Separate effect of oxidation on the subcooled flow boiling performance of Zircaloy-4 at atmospheric pressure

Abstract

In this paper, we present the results of an experimental investigation aimed at elucidating the separate effect of oxidation on the subcooled flow boiling performance (i.e., heat transfer coefficient and critical heat flux) of Zircaloy-4. We also present the results of exploratory tests which we conducted in preparation to this investigation, which demonstrate that even nominally identical unoxidized samples (i.e., with the same exact roughness and wettability) can have very different boiling dynamics and boiling performance under the same exact operating conditions. Such differences can be even larger than those between unoxidized and oxidized samples. Thus, to carefully separate the effect of oxidation, as part of this work, we developed a special testing and characterization protocol. We conducted boiling tests on the same exact Zircaloy-4 sample, from unoxidized conditions throughout its pre-transition, transition and post-transition oxidation stages. All the flow boiling tests were run at atmospheric pressure with degassed and deionized water at 1 bar, with a subcooling of 10 K and a flow rate of 1000 kg/m(2)/s. Together with boiling experiments, we characterized the surface morphology at the micro-and nano-scale in order to clarify how the oxidation-induced changes of surface properties affect the performance and the dynamics of the boiling process. Crucially, to eliminate random effects, we conducted this characterization on the same exact spot of the boiling surface at all stages of oxidation. Our results reveal that, differently from pool boiling studies in the literature, the oxidation of the surface has little or no impact on the dynamics (i.e., on nucleation site density, bubble growth time and departure frequency, and bubble size) and the performance of subcooled flow boiling, except for the post-transition oxide. This oxide has smaller bubble departure diameter and growth time, higher nucleation site density, and a higher critical heat flux limit. This behavior seems to be triggered by micron-scale cracks on the oxidized surface, which make the surface wick liquid.