Cryogenic quenching enhancement of a nanoporous surface

Quenching heat transfer is of fundamental interest in cryogenic chilldown applications. Surface configuration can improve quenching heat transfer and clear understanding of the effect of different surface characteristics is necessary. In the present study, the effect of anodic aluminum oxide (AAO) surface on pool quenching is investigated. The AAO surface and other four types of surfaces are quenched in liquid nitrogen and a theoretical model is applied to further investigate the effect of surface thermal resistance. It is shown that the total chilldown time is decreased from 63.8 s on electropolishing surface to 33.4 s on the AAO surface. By comparisons with other surfaces, it is found that the effect of the MO surface results from two aspects: surface thermal resistance and nanopore structure. For surface thermal resistance, it is indicated by the consistence of numerical and experimental results that the end of film boiling is resulted from the decrease of local surface temperature that is not high enough to sustain the local vapor film, even though the bulk temperature is still high. For the nanopore structure, it is theoretically estimated that the nanopores will be kept filled with vapor at the surface temperature higher than the critical pinning state temperature (94.4 K). This phenomenon of vapor-filled nanopores reduces the liquid-solid contact area and local vapor generation rate, resulting in a higher temperature of the Leidenfrost point (LFP) and lower heat flux in partial nucleate boiling regime. When the surface temperature drops below the critical pinning state temperature, liquid nitrogen infiltrates into the nanopores and a heat flux jump appears. (C) 2019 Elsevier Ltd. All rights reserved.