A Visualization Study Of Water Vapor Condensation Process On Superhydro-Phobic Hierarchical Porous Surfaces

As an ubiquitously seen gas-liquid phase change process in our daily life,water vapor condensation is most likely refered to the heterogeneous nucleation process on a low temperature surface which is cooler than the saturation temperature at a regulated pressure.In the mean while,as a rapid and highly efficient heat transfer method,water vapor condensation is also involved in a variety of industrial applications,such as desalination,power generation,safety protection in nuclear power station and air-conditioning ventilation system,etc.Conventional condensation mode could be divided into two types,filmwise condensation and dropwise condensation.Comparing to the filmwise condensation,the condensates are formed on the surface as discrete droplets(in microscale)with lower thermal resistance than the continuous liquid film which results in the heat transfer enhancement.During the past decade,an emerging mode of condensation has attracted increasing attention,i.e.,droplet jumping condensation.In the this condensation mode,droplets(in micro scale with even lower thermal resistance)could be removed out from the surface by coalescence-induced droplet jumping without any external force which leads to a better condensation heat transfer performance,comparing to the dropwise condensation.However,with the increase in the surface subcooling,the nucleation sites for droplets condensing would increase gradually which might finally result in the failure of the surface function to facilitate the droplet jumping condensation and transition to the dropwise condensation with a deterioration in the condensation heat transfer performance.To study the spatial confinement and separation effect of the hierarchical structures on the droplet behavior of growth and movement and to realize the coalescence-induced droplet jumping at a large surface subcooling,electrochemical deposition is applied to fabricate superhydrophobic hierarchical porous surfaces with different pore diameters on copper tubes and plates in this work.The modified hierarchical porous structures are detailly observed under the scan electronic microscopic and the characteristic parameters(average pore diameters and thicknesses of porous layer)were statically measured and analyzed.The static contact angle and dynamic contact angle hysteresis are measured by the application of contact angle meter.A visualized experimental system of condensation heat transfer at a low pressure is designed and constructed to observe the condensation process in macroscopical and to test the condensation heat transfer performance of these fabricated superhydrophobic hierarchical porous surfaces.It is shown that,the coalescence-induced droplet jumping is realized on these fabricated surfaces in a wide range of surface subcooling(0-20 K).Comparing to the heat transfer performance of conventional dropwise condensation on a smooth hydrophobic surface,the heat flux and condensation heat transfer coefficient of the fabricated superhydrophobic hierarchical porous surface is enhanced nearly 34% and 190% respectively.Nevertheless,the intensity of droplet jumping on the fabricated surface decreases with the increase in the surface subcooling which finally results in the deterioration of the heat transfer performance.The heat transfer performance of the fabricated surface gradually lowers than that of smooth hydrophobic surface with the surface subcooling exceeding 7 K.In addition,the intensity of jumping droplets would decrease with the increase of pore diameter which would also result in the heat transfer performance of the fabricated porous surface deteriorating with the increase in the pore diameter.By combining the variation trend in the weakening of droplet jumping intensity with the increase of pore diameter and the condensation heat transfer performance on the fabricated porous surface,we raise a hypothesis concerning the spatial confinement and separation effect of hierarchical surface on the droplet behavior of growth and movement,and verify it to be reasonable indicreatly.Another visualized experimental system to observe the droplet behavior of growth and movement is designed and fabricated.By this expermentla system,the droplets behavior of growth and movement is observed in micro scale,as well as the droplet distribution,in condensation process.It is shown that,the diameter of droplets that might be able to jump off the surface induced by the coalescence increases with the increase of surface pore diameter and more relatively big droplets are observed on the surface with a bigger pore diameter,which finally results in a higher surface coverage and the detoriation of heat transfer performance.This phenomenon could demonstrate the correctness of that hypothesis(the spatial confinement and separation effect of hierarchical porous structures on the droplets behavior of growth and movement)directly.Furthermore,just this spatial effect effectively prevents the surface from achieving an excessively high droplet nucleation density and transforming into the conventional dropwise condensation through droplet jumping condensation.