Fractal Analysis Of Growth And Heat Transfer Of Droplets In Dropwise Condensation

It is very common that saturated water vapor condenses when meeting cold surfaces in many enginnering fields such as nuclear engineering, chemical engineering, aviation and ariro-space engineering as well as plant surface. There are two types of condensation when water vapor encounters cold solid surfaces:filmwise condensation and dropwise condensation. About which one can be formed depends on surface properties of solid and liquid, in other words, contact angle between solid surface and liquid. This question is investigated firstly in this work. Based on energy balance, expressions of advancing and receding contact angle of droplet are derived by considering both surface roughness factor and liquid wetting behaviors on rough surface. By means of the present expressions, we can well understand experimental data, which could not be well explained by Wenzel equation and Cassie-Baxter equation.Recently, a large number of experiments has shown that heat transfer coefficient of dropwise condensation is larger several or several ten times than that of filmwise condensation. However, the size span of droplets in dropwise condensation, ranging from nanometer to millimeter, is very large; nucleation, growth and coalescence of droplets on condensing surface are also quite random and complex. Therefore, the growth process and heat transfer of droplets are still unclear. In this thesis, these questions are studied from theory and simulation. From the research results, it is found that the relationship between the function of droplet-size distribution and droplet size is of power-law property, and droplet-size distribution on condensing surface, like the pore-size distribution in porous media, is fractal. A fractal model for dropwise condensation heat transfer is developed based on the fractal characteristics of droplet-size distribution on condensing surfaces, and good agreement between the model predictions and experimental data is found.This thesis is organized as five Chapters. In Chapter 1, the progress of nucleation mechanism, growth, heat transfer of droplets and the implementation method of dropwise condensation are reviewed, and fractal geometry theory is introduced. In Chapter 2, contact state between droplet and rough surface is discussed, and the expression of equilibrium contact angle is given. Meanwhile, a fractal model for rough surface is constructed. Chapter 3 focuses on droplet-size distribution with static and dynamic state. In Chapter 4, based on the fractal property of droplet-size distribution, a fractal model of heat transfer of dropwise condensation is established. Finally, a summary of the present work is given in Chapter 5, and some potential research interests are commented for future study of dropwise condensation.