Doctor of Philosophy (PhD)
Nucleate boiling is an attractive method for achieving high heat flux at low superheat temperatures. It is frequently used for industrial applications such as heat exchangers and is being considered to cool advanced central processing units (CPU) which produce heat fluxes on the order of 1 MW/m2 and are becoming increasingly less efficient to cool via forced conduction of air. The issue with implementing nucleate boiling as a cooling mechanism lies in the difficulty of quantifying the numerous and complex mechanisms which control the process. A comprehensive nucleate boiling model has yet to be formulated and will be required in order to safely and reliably cool high performance electronics. Spatially periodic systems with localized asymmetric surface structures (ratchets) can induce directed transport of matter (liquid/particles) in the absence of net force. It was hypothesized that ratchets may enhance pool boiling heat transfer by aiding in the removal of vapor which forms on the heated surface. Therefore, preliminary experiments on pool boiling using asymmetric micro ratchets with de-ionized (DI) water and various concentrations of alumina particles and DI water as the working fluids were investigated. Results indicated that ratchets indeed improve heat transfer performance. However, few conclusions could be drawn regarding the underlying physics. A more advanced boiling system was designed in order to more accurately measure heat transfer, test multiple working fluids, and perform condensing experiments. Various micro ratchet geometries were tested to study the effects of ratchet parameters on pool boiling performance using FC-72 as the working fluid. The samples underwent surface characterization to determine roughness and wettability which are important parameters influencing pool boiling heat transfer. Results indicate that, as in the previous study, micro ratchets significantly improve heat transfer. Various parameters were studied in order to better understand the influence of surface geometry on heat transfer. The results were found to be under-predicted by various models found in the literature.
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Brumfield, Lance Austin, "The Effects of Asymmetric Micro Ratchets on Pool Boiling Performance" (2014). LSU Doctoral Dissertations. 1806.