Experimental Investigation Of Steam Dropwise Condensation On SAM Superhydrophobic Surface At Extra-Low Pressure

Dropwise condensation, with its heat transfer coefficient being several even dozens times higher than that of filmwise condensation, attracts great interests from academia and engineering areas. During recent years, it has become a hot issue among scholars that how to maintain long-time dropwise condensation, especially under the situation of today's energy crisis. It is much better to probe the heat transfer mechanism of dropwise condensation from the microscale viewpoint. In this paper, experiments were conducted to investigate the heat transfer performance, droplets dynamic characteristics as well as droplets temperature distribution of steam condensation at extra-low pressure.In order to promote dropwise condensation, two functional surfaces, hydrophobic surface (SAM-1) and superhydrophobic surface (SAM-2), were prepared with n-octadecyl mercaptan on copper substrates by self-assembled monolayers technology. The contact angles were measured before and after experiments. Also, the topography of these two surfaces was analyzed with SEM.The heat transfer experiments were conducted on the two surfaces under surface subcooling degree ranging from 2K to 8K at 4.5kPa,7.0kPa and 10kPa, respectively. The results indicate that steam condensation heat transfer coefficient decreases with the pressure decreasing; the higher the pressure is, the faster the value decreases as the subcooling degree increases. Besides, the heat flux ratio of SAM-1 to SAM-2 is 1.84 at 10kPa,1.21 at 7kPa and 1.01 at 4.5kPa due to the increasing interface thermal resistance. Heat transfer performance of smooth SAM-1 surface is better than that of rough SAM-2 surface under experimental conditions.A high speed camera was introduced to explore droplets dynamic characteristics in condensation process. By analyzing the pictures captured, the departure radius of droplets increases, and the growth cycle of droplets prolongs as the pressure decreases. The growth cycle of droplets on SAM-2 is much larger than that on SAM-1 as well as the departure radius of droplets.In addition, an infrared thermography camera was adopted to monitor and analyze the surface temperature distribution of droplets when the experimental pressure was at 1kPa-10kPa, with the subcooling degree between 2K-8K. The results reveal that the surface temperature of a droplet, which is the highest at the apex and the lowest at the edge, varies with the droplet diameter; the larger the diameter is, the higher the surface temperature becomes. Surface temperature of the droplet increases when the droplet is moving downward, which gives rise to steam turbulence, fastens steam condensation onto the droplet directly. Steam turbulence, also caused by the coalescence of droplets in vicinity, accelerates the steam condensing onto droplet surface, resulting in local temperature-jump. A special structure with grooves in horizontal direction is constructed on the SAM-2 surface in local area, which can enhance heat transfer performance due to the suction effect on the condensate at that area when droplets depart from the surface near the structured area.