Experimental study on the influence of sub-millimeter copper foams on boiling heat transfer in vertical orientation

초록

Recent years have seen porous metal foams gaining research interest for boosting boiling heat transfer in sustainable energy systems because of their high surface area, nucleation sites, and thermal conductivity. To keep electronic devices compact, surface coatings must be thin to lower thermal resistance and optimize space, with components often placed vertically. Despite extensive research, most studies have focused on millimeter foams in horizontal orientations, leaving the pool boiling behavior of sub-millimeter foams in vertical configurations largely unexplored. In this work, the pool boiling performance of vertically oriented copper foams with thicknesses of 50, 100, and 200 mu m and pore densities ranging from 90 to 130 PPI is investigated. The influence of foam thickness and pore size on boiling characteristics such as onset of nucleate boiling (ONB), heat transfer coefficient (HTC), and critical heat flux (CHF) is quantitatively analyzed. All copper foam surfaces exhibit enhanced CHF compared to the bare silicon surface, while HTC trends vary depending on foam thickness and pore density. Two distinct CHF and HTC trends are observed based on foam thickness, indicating two different mechanisms at work. In one case, at 200 mu m cooper foam thickness, higher wall superheats and poor HTCs are seen, suggesting vapor sublayer dryout caused by coalescence of vapor channels induced by hydrodynamic instabilities. In the other case, at 50 mu m, a non-monotonic CHF trend with lower superheats is observed, similar to micro-pillared surfaces, which results from dryout of the liquid sublayer beneath vapor bubbles. These findings on CHF trends in porous metal foams highlight the unique characteristics of microporous foam structures in a vertical orientation and guide future research aimed at optimizing foam designs for improved boiling heat transfer.

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