a lower surface temperature or on the surface with higher hydrophobicity.