Effects of layer-by-layer assembled PEI/MWCNT surfaces on enhanced pool boiling critical heat flux

Abstract

Layer-by-layer (LbL) assembly is a versatile deposition technique for the fabrication of nanostructured thin films on a substrate. In this study, multilayers of poly(ethylenimine) (PEI) and multi-walled carbon nanotubes (MWCNTs) were deposited onto a stainless steel surface. Since a substrate experiences notable changes in surface characteristics and structures during the LbL deposition process, resulting surface characteristics were thoroughly investigated. It was found that randomly oriented porous structures were developed by the fibrous wires of MWCNTs. As a result, surface roughness increased by 20-30%, and surface wettability increased by 70% for all samples. Both roughness and wettability were improved as the bilayer number increased. After the surface modification process, pool boiling heat transfer experiments were conducted to investigate the thermal margin achievable with the LbL deposition. Critical heat flux (CHF) enhancement was obtained as the number of bilayers increased, with the maximum enhancement evaluated at 94%. The enhancement was mainly attributed to the improved surface characteristics due to the formation of porous structures in the PEI/MWCNT layer. The comparison works with newly proposed CHF correlations supported the wicking effect on the CHF enhancements for the LbL-assembled test samples. Image processing results indicated a desirable structure for improved capillary action, which could affect further wetting to dry spots. High-speed bubble departure images were obtained from which a few msec reduction in rewetting intervals was observed for all LbL-assembled heaters.