SURFACE CHARACTERISTICS+TH

A strict investigation protocol (Fig. 1) was established to ensure the ability to separate oxidation effects from other influences. The Zircaloy-4 surface finish was meticulously controlled throughout the processes of polishing, physical vapor deposition (PVD) sputtering (with Al2O3, Cr, and Ag films to create heater elements), oxidation, subcooled flow boiling, and repetitive procedures. The Zircaloy-4 sample underwent oxidation in a tube furnace with a humid air flow, replicating the oxidation stages observed in nuclear reactors. 

The micro- and nano-scale morphology of the exact same region of Zircaloy-4/FeCrAl was measured before and after Cr-coating. Additionally, the line profile of surface roughness (Fig. 4) clarifies the effect of Cr-coating: the coating is thick enough to cover and smooth out the rugged sub-micron surface features present on the FeCrAl surface (but not on the Zircaloy-4 surface), whereas it is thin enough not to smoothen the micron-scale structures of the surface. The Cr-coating acts like a compact (i.e., non-porous) “snow” layer, conformally coating the larger features and cluttering while smoothing out the smaller ones. 

(Fig. 5) The Cr-coating reduces both the heat transfer coefficient (HTC) and the critical heat flux (CHF) limit, for both Zircaloy-4 and FeCrAl samples. The onset of nucleate boiling (ONB) temperature on both uncoated and Cr-coated Zircaloy-4 surfaces is very similar, ~20 ℃. Such wall superheat aligns with a vapor-trapped cavity radius (predicted using Young-Laplace eq.) of roughly 400 nm. On the uncoated FeCrAl surface, the ONB wall superheat is approximately 14 ℃, consistent with a cavity radius of roughly 650 nm. This suggests that Cr-coating effectively smoothens large cavities, promoting nucleation at higher temperatures. 

High speed video (HSV) images (Fig. 6) were post-processed using in-house ad-hoc algorithms (see VIRTUAL ALGORITHMS DEVELOPMENT page). Under the same heat flux, Cr-coating reduces the number of bubbles while increasing their size. Precisely, classical models predict that the bubble growth rate is proportional to the Jacob number, i.e., to the nucleation superheat. In these conditions, the liquid inertia force holding the bubble attached to the surface is also proportional to the nucleation superheat. Thus, as these bubble-departure-impairing inertial effects may dominate, bubbles nucleating at higher temperature on the coated surface have to grow bigger before the lift and drag forces from the liquid flow can cause them to detach or slide over the surface.