Visualization study on droplet dynamic characteristics in dropwise condensation on inclined super-hydrophobic tube

Abstract

Steam condensation with non-condensable gas exists widely in petro-chemical, power plant and seawater desalination processes. Super-hydrophobic surfaces with high contact angle could be an promising condensing surface to promote DWC and enhance heat transfer. As pure steam condensing droplet completely occupies micro/nano-structures of super-hydrophobic surfaces, super-hydrophobic function becomes “invalid”. For stream-air mixture vapor, the non- condensable gas would occupy the micro/nano-structures instead of water. Droplets form Cassie or transition wetting mode, holding or partly holding super-hydrophobic feature. Inclination angle of the tube can influence the shape, size and dynamic characteristics of the condensing droplets, thus changing the heat transfer performance. Therefore, for getting deeply insights of the effect of the non-condensable gas on condensation heat transfer, it is necessary to study the condensing drops characteristics on super-hydrophobic tube for different inclined angles. According to dropwise condensation model, increasing contact angle hysteresis, departure droplet size, sweeping period would reduce the condensation heat transfer efficiency. In this paper, dropwise condensation is performed on super-hydrophobic copper tube, departure droplet size, sweeping period and the phenomenon of contact angle hysteresis are thoroughly studied and are compared with the case of hydrophobic surface or pure steam to explore the influence of inclined angle and surface wettability on droplet dynamics to achieve condensation heat transfer enhancement. The experimental result shows: (1) On the top of copper tube, for the condensation of steam with 10% non-condensable gas, the contact angle hysteresis on super-hydrophobic surface is smaller than that on hydrophobic surface. Compared with pure steam condensation on super-hydrophobic surface, the contact angle hysteresis of mixed steam is smaller because the non- condensable gas fills the micro/nano-structures. However, on the middle and bottom parts of the tube, the contact angle hysteresis on super-hydrophobic surface is greater than that on hydrophobic surface for the condensation of both pure steam and mixed steam. This phenomenon indicates that the contact angle hysteresis is determined by surface wettability instead of inclination angle. (2) The departure droplet size on super-hydrophobic surface increases with the inclination angle and the opposite trend is found on hydrophobic surface. With the increase of inclination angle, the support force provided by the tube would increase and change direction. So the wettability of the surface and departure droplet size would be changed. (3) The sweeping period of droplet on super-hydrophobic surface is longer because the micro/nano-structures increase the adhesion between the surface and the droplets. With the increase of the inclination angle, the sweeping period decreased slightly, shorter than the sweeping time when the tube is vertical. The sweeping period of vapor with non-condensable gas condensation is greater than that of pure steam. The presence of non-condensable gas increases thermal resistance, thus slowing down the drop growth rate and increasing the sweeping period. To sum up, on the top of copper tube, the contact angle hysteresis on super-hydrophobic surface is smaller than that on the hydrophobic surface. However, the totally opposite trend is found on the middle and bottom parts of the tube. The departure droplet size on super-hydrophobic surface increases with the inclination angle. The sweeping period of droplet on super-hydrophobic surface is longer. This work offers a new avenue to further enhance heat transfer of condensation with non-condensable gas.