Subcooled water quenching on a super-hydrophilic surface under atmospheric pressure

Abstract

The goal of this work is (i) to evaluate the cooling rate on a super-hydrophilic surface as a function of the subcooled degree Delta T-sub of the liquid coolant, (ii) to analyze the contact heat transfer q ''(c) of the liquid-solid contact, and (iii) to investigate the mechanism of microbubble emission boiling (MEB). We fabricated a super-hydrophilic surface by anodic oxidation of a zirconium vertical rod, so called completely wettable surface (CWS), which had surface microstructures with super-hydrophilicity. The CWS results in a decrease of the cooling time t(cool) as compared with the Bare Zirconium surface (BZS) results under small Delta T-sub (t(cool) similar to 50% decrease for Delta T-sub = 0, 15, and 40 K, respectively). However, its surface effect is limited in the case of large Delta T-sub (t(cool) similar to within 5% for Delta T-sub = 60 and 75 K). The fast quench on the CWS under Delta T-sub, explained by the increase in minimum film-boiling temperature T-MFB and rewetting velocity U, is due to the liquid-solid contact. We evaluate the contact area A(c) and volumetric absorption rate of the liquid dV/dt by conducting liquid absorption experiments. The increase in A(c) and dV/dt contribute to an increase in q"(c), by forming the liquid film at the liquid-solid contact spot. The orders of the time scale between capillary-wicking and liquid-solid contact are comparable. Destabilization of the large vapor bubble is caused by an increase in q"(c), which is a major reason for MEB generation, and this mechanism enables the q" to be significantly high on the CWS under subcooled quenching. (C) 2017 Elsevier Ltd. All rights reserved.