Dropwise condensation heat transfer of the surface with micro columns

Abstract

Condensation is the process of changing a substance from a gaseous state into a liquid state, which is a phase-change process and has been widely applied in many fields. Dropwise condensation occurs on hydrophobic and superhydrophobic surfaces, and thus, has a much higher heat-transfer coefficient than film condensation. In this study, four types of micro-structured surfaces including cylinder, cube, cone and pyramid with the same height of 6μmwere made on silicon surfaces. Due to the different surface morphology, the micro-structured surfaces with cylinders and cubes had superhydrophobic properties with the contact angle of 158.4° and 155.5°, respectively; while the micro-structured surfaces with cones and pyramids had hydrophilic properties with the contact angle of 86.4° and 44.4°, respectively. Through the experimentally study of the heat transfer and dynamic characteristics of droplets on the micro-structured surfaces, the results showed that under the same conditions (steam pressure is 0.1MPa, saturated steam temperature is 100 °C and steam flow rate is 0.298 ± 0.02 m3/h), the heat flux increased linearly with the increase of the cooling water temperature difference and the subcooling, respectively. Moreover, condensation heat transfer coefficient decreased as the increase of the subcooling. Because of less sharp edges and bigger contact angles, the cylindrical micro-structured surface had more excellent droplet mobility leading to faster condensation process and heat transfer. Through the comparison of the droplet growth in the horizontal and vertical directions, it was found that the micro-structured surface with more sharp edges had a larger droplets density. However, the sharp edges prevent the coalescence and shedding which slowed down the heat transfer. Therefore, the cylindrical micro-structured surface spent the shortest time in the whole growth process and thus had the best heat transfer performance.

Keywords

• Dropwise condensation
• Heat transfer
• Superhydrophobic
• Micro-structured surface