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How surface wettability affects boiling heat transfer is still a challenging problem at nanoscale. In this work, a three-dimensional analysis of surface potential energy is used to address its relationship to surface wettability and nanoscale pool boiling characteristics of liquid molecules. It is found that the major reason for the enhanced heat transfer by increasing the surface wettability is essentially the improvement of surface potential energy, of which the influence is summarized using logic order. Moreover, the nanoscale boiling of water on copper surfaces with various wettability is investigated, which reveals that the increase in surface wettability can improve the heat transfer coefficient and critical heat flux under high superheat conditions in both nano- and macroscale systems. Besides, the effects of surface potential energy on contact angle, Kapitza resistance, heat transfer coefficient, and critical heat flux are analyzed systematically to demonstrate their relationships. A nondimensional analysis is finally implemented to correlate the normalized surface potential energy (P), the Nusselt number (Nu), and the normalized critical heat flux (J): Nu = 0.081P0.34, J = 0.827P0.50 (P 2.387). This work provides insights into the enhancement mechanisms for the phase-change heat transfer of liquid molecules over surfaces with different wettability.

The boiling of liquid water on the superalloy surface is important in applications such as steam and gas turbines, for which it is necessary to explore the mechanisms of the Leidenfrost phenomenon. The molecular dynamics simulation method provides an effective way to study this phenomenon at the nanoscale. Previous studies, however, have not revealed the Leidenfrost effect of liquid molecules on a superalloy surface. This work investigated the effects of surface temperature and impact velocity on the nanoscale Leidenfrost phenomenon of water droplet over the Fe-Cr superalloy plate. When the surface temperature is higher than 798 K, the Leidenfrost phenomenon occurs on the superalloy surface with apparent temperature, stress, and pressure differences. The results indicate that the Leidenfrost phenomenon is related to the high surface temperature, rather than its impacting velocity. Furthermore, it is found that the Leidenfrost phenomenon is the combined results from the temperature and pressure differences exist in the vapor film, as enough kinetic energy can be generated by the thermal motion to cause the bounce phenomenon of droplet. The revealed mechanisms are critical for the design of a gas turbine, in which vapor film could be dominant in determining the film cooling effectiveness of gas/vapor on the blade surface.

Surface temperature and wettability can synergistically affect the performance of explosive boiling which is important for many industrial processes. This work investigated the explosive boiling of water film over copper surfaces with different wettability under various surface temperature conditions. The results show that the water molecules on the hydrophilic surfaces have higher temperature gradient and initial heat flux, shorter onset time of boiling, and lower Kapitza resistance than those on the hydrophobic surface. Meanwhile, the temperature gradient, the heat flux in the water films, and the onset time of explosive boiling are promoted by elevating the surface temperature. These indicate that the hydrophilic surface under high surface temperature conditions is most favorable for enhancing the explosive boiling heat transfer. Analysis of the onset of boiling reveals that the effect of wettability on the explosive boiling is more prominent under low surface temperatures but is limited under high surface temperatures. This work provides insights into the mechanisms of liquid molecules behaviors on plate surfaces with different wettability and surface temperatures for phase-change heat transfer enhancement and process intensification.