Design And Preparation Of Bionic Interface For Vapor Enrichment With Small Subcooling

Desalination is an important solution to the crisis of fresh water resources;solar energy is a renewable energy source with the advantages of cleanliness and longevity.Using solar energy for desalination is an effective measure to solve the water shortage and energy crisis.In a solar desalination system,seawater is evaporated into vapour under the effect of solar photothermal on the evaporation side and then collected as fresh water by cooling on the condensation side.At present,the solar photothermal utilisation rate on the evaporation side has reached more than 95%,and a sufficiently large photothermal utilisation efficiency can make the evaporation side continuously produce vapour,while the maximum vapour collection efficiency on the condensation side is only about 50%,which becomes a major factor limiting the efficiency of desalination systems.Unlike large industries such as smelters and power plants where the vapour and condensing surface has a large subcooling degree and where the vapour is easily nucleated on the surface.The solar desalination vapour recovery surface is subject to thermal radiation and the temperature difference between the vapour temperature and the cooling surface temperature is small(only about 10 K).This results in a high barrier for vapour nucleation on the condensing surface,while the droplets have difficulty rolling on the conventional collecting surface.The low frequency of condensing surface renewal severely limits the efficiency of vapour collection.The water collection characteristics of plants and animals in nature,which have evolved over time,provide inspiration for the design of efficient condensation and water collection surfaces,such as hydrophilic surfaces that reduce the nucleation barrier for droplet nucleation and hydrophobic surfaces that reduce the droplet rolling angle for droplet transport.To this end,this thesis regulates the kinetic properties of condensate droplets by modifying the surface chemistry or rough structure of a bionic functional surface prepared.The ultimate goal is to improve the condensation characteristics under small temperature difference conditions.The details of the study and the main findings are as follows:(1)A multi-stage solar desalination experimental system was designed and constructed.To investigate the efficiency of multi-stage temperature transfer and the effect of different number of stages on the water production performance of this system.The multi-stage structure provides significantly better utilisation of temperature compared to the 1-stage desalination system.With regard to the 120-minute experimental sample of this system,the freshwater production of the 3-stage and 6-stage desalination systems was increased by 88.7% and 127.8% respectively compared to the 1-stage system.Therefore,the multi-stage desalination system was valuable for development and utilization.(2)Biomimetic superhydrophilic aluminium surfaces and hydrophobic fluorinated aluminium surfaces were prepared to investigate the effect of functional surfaces with different wettability on the water collection efficiency of desalination systems.A condensation experimental platform was built to investigate the condensation characteristics of the above surfaces and aluminium surfaces.The three surfaces were used as the condensation side of a multi-stage solar desalination experimental system.The investigation showed that the hydrophobic fluorinated aluminium surface is the first to produce fresh water,but has a slow water production rate;the superhydrophilic aluminium surface produces fresh water later,but has a fast fresh water production rate.At the end of the experiment,each type of surface produced approximately the same amount of fresh water.This suggests that homogeneous functional surfaces play a limited role on the condensation side and have shortcomings in the application of freshwater harvesting.(3)Based on molecular dynamics simulation methods,a functional surface was designed that can achieve self-driven of droplets along a curve.The mechanism and properties of droplet motion on this surface were investigated.In addition,the effects of surface wettability differences,the width of the curve track and the temperature on droplet motion were assessed.For the same wettability difference,the hydrophobic-hydrophobic combination of the curved track and the background region provides a greater driving force for droplet motion than the hydrophilic-hydrophobic surface and the hydrophilichydrophilic surface;For the same wettability difference,the hydrophobic-hydrophobic combination of the curved track and the background region provides a greater driving force for droplet motion than the hydrophilic-hydrophobic surface and the hydrophilichydrophilic surface;Increasing the width of the curved track leads to increasingly faster droplet motion.However,a too large track width reduces the contact area between the droplet and the boundary,which slows down the droplet motion or even causes it to stagnate;Maintaining the droplet in a liquid state,an increase in temperature increases the internal energy of the molecules,accelerating the droplet’s motion along the curved track.This provides microscopic support for the creation of macroscopic surfaces.