Boiling Enhancement on Engineered Surfaces
Authors:   , Rahman Md Mahamudur , McCarthy Matthew 
Published: July 2021

The realization of next generation technologies to enhance boiling heat transfer is of critical importance due to its impact on energy, the environmental, and water resources, as well as thermal management needs of high-power electronics. Recent studies have shown that high surface area coatings comprised of micro and nano scale structures can be used to substantially increase performance during boiling. While exciting results have been reported in the literature, there is little understanding, and even less consensus, on the fundamental underlying mechanisms by which structured coatings enhance boiling. Additionally, the fabrication schemes used in the lab setting to create these structures are not scalable to large areas, complex geometries, or materials traditionally used in heat transfer applications. The focus of the Ph. D. dissertation research is on: (1) a detailed characterization of the effects of structured surfaces on boiling performance, including the role of capillary wicking, as well as the effects of surface morphology, material, and length scales; (2) scalable nanomanufacturing of structured coatings using nanoscale biological templates, and (3) the realization of novel surfaces for boiling enhancement that are resistant to degradation. The Tobacco mosaic virus (TMV) has been used as a nanoscale building block to create nanostructured and hierarchically structured surfaces with a wide variety of morphologies. Utilizing a simple technique for characterizing surface wicking, an experimentally validated model relating the maximum heat flux from a boiling surface to its inherent "wickability" has been shown for the first time. For structured superhydrophillic surfaces with characteristic lengths much smaller than the inherent flow structures (~ 1mm), capillary wicking through surface structures is the single factor determining the critical heat flux (CHF). Separately, it has been shown that hierarchically structured surfaces with length scales comparable to the flow yield more complicated enhancement mechanisms with each length scale contributing differently. Nanoscale structures enhance CHF, while micro-to-milli scale structures enhance nucleation and therefore heat transfer coefficient (HTC). The combination of the two has been show to enhance CHF and HTC by over 238% and 540% respectively. TMV biotemplating has been leveraged not only for fundamental studies of boiling enhancement but also demonstrated here for the scalable nanomanufacturing of structured coatings. Biotemplating has been used to conformally coat a variety of materials using a cheap and sustainable room-temperature solution-based procedure. Repeatable boiling heat transfer enhancements of over 200% have been demonstrated on aluminum, silicon, copper, and stainless steel surface. Such solution-based techniques are easily extended to complex surfaces and large areas using in-situ depositions for retrofittings existing systems and rejuvenating surfaces that have fouled and degraded. Finally, the use of engineered thermal gradients across surfaces comprised of heterogeneous materials has been demonstrated. By promoting ordered pathways between nucleating bubbles and replenishing liquid, engineered thermal gradients have been shown to be as effective as structured surfaces in boiling enhancement, yet inherently resistant to foiling. These planar "bi-conductive" surfaces have been characterized and their geometries optimized based on analysis of the bubble departure phenomena and surface tension effects.