Dra. María Fernandino

Norwegian University of Science and Technology, Noruega

Heat transfer enhancement during droplet evaporation and condensation by controlling the surface geometry

When a liquid droplet is deposited onto a heated surface, four very distinctive behaviors are observed depending on the wall temperature. At low temperatures, a single-phase regime is observed and the heat is transferred by conduction from the wall to the liquid. At higher temperatures, the nucleate boiling regime is observed and small bubbles are formed inside the droplet. The heat transfer will increase until the formation of the bubbles is so rapid that a vapor blanket or film begins to form on the surface. In this regime, nucleate boiling and a vapor film will coexist, but the fraction of film boiling to nucleate boiling will increase with increasing wall temperature. Above a certain temperature or Leidenfrost temperature, film boiling regime commences and the drop is levitated by formation of a thin vapor continuous cushion. The droplet hovers over the surface in a frictionless regime, and the cooling efficiency is reduced. Understanding how to enhance the heat transfer by displacing  the Leidenfrost temperature limit to higher values is essential for enabling efficient cooling technologies. In the last years, significant work has been done in developing new surfaces capable of overcoming the Leidenfrost limit. However, which mechanisms control the Leidenfrost limit remains not well understood.

Another related phenomenon involving heat transfer in the presence of liquid droplets is the case of dropwise condensation, observed every day in nature and widely utilized in industry. Superhydrophobic surfaces have been studied in detail for enhancing condensation heat and mass transfer. However, the applicability of such surfaces remains limited to a narrow range of operation.

This work will focus on the development of novel surfaces using micro-/nano-fabrication techniques for improving heat transfer during evaporation and condensation. It will be shown that changes in the micro-/nano-structures have a strong influence on the macroscopic behavior which can enhance or deteriorate the heat transfer from the surface to the liquid phase.
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