Dropwise Condensation Heat Transfer Model And Droplet Dynamic Characteristic On The Nanocone-structured Surface

In the field of heat transfer,dropwise condensation has a high heat transfer coefficient.Dropwise condensation occurs on the superhydrophobic surface due to the natural characteristics of the superhydrophobic surface.The superhydrophobic surface with a nanocone structure achieves the efficient self-departure of condensed droplets more easily,and greatly improves the condensation heat transfer coefficient.Therefore,the dropwise condensation process and droplet dynamic characteristic on the nanocone-structured surface are investigated in this work.Firstly,we study the droplet growth rate in two modes.One keeps the constant contact angle(CCA)by increasing the bottom contact area,and the other keeps the constant bottom(CB)diameter by increasing the contact angle,and the mathematical models are obtained.The various factors affecting the droplet growth rate is analyzed,and the sensitivity analysis about these factors is performed.It is found that the contact angle has the most significant effect.Secondly,we present a comprehensive model for dropwise condensation heat transfer(DCHT)on a nanocone-structured superhydrophobic surface.Condensated droplet heat transfer,wetting morphology,size distribution,and dynamics are incorporated into the developed model.Experimental data in existing literatures are used to verify the theoretical model.The influences of the thermal physical properties and geometry of nanocones on the droplet growth rate and DCHT coefficient are then analyzed.We conclude that the droplets have rapid growth rate and better performance of DCHT when nanocone thermal conductivity is greater than 100 W /(m·K),the tip size is about 15 nm,height is in the range of 0.1 to 0.3 ?m and interspaces are less than 50 nm.A sensitivity analysis of the factors influencing the DCHT coefficient is further performed.These results provide theoretical guidance to optimize the design of nanocone-structured surfaces.Moreover,the Cu-based nanocone-structured surface coated with ZnO thin film was prepared by a new synthesis method in this paper.The scanning electron microscope and three-dimensional microscopic imager are used to observe the dynamic behaviors of droplets on the nanocone superhydrophobic surface,such as growth,merging and bouncing,and the results are compared with smooth copper surfaces.It is found that the self-jumping of droplets are more likely to occur on the nanocone surface,so it has a higher heat transfer coefficient.