Advances in vapor dropwise condensation heat transfer

Abstract

Vapor condensation on solid surface is widely observed in nature and employed in industry, among which, dropwise condensation (DWC) has attracted a lot of attentions due to its high performance in heat transfer and wide applications in industries. With remarkable advances in functional surface engineering, more and more progresses have been obtained in this field. Therefore, it is necessary to carry a review for those investigations obtained by different methods and on various aspects of DWC. An introduction of the wetting of droplets is firstly presented, and then this review article turns to theoretical modelling of heat transfer, followed by numerical and experimental works. No doubt that it is impossible to cover all the efforts since 1930, when DWC was observed for the first time. Thus, the present work is mainly devoted to delivering two issues, by summarizing several representative questions. One is the origin of classical heat transfer model and the other is a framework of DWC research. The wetting of both macro-droplets and nano-droplets matters significantly in the whole life of a condensed droplet. For the former, contact angle and wetting status are introduced, while for the latter, three hypotheses for nano-droplet formation are responded. "The hypothesis of film break-up" has been abandoned; the widely accepted "hypothesis of specific nucleation cites" is demonstrated here based on solid evidence from molecular dynamics (MD) simulations. To be honest, it is not easy to sufficiently abandon "the hypothesis of coexistence of droplet and film", which may still be regarded as an open question. For heat transfer modelling, via a comparison with Nusselt's theory of filmwise condensation, it is shown that heat transfer modelling for DWC, as a challenging topic, is remarkably different from those for convention heat transfer, or even filmwise condensation. The development of this topic presents a long story. The efforts in early stage are devoted to understanding basic mechanisms and improving measurement accuracy. With the advances in superhydrophobic surfaces, the enhancement of DWC is obtained by advanced manipulation of condensed droplets wetting, especially by efficient removal of condensed droplets. These works are divided into three types according to the differences in their motivations: For accurately modelling of solid-liquid interaction, for the effect of coalescence-induced droplet jumping, and for new-facing situations of industry applications. To optimize DWC, it is necessary to sufficiently understand its mechanism of droplet wetting, which is mainly performed by numerical simulations, especially by MD simulation method and lattice Boltzmann method (LBM). Before a summary of the latter one, the contribution of MD simulations is credited by presenting their exploration on the fundamental behaviours of condensed nano-droplets, especially the formation, wetting, growth, coalescence and self-driven departures. The efforts and advances above are actually inspired by, and will be applied to as well, experimental investigations. Firstly, a summary is given for the coating techniques for super-hydrophobic surfaces, with some comments. Then, the development of experimental work is presented by dividing its history into three stages: Initial pioneering for mechanism and measurement, coating-inspired DWC, and interdiscipline-boosted DWC. The advances of DWC with non-condensable gas are introduced as well, which is expected to help DWC be applied extensively. Finally, a summary of the progresses and possible challenges are given, followed by the advices on several specific questions. Our revisit to "the DWC museum" is concluded by an outlook full of promise, with the application of emerging technologies in this classical field.