The surface characteristics of materials significantly influence the thermohydraulic behavior of industrial systems at micro- and nano-scales. Over time, surfaces naturally undergo changes, including oxidation, corrosion, degradation, and deposition of contaminants. Humans also intervene to counteract or accelerate such changes through coating, etching, electroplating, annealing, etc. Studying the impact of surface characteristics is crucial for understanding the overall TH performance of industrial systems.
EXAMPLE 1: SEPARATE EFFECT OF ZIRCALOY-4 OXIDATION ON SUBCOOLED FLOW BOILING
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.
Fig. 1. Schematic of sample fabrication procedure.
Fig. 2. Oxide weight gain and corresponding surface appearance & thickness visualization.
Boiling experiments and surface characterization were conducted on the same exact sample throughout the different kinetic stages of oxidation. The oxide formation was monitored by checking the oxide weight gain on a daily basis and focused ion beam (FIB) thickness measurements (Fig. 2).
The micro- and nano-scale morphology of a sample was characterized before and after surface modifications (i.e., oxidation) on the exact same region (Fig. 3). This expedient is important to eliminate characterization uncertainties arising from randomly selected surface regions. To elucidate the separate effect, the surface was characterized in several steps with increasing spatial resolution: micron-texture (3D optical profilometer), sub-micron topography (atomic force microscope (AFM)), nano-image (scanning electron microscope (SEM)), wettability, and wicking. These pieces of information were then correlated with boiling parameters to identify how oxidation might alter the surface properties and, consequently, influence subcooled flow boiling boiling processes.
Fig. 3. Surface characterization.
J. H. Seong, C. Wang, B. Phillips, M. Bucci*
Separate effect of oxidation on the subcooled flow boiling performance of Zircaloy-4 at atmospheric pressure
IJHMT, 188, 122620
EXAMPLE 2: EFFECT OF Cr-COATING ON BOILING, AS ACCIDENT TOLERENT FUEL (ATF) MATERIAL
Fig. 4. Line profile of surface roughness
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.
Fig. 5. Boiling curves
Fig. 6. HSV images
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.
J. H. Seong, C. Wang, B. Phillips, M. Bucci*
The effect of PVD-coated chromium on the subcooled flow boiling performance of nuclear reactor cladding materials
Applied Thermal Engineering (ATE), 213, 118670
34141 대전광역시 유성구 대학로 291 (구성동 한국과학기술원 373-1) 기계공학동 www.kaist.ac.kr