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 ΔTsub 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 tcool as compared with the Bare Zirconium surface (BZS) results under small ΔTsub (tcool ∼ 50% decrease for ΔTsub = 0, 15, and 40 K, respectively). However, its surface effect is limited in the case of large ΔTsub (tcool ∼ within 5% for ΔTsub = 60 and 75 K). The fast quench on the CWS under ΔTsub, explained by the increase in minimum film-boiling temperature TMFB and rewetting velocity U, is due to the liquid-solid contact. We evaluate the contact area Ac and volumetric absorption rate of the liquid dV/dt by conducting liquid absorption experiments. The increase in Ac 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.