Sun Jie, Wang Hua Sheng
Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China.
School of Engineering and Materials Science, Queen Mary University of London, London E1 4NS, UK.
Sci Rep. 2016 Oct 10;6:35003. doi: 10.1038/srep35003.
We use molecular dynamics simulation to investigate the early and developed stages of surface condensation. We find that the liquid-vapor and solid-liquid interfacial thermal resistances depend on the properties of solid and fluid, which are time-independent, while the condensate bulk thermal resistance depends on the condensate thickness, which is time-dependent. There exists intrinsic competition between the interfacial and condensate bulk thermal resistances in timeline and the resultant total thermal resistance determines the condensation intensity for a given vapor-solid temperature difference. We reveal the competition mechanism that the interfacial thermal resistance dominates at the onset of condensation and holds afterwards while the condensate bulk thermal resistance gradually takes over with condensate thickness growing. The weaker the solid-liquid bonding, the later the takeover occurs. This competition mechanism suggests that only when the condensate bulk thermal resistance is reduced after it takes over the domination can the condensation be effectively intensified. We propose a unified theoretical model for the thermal resistance analysis by making dropwise condensation equivalent to filmwise condensation. We further find that near a critical point (contact angle being ca. 153°) the bulk thermal resistance has the least opportunity to take over the domination while away from it the probability increases.
我们使用分子动力学模拟来研究表面冷凝的早期和发展阶段。我们发现，液-气和固-液界面热阻取决于固体和流体的性质，这些性质与时间无关，而冷凝物的体热阻取决于冷凝物厚度，该厚度随时间变化。在时间轴上，界面热阻和冷凝物体热阻之间存在内在竞争，并且由此产生的总热阻决定了给定气-固温差下的冷凝强度。我们揭示了竞争机制，即在冷凝开始时界面热阻占主导并在之后保持，而冷凝物体热阻随着冷凝物厚度的增加逐渐占据主导。固-液键合越弱，这种主导地位的转变发生得越晚。这种竞争机制表明，只有当冷凝物体热阻在占据主导地位后降低时，冷凝才能有效地增强。通过使滴状冷凝等效于膜状冷凝，我们提出了一个用于热阻分析的统一理论模型。我们进一步发现，在临界点附近（接触角约为153°），体热阻占据主导地位的机会最少，而远离该点时概率增加。
