Fundamental Study of Nucleate Boiling on Nanostructured Interfaces

[853785]在传热表面上沸腾的流体需要以微或纳米级空腔形式的成核点来促进气泡生长。通过在表面沉积纳米棒，可以形成富含有效成核位点的传热表面。这可以显著改善传热过程。具体来说，最近的研究表明，纳米棒可以降低核沸腾开始时的表面温度，降低沸腾开始后的表面温度（即增加两相传热系数），并增加最大允许传热率。然而，迄今为止，对于沉积纳米棒影响核沸腾过程的原因还没有明确的认识。该项目旨在通过完成以下任务，对纳米棒表面改善核沸腾传热的原因有一个基本的理解。(1)对无微尺度表面缺陷的纳米棒膜进行了成核沸腾的实验研究。这种薄膜仅由纳米棒间隙之间形成的相互连接的纳米孔网络组成，并且不包含微尺度的表面空腔。这个控制实验将确定当气泡与微尺度空腔分离时，气泡是否能在低过热下从纳米孔成核。(2)对具有孤立微腔的表面进行实验研究。这将使我们能够确定当微腔与纳米孔网络分离时，它们是否能在低过热下产生稳定的气泡成核。(3)研究纳米孔的润湿性、纳米棒的尺寸和间距以及微缺陷的密度/尺寸对气泡沸腾过程的影响。我们还将进行高速、微观的流动可视化来观察气泡的生长，并测量各种操作条件下气泡的释放频率和偏离直径。这项研究的智力价值是基于这样一个事实，即在受热表面上，非常有限的核沸腾传热数据可用于定义良好和工程纳米结构的空腔。收集的数据将揭示控制气泡成核和热传递改善的机制，表面形貌的长度尺度递减。它还将有助于开发新的传热表面，并为工程师提供更好的方法来改善热性能，也许是显著的。这项研究的广泛影响是基于这样一个事实，即越来越多的行业（微电子、航空航天、化学、低温、能源）迫切需要技术来增加各种高热流密度设备的传热。这项研究的结果将极大地扩展纳米和微尺度空腔上的核沸腾的科学知识体系，并使创新的传热表面的发展能够在涉及沸腾的各种过程中具有实际应用。为整合研究与教学，将开发特别设计的虚拟实验室。外联活动包括向高中生进行示范；这将有助于吸引各种各样的年轻学生从事科学和工程方面的职业。

0853785Peles Boiling of fluids on heat transfer surfaces requires nucleation sites in the form of micro- or nanoscale cavities to promote bubble growth. Heat transfer surfaces, rich in effective nucleation sites, can be formed by depositing nanorods on the surfaces. This can significantly improve the heat transfer process. Specifically, it has been recently shown that nanorods can reduce the surface temperature at the onset of nucleate boiling, reduce the surface temperature following boiling inception (that is, increase the two-phase heat transfer coefficient), and increase the maximum allowable heat transfer rate. However, so far there is no clear understanding regarding why the deposited nanorods affect the nucleate boiling process. This project seeks to develop a fundamental understanding of why nanorod surfaces improve nucleate boiling heat transfer by completing the following tasks. (1) perform an experimental study of nucleate boiling of a nanorod film without microscale surface defects. Such a film is comprised of only an interconnected network of nanopores formed between the nanorod interstices, and does not contain microscale surface cavities. This control experiment will determine whether or not bubbles can nucleate at low superheats from nanopores when they are uncoupled from microscale cavities. (2) Perform experimental studies of surfaces with isolated micro cavities. This will allow us to determine whether or not the micro cavities can generate stable bubble nucleation at low superheats when they are uncoupled from the nanopore network. (3) Investigate the role of the wettability of the nanopores, nanorod size and spacing and the density/size of micro-defects on the bubble ebullition process. We will also perform high-speed, microscopic flow visualization to observe bubble growth, as well as measure the bubble release frequency and departure diameters under various operating conditions. The Intellectual merit of the research is based on the fact that very limited nucleate boiling heat transfer data are available for well defined and engineered nanostructured cavities on heated surfaces. The data to be gathered will reveal the mechanisms that control bubble nucleation and heat transfer improvement on surfaces with morphologies of diminishing length scales. It will also aid the development of new heat transfer surfaces, and provide engineers with superior methods for improving thermal performance, perhaps significantly. The Broader impacts of this research are based on the fact that a growing number of industries (microelectronics, aerospace, chemical, cryogenics, energy) are in pressing need of techniques to increase heat transfer in various high heat flux devices. The results of this study will greatly extend the body of scientific knowledge of nucleate boiling over nano- and microscale cavities, and enable the development of innovative heat transfer surfaces that could have practical applications in a variety of processes that involve boiling. To integrate research and teaching, specially-designed virtual labs will be developed. Outreach includes demonstrations to high school students; this will help to attract a diverse cadre of young students to careers in science and engineering.