Approved for public release, distribution is unlimited / This study concentrates on the empirical characterization of a staggered pin-fin array heat exchanger placed in a modular, rectangular wind tunnel. A full analysis of the heat transfer and pressure drop behavior was conducted on various pin-fin shapes, sizes, and configurations. The study was based on airflow over a wide range of Reynolds numbers in the laminar regime. The empirical data gathered can be used to corroborate and develop better numerical models to characterize the performance of such heat exchangers as well as scale down to the micro level for comparison with micro-heat exchangers. / Lieutenant, United States Navy

Aluminum and aluminum-alumina powder mixtures were used to produce pyramidal fin arrays on aluminum substrates using cold spray as an additive manufacturing process. Using aluminum-alumina mixtures instead of pure aluminum powder could be seen as a cost-effective measure, preventing nozzle clogging. The fin geometries that were produced were observed using a 3D digital microscope to determine the flow passages width and fins geometric details. Heat transfer and pressure tests were carried out using different ranges of appropriate Reynolds numbers for the sought commercial application to compare each fin array and determine the effect of alumina content. It was found that the presence of alumina reduces the fins’ performance when compared to pure aluminum but that they still outperform traditional fins. Numerical simulations were performed and were used to explain the obtained experimental results. The numerical model opens up new avenues in predicting different parameters such as pressure and substrate temperature.

<p>Heat sinks have the capability of increasing operating heat flux limits for improved thermal management in the immersion cooling of electronics using dielectric fluids. However, even for arrays of simple, straight fins, the generation of vapor between and along fins during pool boiling lead to performance effects that are not well understood. Further investigation of the heat-flux-dependent variation of boiling modes that can manifest along the fin height is required. Although methods for the prediction of fin boiling heat transfer exist that incorporate a variable heat transfer coefficient determined from a flat surface, they have been developed and assessed for single, isolated fins under the assumption that the sides of the fin at any location behave like that of a flat surface. As a result, when applied to fin arrays, these methods may not always be accurate for the full range of heat flux operation along boiling curve up to the critical heat flux, due to the fins interfering with each other when arranged in arrays of differing spacing and height. To establish when the fins in an array can be described as isolated and having the flat surface boiling behavior, pool boiling experiments are performed using copper heat sinks in two fluids with vastly different properties: HFE-7100 and water. The spacing and height of the longitudinal fins are varied across a range from much larger to less than half of the scale of the capillary length scale of both fluids, <em>L</em>_<em>b</em>. High-speed visualizations enable the identification of different boiling regimes to identify correspondence between flow observations and the boiling performance, such as when there is bubble confinement from fin interference. Trends in the pool boiling data are also compared, noting changes in superheat at various heat fluxes to establish when fin height or spacing affects boiling behavior. The experimental boiling performance is compared to predictions developed assuming isolated fins so as to identify the spacings and heights for which the fin arrays follow this behavior. Overall, the data from both fluids strongly support a hypothesis that <em>L</em>_<em>b</em> is the key length scale. Heat transfer from fin array heat sinks with heights and spacings above <em>L</em>_<em>b</em> are shown to be accurately predicted in both fluids. However, spacings smaller than <em>L</em>_<em>b</em> lead to bubble confinement which affects the superheat, particularly at low heat fluxes, while heights shorter than <em>L</em>_<em>b</em> are unable to support multiple boiling regimes along the fin sidewall. This work identifies the capillary length as the key length scale at which confinement and height effects need to be considered for accurate predictions of immersion cooling applications.</p>