| Summary: | Boiling is a vital process used to transfer heat effectively via harnessing the large latent heat of vaporization for a variety of energy and thermal management applications. The boiling heat transfer performance is described mainly by critical heat flux (CHF) and heat transfer coefficient (HTC), which quantifies the operational heat flux limit and the efficiency of boiling heat transfer, respectively. The goal of this thesis is two-fold: fundamental understanding on the mechanisms associated with CHF and significantly enhancing pool boiling heat transfer. First, we addressed the large discrepancy of experimental CHF values on flat surfaces reported in the literature by accounting for hydrocarbon adsorption and oxidation of metallic surfaces during boiling. Accordingly, we developed an experimental protocol based on this understanding on the causes of spread in CHF values and used the protocol throughout this thesis for consistent experimental measurements.
We subsequently investigated the effects of surface structures on enhanced CHF during pool boiling of hemi-wicking surfaces. We systematically designed micropillar surfaces with controlled roughness and wickability, and combined the results with scaling analysis to obtain a unified descriptor for CHF. This unified descriptor represents the combined effects of the extended contact line length and volumetric wicking rate, which shows a reasonable correlation with CHF values with our experiments and literature data.
Next, we engineered boiling surfaces to achieve simultaneous CHF and HTC enhancements. We developed a microtube structure, where a cavity is defined at the center of a pillar, to enhance the heat transfer characteristics in controllable manner. In addition to uniform microtube arrays, we designed a surface with microtube clusters interspersed with micropillars, referred to as tube-clusters in pillars (TIP), to mitigate the earlier boiling crisis of uniform microtube arrays due to the extensive bubble coalescence. While uniform microtube arrays and TIP surfaces showed significant enhancement of both CHF and HTC compared to a flat surface, there was an intrinsic trade-off between CHF and HTC associated with the nucleation site density. Accordingly, we proposed hierarchical TIP (h-TIP) surfaces to control vapor nucleation with multi-scale structures while providing capillary wicking. These surfaces showed CHF and HTC enhancements up to 138 and 389%, respectively, compared to a flat surface.
Finally, we investigated the use of sandblasting as a scalable surface engineering technique for enhanced pool boiling heat transfer for industry-scale applications. Pool boiling results along with surface characterizations on silicon surfaces showed that surface roughness and volumetric wicking rates increased with the sandblasting abrasive size. As a result, CHF and HTC values enhanced up to 192.6 and 433.6% compared to a flat surface, respectively.
This thesis provides important insights to understand the role of surface properties and structures on pool boiling heat transfer, thereby providing guidelines for the systematic design of surface structures for enhanced pool boiling heat transfer.
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