Experimental Study On Enhanced Condensation Heat Transfer Of Hydrophilic-Hydrophobic Two-Layer Structure

Steam condensing heat transfer is widely used in electric power,air conditioning,air refrigeration,heat pump and chemical industries.Strengthening condensation heat transfer is of great significance for improving energy efficiency and protecting the environment.Dropwise condensation has received extensive attention due to its high heat transfer performance.In the process of dropwise condensation,the droplet is the main thermal resistance during the heat transfer process,and its dynamic behavior is closely related to the surface wettability.Therefore,the behavior of the condensation droplet can be adjusted by adjusting the surface wettability,thereby enhancing the condensation heat transfer.From the perspective of bionics,the superhydrophobic-superhydrophilic composite structure(SHI-SHO)was innovatively prepared based on the basic idea of collaborative drainage.One layer is a super-hydrophobic condensation surface prepared by chemical etching and low energy substance modification,and the other is a super-hydrophilic mastoid structure surface sintered with copper powder(particle diameter is～75μm)and treated with hydrogen peroxide.The condensing heat transfer performance of this combined surface under different subcooling degrees,different cooling water flow rates and different heating power was studied.It was found that the super-hydrophobic surface(SHO)has a micro-nano two-level rough structure,and the droplets can exhibit the Cassie wetting mode.The experimental data shows that the super-hydrophilic mastoid has a strong liquid absorption capacity.When the mastoid is placed under positive gravity,the droplets(about 1.5μm in diameter)can be completely absorbed within 15.00ms.The condensation heat transfer performance of steam on the HI surface,SHO surface and SHI-SHI surface was studied at the same time.The function synergy and scale matching regularity of the two-layer hydrophilic-hydrophobic structure in condensation heat transfer are obtained.The experimental results show that the condensation heat transfer coefficient on the SHI-SHO surface is the largest,followed by the SHO surface,and the HI surface is the smallest.The reason is that dropwise condensation can be achieved on the SHO surface,and the heat transfer resistance is small.And the super-hydrophilic mastoid has a porous structure inside,which can absorb the droplets on the surface in time.This can further increase the drop-off frequency of the droplets,improve the self-renewal capacity of the condensation surface,and reduce the thermal resistance,which ultimately leads to a larger condensation heat transfer coefficient of the composite surface.When △T=5.0K,the condensation heat transfer coefficient of SHI-SHO surface is 4.8 times and 1.8 times higher than that of HI and SHO,respectively.The movement of the droplets was visually observed and four methods of detachment of condensed droplets on the SHI-SHO structure were proposed:adsorption detachment,bouncing detachment,sliding detachment(scouring action),and fluid detachment from the tip of the mastoid.Aiming at the adsorption detachment of droplets,Gibbs free energy and surface energy theory was used to establish a droplet dynamics analysis model,which explained the unique droplet dynamics behavior of droplets on the two-layer structure surface.In addition,the effect of cooling water flow and heating power on the condensation heat transfer performance was studied.The experimental results show that the condensation surface has a larger heat transfer coefficient at low cooling water flow and high bottom heating power.This innovative design provides a reference for improving the condensation heat transfer performance in industrial production.