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.
