# A Review of Research on Dropwise Condensation Heat Transfer
## 1 Introduction  
Heat transfer enhancement is a hotspot in the research of energy conservation, and with hydrology development, more and more materials and methods are introduced in this traditional subject. For enhancing heat transfer between the fluid and surface, much research introduced some new materials into the fluid, such as nanofluids [1; 2; 3]. Using new materials to modify the surface can also considerably improve the heat transfer performance, and condensation heat transfer is one of the prime beneficiaries. The condensation phenomenon exists widely in our daily life and industrial production, such as power generation, water desalination, and condensation of blue gas. There are two different condensation forms of condensing surfaces due to the different wettability. For filmwise condensation (FWC), the condensate can wet the surface well. During the condensation process, a continuous liquid film flowing downward will be formed and the steam must pass through this film to exchange heat of the surface. It will increase the thermal resistance of heat transfer, and thus the heat transfer capability is limited. Dropwise condensation (DWC) was first proposed by [PERSON] [4] in the 1930s. Compared with FWC, DWC cannot form a continuous liquid film; thus, there is no liquid film thermal resistance. DWC involves droplets nucleation, growth, coalescence, and departure (shedding, sliding, or jumping) of droplets at a critical size (\(\sim\)1 mm). DWC has a better heat transfer effect because the dropping of liquid drops increases the disturbance of condensation heat transfer. Therefore, it was recognized as an effective method for enhancing condensation heat transfer. Condensation forms are influenced by the surface characteristics (structural, material) and condensate characteristics. DWC can be promoted by adding condensable gas to the steam or surface modification technique. When the mixed steam (steam is added withalcohol, ammonia, methanol, or refrigerant that can condense and has a low boiling point) condenses on the surface, Marangoni disturbance will occur in the liquid film. Because the mixed liquid with different boiling points has different surface tension, it will bring about the rupture of the liquid film and produce Marangoni dropwise condensation [5,6]. Adding other gases to the steam will inevitably cause the pollution of the steam, so it does not hold well for all cases. By contrast, the surface modification techniques promote dropwise condensation by coating the surface, changing the material near the surface, or changing the structural characteristics of the surface. It has better industrial application prospects because it is easier to realize and has better applicability for complex surfaces and complex working conditions. Promoting dropwise condensation is aimed at enhancing heat transfer of condensation but realizing DWC does not mean that the heat transfer has been enhanced. For example, for the condensate frost problem of refrigerating machine, if the drops cannot fall off in time, they will solidify on the surface, which may lead to a sharp decline in heat transfer efficiency. Accelerating droplet shedding rate will avoid solidifying the refrigerator and have an important significance of enhanced heat transfer of DWC. The heat transfer of DWC reaches a maximum value of the initial stage of condensation. With the growth of droplets, the heat flux gradually decreases, and the instantaneous heat flux when droplets depart also reaches a high point [7]. Therefore, the smaller the drop radius of departure is, the better the heat transfer effect will be. On the other hand, in the process of condensation, the ideal continuous DWC occurs only under the condition of low subcooling. With the increase in the heat flux, the condensation mixing of FWC and DWC will occur. Accelerating droplet shedding rate can delay the transition from DWC to FWC, which maintains the DWC at a higher subcooling degree.