| Condensation is ubiquitous gas-liquid phase change phenomenon in nature and is widely applied in industrial fields such as chemical industry,heat exchange,water collection,seawater desalination,etc.Enhancing condensation heat transfer performance is conducive to improving energy utilization efficiency.Dropwise condensation has received great attention because of its much higher heat transfer performance compared with filmwise condensation.Previous research has demonstrated that groove structures can promote the drag of condensate by surface tension and accelerate the removal of condensate from the surfaces,which can enhance the condensation heat transfer.Enhancing filmwise condensation via groove structures has been studied intensively,while the investigation of enhancing dropwise condensation via groove structures is comparatively scant.Therefore,study on heat transfer enhancement of dropwise condensation on grooved surfaces was conducted in this dissertation.Firstly,wettability-interval grooved surfaces with hydrophilic grooves and hydrophobic ridges were prepared.Besides,the humid air condensation experimental platform was built to visually evaluate the condensate dynamic behaviors and measure the condensation heat flux of different functional surfaces including wettability-interval grooved surface,plain hydrophilic surface,plain hydrophobic surface,hydrophilic grooved surface,and hydrophobic grooved surface.The experimental results demonstrated that:the phenomena of spontaneous suction and directional drainage via hydrophilic grooves occurring only on the wettability-interval grooved surface in humid air could not only remove condensate quickly but also suppress the formation of the flooded liquid film,which were superior to the enhancement of heat transfer performance compared with the ordinary gravity-driven droplet sweeping on homogeneous surfaces.The heat flux of the wettability-interval grooved surface at subcooling 12 K could reach 1280 W/m~2,which was 1.25 times that of the plain hydrophobic surface,and 15%higher than that of the hydrophobic grooved surface.Furthermore,inspired by Sarracenia,which can harvest water efficiently due to its hierarchical microgrooves that differ from conventional groove structures,a three-tier grooved surfaces were designed and prepared,which provided an alternative approach to enhance the condensation heat transfer performance.The visual experimental apparatus was built and verified to precisely explore the effect of wettability and subcooling degree on the condensate dynamic behaviors and heat transfer characteristics.The main conclusions are as follows:(1)Subcooling degree was an important factor affecting the condensate dynamic behaviors and heat transfer characteristics.An obvious shift from dropwise condensation to filmwise condensation was observed on the hydrophobic grooved surfaces as the subcooling degree increased.The condensation heat flux increased with the increasing subcooling degree,while the heat transfer coefficients decreased with increasing subcooling degree and converged to the value of Nusselt theory.In addition,surface wettability was another significant factor dominating the condensation heat transfer performance,and the heat transfer coefficients on the hydrophobic grooved surfaces were higher than those on the hydrophilic grooved surfaces.(2)For hydrophobic three-tier grooved surfaces,the dynamic behaviors of condensate varied from subcooling degree.At low subcooling degree(0K～5K),unlike the conventional hydrophobic one-tier grooved surfaces,droplet jumping could be induced by the coalescence of the droplets between adjacent grooves on the hydrophobic three-tier grooved surfaces.The sidewalls of the hierarchical grooves could make the droplets adjoining the walls jump obliquely along the sidewalls and trigger droplet-jumping-induced sweeping,which was an efficient droplet removal phenomenon.At medium subcooling degree(5K～8K),the hierarchical grooves functioning as inhibiting the growth of large liquid blocks came into effect.Stable Cassie state of droplets were achieved by the support from the hydrophobic groove-III.The lateral growth and movement of the droplets were hindered by the sidewalls of hydrophobic groove-II and groove-I.The rupture of the large liquid blocks could be obtained by the suction effect of the grooves.At high subcooling degree(8K～15K),the hydrophobicity of the hydrophobic three-tier grooved surfaces gradually digressed and the whole surface changed into filmwise condensation.(3)The condensation heat transfer coefficient of the three-tier grooved surfaces was higher than that of the conventional one-tier grooved surfaces.Especially,the hydrophobic three-tier grooved surfaces had the highest heat transfer coefficient for the reason that the hierarchical grooves could promote droplet jumping at low subcooling degree and inhibit the lateral growth of liquid blocks into large liquid films at medium subcooling degree,which was 30%higher than that of the conventional one-tier grooved surfaces.Therefore,the hydrophobic three-tier grooved surfaces could enhance condensation heat transfer performance at low and medium subcooling degrees. |
