Study On The Effect Of Substrate Property And Relative Humidity On Dropwise Condensation

Condensation is one of the most common physical phenomena in nature and is widely used in the fields of dehumidification,power generation and desalination.There are mainly two condensation types on a surface including the film condensation and the dropwise condensation due to the different surface wettability.Compared with the film condensation,the latter has a better heat transfer.In order to improve the condensation efficiency,people have done a lot of research in different materials and rigid substrates of physical and chemical treated.However,there are few experiments of condensation on soft substrates.And previous investigations of condensation have only been performed at a constant relative humidity,the effect of relative humidity on condensation is not clear.Moreover,the coalescence of droplets plays a key role in droplet growth during condensation.The condensation process can be better understood by studying the dynamics of the droplet coalescence.Accordingly,the main research content of this thesis is divided into three parts:Part?:The effect of substrates stiffness on dropwise condensation was studied,the results show that the condensation rate and surface coverage increase with the softness of substrates.As the surface coverage reaches a constant average value,the surface coverage on rigid substrates begins to fluctuate violently due to the sudden coalescence of large droplets and nucleated of new small droplets.However,the oscillation of surface coverage on softer substrates decreases dramatically as the wetting ridge between two droplets can delay droplet coalescence.Part ?:The effect of relative humidity on dropwise condensation on rigid substrates was studied.The condensation rate and surface coverage increase with the relative humidity.Compared with relative humidity of 90%,the surface coverage at 60% is only slightly lower,but decreases highly at 30%.Moreover,the oscillation of surface coverage was only not found in low relative humidity of 30% within the experimental time we monitored,which can be attributed to the inhibition of droplet growth and thus the coalescence droplets are not large enough to have a significant effect on surface coverage.Part ?:The effect of relative humidity,wettability and stiffness of substrates on coalescence of two sessile microdrops was studied.It's shown that the coalescence contains two fast regimes,in which a liquid meniscus bridging the parent droplets rapidly grows,forming an ellipsis-like composite droplet,and a slow regime,in which the nonspherical composite droplet relaxes to the equilibrium hemispherical cap.Whereas the fast bridging regimes last less than two milliseconds and are almost independent on surface wettability and stiffness,the relaxation regime,which was only observed on sufficiently hydrophobic and rigid surfaces,continues for a few tens of to several hundreds of milliseconds depending on surface properties.It's further demonstrated that the slow droplet relaxation can be described neither by the bulk hydrodynamics nor by a microscopic model concerning liquid evaporation near the droplet edge,but by the molecular kinetic theory for the motion of the three-phase contact line.