Experimental Study Of Bead-like Condensation On The Surface Of Columnar Structures At The Micrometer Scale

According to the different wettability of a surface,the condensation form on the surface can be classified as dropwise condensation and filmwise condensation and the heat transfer coefficient of dropwise condensation is several times or even tens of times higher than that of film condensation.Dropwise condensation is easier to formed on a superhydrophobic surface to greatly improve the condensation heat transfer coefficient.Therefore,the optimization of the microstructure on the surface has become the research hotspot in the heat transfer field.For this reason,a mathematical growth model of single droplet was established and the heat transfer model of the dropwise condensation on a microstructured surface was proposed in this paper.Moreover,the visual experimental platform was setup to investigate the process and dynamic characteristics of dropwise condensation on the surface of micro-structure.The main context are as follows:(1)In this paper,we firstly studied two modes of droplet growth: increasing the bottom contact area to keep constant contact and increasing the contact angle to keep constant bottom diameter to establish the growth model of a single droplet.Moreover,through the analysis of various factors affecting the growth rate of droplets,it was found that the influence of contact angle is the most significant.(2)A heat transfer model of dropwise condensation on the surface of micro-structure was proposed,including heat transfer and size classification of droplets and through the comparison between mathematical model and the existing literature,the theoretical results are basically consistent with the experimental results.Furthermore,the effects of the geometrical parameters of the micro-structure on the droplet growth rate and heat transfer coefficient are analyzed.It could provide a theoretical guidance for the optimal design of micro-structured surface.(3)Four kinds of microstructured surfaces including cylinder,square column,cone and pyramidal column were made by femtosecond laser and a visual experimental platform was built to study the condensation heat transfer performance and droplet dynamic characteristics of water vapor on two kinds of micro-structured surfaces.The results show that the condensation heat transfer flux and condensation heat transfer coefficient of the four kinds of surfaces show the same changing trend with the variation of subcooling.Moreover,the heat flux increases and the heat transfer decreases with the increase of subcooling,and the heat flux and heat transfer coefficient increase with the increase ofcooling water flow.(4)Comparing the growth on the vertical and horizontal microstructured surfaces captured by the high-speed camera in the visual experimental platform,it was found that the horizontal surface with pyramidal columns spent the shortest time for droplet growth because of no tilt angle and different hysteresis.Morever,the vertical surface with cylinders spent the shortest time because the self-migration of the droplet can be achieved on a superhydrophobic surface but it cannot be achieved on a hydrophilic surface.