چكيده به لاتين
Condensation occurs on surfaces in two distinct modes including dropwise and filmwise one. The filmwise condensation is the most common form of this regime in which a thin condensate layer covers the surface. In dropwise condensation, the condensate forms as distinct droplets on the surface. Growing, coalescence and finally falling up of the droplets in dropwise condensation is so that some of the surface is bare of the condensate in every instant. Due to lower thermal resistance between the vapor and the surface occurring in dropwise condensation, the heat transfer coefficient of this regime of heat transfer is much higher than the filmwise condensation. Dropwise condensation occurs on hydrophobic surfaces. Micro-nano structures have the main role in the appearing the hydrophobicity. Therefore, the main step in studying dropwise condensation is an investigation on the role of micro-nano structures on the formation of hydrophobicity. In this study the role of micro-nano structures on dropwise condensation and hydrophobicity is studied from theoretical and experimental point of view. To do this, first, hydrophobic and superhydrophobic surfaces are fabricated using some methods like as etching, oxidation, electrodeposition, and anodization. The effect of micro-nano structures are studied using different tools such as SEM images and X-ray difractometry in this section. In the theoretical study, the effects of different parameters such as the shape, dimensions, dispersion, roughness arrangement and morphology of a droplet are studied. From this section it has been revealed that by increasing the height of micro-nano structures (by about 8 times), independent from the morphology of the droplet, heat transfer coefficient is decreased (up to about 7 times). In the experimental study section, first, a condensation experimental apparatus is designed and fabricated based on the requirements revealed from the literature. The results show that the coating time 30 min and concentration 0.025 M of octadecanethiol improves heat transfer by about 5 times on a flat coated surface with respect to a flat surface. The electrodeposition method is used to produce the micro-nano structures on the copper surfaces in this section. The results show that in longer electrodeposition times (135 and 75 sec) and in low subcooling temperatures, dropwise condensation has a better performance than filmwise condensation (by about 2.5 times).
