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Date of Publication:2017-01-01

Journal:科学通报

Affiliation of Author(s):化工学院

Volume:62

Issue:10

Page Number:1082-1089

ISSN No.:0023-074X

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 noncondensable 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 noncondensable 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.

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