Acoustic sensing for investigating critical heat flux enhancement during pool boiling on electrodeposited copper foams

by Pandey, H.; Mehrabi, H.; Williams, A.; Mira-Hernández, C.; Coridan, R. H.; Hu, H.

Nano-engineered surfaces have shown great promise in improving boiling heat flux and heat transfer coefficient for thermal management. Electrodeposited copper foams have attracted lots of scientific and engineering interest owing to their multi-tier structures. In this study, hierarchical copper foams with ultra-wicking properties were fabricated using cathodic deposition at different pH, including both acidic (pH 0) and basic (pH 10 and 12) solutions. Pendant droplet wicking tests and pool boiling experiments were performed to characterize the wicking flux and the critical heat flux (CHF) of the fabricated copper foams, respectively. The CHF enhancement mechanism in hierarchical copper foams was investigated with high-speed optical imaging and acoustic emission (AE) sensing during both the wicking and boiling tests. The results show that the wicking flux of the electrodeposited copper foams increases with the pH due to smaller dendrites and crystallites. Boiling CHF is also higher on the base-deposited copper foams (pH 10 and pH 12) than on the acid-deposited foams (pH 0). Consistent acoustic signatures are observed between wicking and boiling, i.e., smaller AE amplitudes are observed at higher wicking flux and higher CHF, which supports wicking-based CHF enhancement theories. A spike in the AE amplitude is observed at CHF during water boiling on copper foam surfaces due to the rapid growth of the vapor layer blanketing the boiling surface. During the nucleate boiling regime, the AE amplitude decreases with the heat flux in the low heat flux stage but increases with the heat flux in the high heat flux stage. This relationship is found to be due to the interplay of bubble size and bubble count in the stress field that bubbles exert on the heater surface. By analyzing the maximum bubble size, bubble count, and the power and dominant frequency of vapor fraction at varying heat fluxes from high-speed videos, our work shows no obvious effect of the surface structures on the dynamics of the apparent liquid-vapor interface. Combining the optical and acoustic analysis, it is concluded that the key role that surface structures play in CHF enhancement is improving capillary wicking underneath the bubbles rather than affecting the dynamics of the apparent liquid-vapor interfaces. This study not only explores the relationship between electrochemical control and wicking properties but also demonstrates the role of wicking in structure-enhanced boiling CHF through acoustic emission analysis.

Applied Thermal Engineering
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