Phase Change Condensation Heat Transfer On Multi-scale Surfaces

Phase change condenstion heat transfer is widely found in power plants,refrigeration,electronic cooling and other industrial fields,and the enhancement of its heat transfer is conducive to the safe operation of these field devices.Moreover,excellent phase change heat transfer performance of the equipment to save space,reduce the cost of investment,environmental protection and efficient use of energy also has important significance.It is well known that phase change condensation heat transfer has an important connection with the surface material of condensation.From an academic point of view,the cross of phase transition heat transfer and material science,is the future trend of development.In particular,the association of phase change heat transfer with the multi-scale surface of biomimetic functionalization is a new discipline growth point in the world and has certain leading role.The main processes that occur in many micro-and nano-structures of different scales in animal and plant communities are different.Because animals and plants do not need to transfer high heat flux density like "phase-change heat transfer devices," the current cross-study between heat transfer and bionics is still rare.However,abundant multi-scale interface phenomena(non-thermal conditions)in nature can inspire us to take inspiration and solve the conflict that the two-phase kinetic process relies on a single scale in phase transition heat transfer.Therefore,based on the natural bionic multi-scale structure,the superhydrophilic-hydrophobic hybrid surfaces with different specifications were constructed.The spacing between the super-hydrophilic grid lines on the combined surface was 1.5 mm,2.5 mm and 3.5 mm,respectively.At the same time,the superhydrophilic surface and the hydrophobic surface were prepared.The droplet spreading characteristics on superhydrophilic surface and the high temperature characteristics of the hydrophobic surface were tested.Hydrophobic surface drops condensate droplet growth combined with exfoliation process was analyzed.The heat transfer performance of the hybrid surfaces was compared with the smooth surface and the hydrophobic surface,and the steam condensation process on hybrid surface was visualized by a high speed photographer.The study found that the super-hydrophilic surface has a good droplet spread characteristics,droplets from contact with superhydrophilic surface to fully spread out only lasted for a short time of 10.5 ms.Hydrophobic surface in the atmospheric pressure 4 hours boiling water harsh environment can still maintain hydrophobic characteristics,with good physical and chemical properties,to meet the high temperature conditions of steam condensation heat transfer.Superhydrophilic-hydrophobic hybrid surface can better regulate the size of condensed droplets,and its condenstion heat transfer performance is better than smooth surface and hydrophobic surface.At ?T= 4.3 K,the heat transfer coefficients of the 2.5 mm grid spacing hybrid surface are 2.2 times and 1.6 times the smooth surface and the hydrophobic surface,respectively.The superhydrophilic-hydrophobic hybrid surface with smaller grid spacing can achieve smaller droplet radius,but not as small as the spacing of the mesh.With the decrease of the hybrid grid spacing,There will be a temporary continuous liquid film generation on the hydrophobic part of the combined surface,the existence of continuous liquid film is not conducive to enhanced heat transfer.In the hybrid surface of three superhydrophilic grid lines,the combined surface heat transfer performance with the grid spacing of 2.5 mm is the best.At ?T=9.0 K,the heat transfer coefficient of the 2.5 mm grid spacing surface is 1.2 times and 1.8 times the heat transfer coefficient of 1.5 mm grid spacing surface and 3.5 mm grid spacing surface t,respectively.