Clustering And Condensing Of Vapor Molecules Near The Cooling Surface Based On In-Situ Infrared Spectroscopy

Condensing and trapping of water molecules is an important step to strengthen industrial heat transfer and alleviate water shortage in arid areas.In recent years,biological surfaces and bionic engineering surfaces for condensation and collection of water vapor keep emerging,a large number of researchers have fully studied the composite structure and gradient structure.However,most of the studies focus on the macroscopic wettability and movement mechanism of droplets on the structural surface,but systematic studies on the clustering,distribution,evolution and growth law of vapor molecules toward the cooling surface before the formation of the smallest droplets are relatively lacking.Some studies show that before condensation,vapor molecules may first cluster in the near-wall gas phase,and the utilization and strengthening of this accumulation phenomenon will provide new ideas for guiding the design of micro-nano structures on the engineering surface.Therefore,this paper first studies the distribution law of vapor molecules in the normal space along the wall surface,obtains the evolution characteristics of vapor molecules in the near-wall space,and then further analyzes the evolution characteristics and condensation characteristics under the influence of various surface structures.The main work is as follows:By using the attenuated total reflection Fourier transform infrared spectroscopy and the displacement platform,IR spectra in the thin layer near the cooling surface are obtained.It is found that when saturated vapor condenses on the cooling surface,vapor molecules in the gas phase will form clusters.The analysis shows that the smallest droplets go through the transition process of clustering in the gas phase before the formation,and the clusters are deposited on the cooling surface to complete the phase transition.When it's close to the cooling surface,water clusters gradually increases and may tend to be compact spherical shape.The clusters at 600?m away from the cooling surface contain about several hundred water molecules on average.Theoretical calculation shows that with the increase of cluster size,the OH stretching vibration of?H₂O?_n evolves to the low frequency,which also reflects the change of molecular interaction and structure within the cluster.In order to further verify the clustering and distribution characteristics of vapor molecules along the vertical direction of the cooling surface,the common condensing medium--ethanol was used as the experimental system.Firstly,the molecular simulation shows that the ethanol clusters?n=2,3,4,5,6?move towards the low frequency with the increase of the size n,and the red shift was found near the cooling surface by attenuated total reflection experiments.This indicates that the cluster evolution mechanism of ethanol is similar to the condensation process of water molecules.But at the same time,the evolution rules of the two condensates are still different.The analysis shows that compared with water molecules,the steric hindrance of ethyl group on the ethanol molecule makes it more difficult to form clusters,resulting in a smaller distribution range of the ethanol clusters along the cooling surface than that of the water vapor condensation.This phenomenon may also predict that the thickness of the gas-heat transfer boundary layer in the process of ethanol steam condensation is less than that of water vapor condensation.On the basis of the spatial distribution of clusters near the cooling surface,the mechanism of microstructure in near-surface space affecting the condensation process was explored.Through the model analysis,it can be speculated that the reason why typical biological water-collecting structure can strengthen condensation is that the hydrophilic microstructure make the curvature radius of cluster increase when cluster adheres,and directly reache or exceed the critical radius,so that the cluster enters the high-speed growth period and spontaneously grows to the macroscopic droplets.According to the cluster distribution in near-surface space,there is an optimal height range for the microstructure on the surface.This is a new idea for the design of microstructure on the engineering surface.