Experimental Investigation Of Steam Dropwise Condensation On Hydrophobic Surface

The steam condensation heat transfer has broad applications in the fields of petrolium, chemical, spaceflight, power generation, air-conditioning and refrigeneration. The improvement of the heat transfer performance of steam condensation also is of great significance in saving energy sources, raw materials and project expenditures. Dropwise condensation, with its heat transfer coefficient being several even dozens times higher than that of filmwise condensation, attracts great interests from academic and engineering areas. During recent years, it has become a hot issue among scholars that how to modify the condensing surface for a long time to maintain dropwise condensation, especially under the situation of today’s energy crisis. In this paper, a visualization of condensation heat transfer experiment device is installed, and experiments were conducted to investigate the heat transfer performance and droplets dynamic characteristics on two different surfaces.Hydrophobic coating of n-octadecyl mercaptan self-assembled monolayers (SAM-1) were prepared by self-assembled monolayers technology, and graphene film (SAM-2) were prepared by roll to roll production technology on substrates of copper testing pieces. The contact angles were measured using a self complied program on the two surfaces before experiments, which are 108.5° on SAM-1 and 89.7° on SAM-2.Visual Basic program was used to calculate the ROSE dropwise condensation model, and the calculation results are consistent with ROSE’s. At different degree of subcooling (0.5-20K) and different pressures (5,10,20,30,40,50,60,80 and 101 kPa), the trends of heat flux and heat transfer coefficient of smooth hydrophobic surfaces are analyzed. Under the same experimental conditions, at the same degree of subcooling, the heat flux and heat transfer coefficient increase with the increase of system pressure; at the same system pressure, the heat flux increases and the heat transfer coefficient decreases with the increase of subcooling degree; and the decline rate of the heat transfer coefficient is getting smaller with the increase of the subcooling degree. Mainly because when the system pressure is high, there is sufficient amount of steam per unit volume in the experimental system could be involved in condensation progress; and the vapor-liquid interfacial thermal resistance is correspondingly smaller, thus it had a better heat transfer performance.The vapor condensation heat transfer characteristics on two different surfaces were experimentally studied under system pressure 40 kPa, subcooling degree range of 1-26 K and cooling water flow of 11/min. The results indicate that condensation heat flux and condensation heat transfer coefficient on both surfaces have the similar trend that the stream condensation heat transfer coefficient decreases and the heat flux increases with the increase of subcooling degree. A high speed camera was used to explore droplets dynamic characteristics in condensation process. It showed that dropwise condensation was formed on the surface of n-octadecyl mercaptan self-assembled monolayers SAM-1, and dropwise-filmwise coexistence state was formed on the surface of graphene film, which obviously had a contact angle hysteresis and smaller contact angle. Under the same experimental conditions, the heat flux of n-octadecyl mercaptan smooth hydrophobic surface is 1.6 times that of the graphene surface, and when the subcooling degree is small (less than 2K.), the heat flux of hydrophobic surface could be increased by 2 to 3 times of that of the graphene surface.