Research Of The Enhancement Of Condensation And Heat Transfer By Surfaces Wettability Modification On Cooper Substrates

Condensation heat transfer is widely used in many industrial engineering fields,such as condensers for power plants,condensers for refrigeration system and so on.Condensation form can be classified into filmwise condensation（FWC）and dropwise condensation（DWC）.Most heat transfer surfaces applied on engineering fields are hydrophilic surfaces which have a higher density of condensation nucleation and a higher rate of condensate conduction.Unfortunately,FWC is also formed on these hydrophilic surfaces.With the liquid film covering the surfaces,the condensation heat transfer resistance is increased and the heat transfer efficiency is weakened.Compared with FWC,DWC is a more effective way of condensation.Since condensate exhibits in the form of droplets on hydrophobic/superhydrophobic surfaces,it allows more areas of the condensation surface to be exposed to the vapor.Nevertheless,hydrophobic/superhydrophobic surfaces still have shortcoming,such as a lower density of condensation nucleation and a lower droplet departure rate.To combine the advantages and disadvantages of the hydrophilic and hydrophobic surfaces applied on condensation,hybrid wettability surface is proposed.With a reasonable pattern design,hybrid wettability surface can effectively balance the advantages and disadvantages of hydrophilic and hydrophobic surfaces and enhance the condensation heat transfer theoretically.Therefore,research on the influence of the wettability modification of the hydrophilic metal surface and hybrid wettability surface on condensation heat transfer have great significance.This article mainly focused on the effect of superhydrophobic surface and hybrid wettability surface on the condensation heat transfer of vertical surfaces.In this regard,a condensation experiment system was built,and the PTFE nanoparticle coating was applied on the copper surfaces to fabricate superhydrophobic surfaces by spraying technology which has less technical difficulty and low cost.To investigate the effect of the patterned hybrid wettability surface on the condensation heat transfer,a Photosensitive Film Protection method was applied to prepared the experimental surfaces with parallel-stripes patterns dip to the width direction of the copper substrates with inclined angles of 60°and 90°.At the same time,a Laser Processing method was used to prepared vertical-stripes-patterned and tree-patterned hybrid wettability surfaces with different superhydrophobic region widths（0.4mm-1mm）.Eventually,visual condensation heat transfer experiments were conducted to observed the condensation phenomena,obtain the heat transfer coefficient,and explore the relationship between the condensation phenomena and the heat transfer effect of different surfacesThe results of the experiment indicated that the wetting model of droplets formed on the superhydrophobic surface tended to represent the Wenzel wetting model which contributed to the difficulty of droplets departing from the surface.The maximum diameter of the condensation droplet observed on the superhydrophobic surface was 3.99mm.As a result,the heat transfer coefficient（HTC）of the superhydrophobic surface was lower than FWC when the condensation heat transfer temperature differenceΔT_sub was in the range of 0-20K.The condensation heat transfer of the superhydrophobic surface and the hybrid wettability surfaces fabricated by Photosensitive Film Protection method had a strong correlation with the droplet and liquid bridge departure rate respectively.Therefore,the HTC of the superhydrophobic and hybrid wettability surfaces increase with the increase ofΔT_sub.Hybrid wettability surfaces fabricated by Photosensitive Film Protection method exhibited obvious liquid bridge phenomenon during condensation which had significant effect on the HTC of the surfaces.However,the liquid bridge phenomenon was not obvious on the hybrid wettability surfaces fabricated by Laser Processing method,and this was attributed to the higher liquid capacity and condensate drainage rate of the hydrophilic region of the laser-machined surfaces.The insignificance of the laser-machined hybrid wettability surfaces contributed to the HTC decreased with the increase of theΔT_sub.However,the hybrid wettability surfaces fabricated by Photosensitive Film Protection method were able to enhance the HTC of the superhydrophobic surface.The highest HTC of16.64k W?m^-2?K^-1 whenΔT_sub of 11.3K were observed on the hybrid wettability surface（60°-parallel-stripes pattern）,whose HTC was 2.14-fold compared with the superhydrophobic surface’s.While an HTC of 13.63 k W?m^-2?K^-1 which was 1.68-fold compared with the superhydrophobic surface was observed on the hybrid wettability surface（90°-parallel-stripes pattern）whenΔT_sub of 13.8K.Moreover,the hybrid wettability surfaces fabricated by Laser Processing method were able to enhance the HTC of FWC.The optimal pattern design of the laser-machined surfaces was the vertical stripes pattern with the hydrophilic-stripe width of1mm and superhydrophobic-stripe width of 0.8mm,whose HTC was 18.44k W?m^-2?K^-1 and1.63-fold compared with the FWC’s whenΔT_sub of 11.27K.The Suboptimal pattern design of the laser-machined surfaces was the vertical stripes pattern with the hydrophilic-stripe width of1mm and superhydrophobic-stripe width of 0.6mm,whose HTC was 50.03k W?m^-2?K^-1 and1.97-fold compared with the FWC’s whenΔT_sub of 0.45K.Meanwhile,the experiments conducted on the laser-machined hybrid wettability surfaces indicated that the effect of the width of superhydrophobic stripes of the hybrid wettability surfaces on the condensation heat transfer was reflected in the balance of limiting the diameter of the condensate droplets on the superhydrophobic region and the increasing area ratio of the hydrophilic region.Moreover,while having the same hydrophilic area ratio,the tree pattern design was more reasonable than the vertical stripes pattern design.