Enhanced pool boiling heat transfer by metallic nanoporous surfaces under low pressure

The nano-structure enhanced pooling boiling at standard atmosphere has been fully studied in literature but it is rarely investigated under low pressure. However, the boiling behaviors and bubble dynamics of nano-structures highly depend on the working pressure. This study investigated the enhanced pool boil-ing (under low pressure) of a nanoporous copper surface (NCS) fabricated by the hot-dip galvanizing & dealloying. The boiling curves and bubble characteristics of the NCS under low-pressure boiling conditions (25 kPa and 65 kPa) were evaluated by a test set up with high-speed visualization. To validate the present experimental plain-wall HTC under low pressure, three predicted models (Labuntsov, modified Rohsenow, and Stephan-Abdelsalam) were applied to obtain the theoretical HTCs for comparison. Besides, the opti-mization of the galvanization time has been conducted. Experimental results showed that the NCS signif-icantly improved the pool boiling heat transfer and mitigated the negative influence of sub-atmospheric conditions. The enhanced mechanisms are the augmented nucleate site density, decreased bubble depar-ture diameter, and increased bubbles generation frequency induced by the promoted hydrophilicity and rough morphology of the NCS. Among the three analyzed samples, the NCS-50 0 degrees C-3 min surface yielded the lowest wall superheat at the onset of boiling (ONB) and the highest heat transfer coefficient (HTC). In addition, the nanoporous surface leads to higher HTC enhancement upon the plain surface at a lower pressure. Specifically, the HTC enhancement percentage was increased from 71.6% (at 65 kPa) to 139% (at 25 kPa) as the pressure decreased. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study investigated the enhanced pooling boiling of a nanoporous copper surface under low pressure conditions. The experimental results showed that the nanoporous surface significantly improved the pool boiling heat transfer and mitigated the negative influence of sub-atmospheric conditions. The study also found that the nanoporous surface led to higher heat transfer coefficient enhancement at a lower pressure.