HangJin Jo

Nucleate boiling performance on nano/microstructures with different wetting surfaces

A study of nucleate boiling phenomena on nano/microstructures is a very basic and useful study with a view to the potential application of modified surfaces as heating surfaces in a number of fields. We present a detailed study of boiling experiments on fabricated nano/microstructured surfaces used as heating surfaces under atmospheric conditions, employing identical nanostructures with two different wettabilities (silicon-oxidized and Teflon-coated). Consequently, enhancements of both boiling heat transfer (BHT) and critical heat flux (CHF) are demonstrated in the nano/microstructures, independent of their wettability. However, the increment of BHT and CHF on each of the different wetting surfaces depended on the wetting characteristics of heating surfaces. The effect of water penetration in the surface structures by capillary phenomena is suggested as a plausible mechanism for the enhanced CHF on the nano/microstructures regardless of the wettability of the surfaces in atmospheric condition. This is supported by comparing bubble shapes generated in actual boiling experiments and dynamic contact angles under atmospheric conditions on Teflon-coated nano/microstructured surfaces.

Boiling on spatially controlled heterogeneous surfaces: Wettability patterns on microstructures

We investigated nucleate boiling heat transfer with precisely controlled wetting patterns and micro-posts, to gain insights into the impact of surface heterogeneity. To create heterogeneous wetting patterns, self-assembled monolayers (SAMs) were spatially patterned. Even at a contact angle <90°, bubble nucleation and bubble frequency were accelerated on SAM patterns, since this contact angle is larger than that found on plain surfaces. Micro-posts were also fabricated on the surface, which interrupted the expansion of generated bubbles. This surface structuring induced smaller bubbles and higher bubble frequency than the plain surface. The resistance provided by surface structures to bubble expansion broke the interface between the vapor mushroom and the heating surface, and water could therefore be continuously supplied through these spaces at high heat flux. To induce synergistic effects with wetting patterns and surface structures on boiling, we fabricated SAM patterns onto the heads of micro-posts. On this combined surface, bubble nucleation was induced from the head of the micro-posts, and bubble growth was influenced by both the SAM pattern and the micro-post structures. In particular, separation of the vapor path on the SAM patterns and the liquid path between micro-post structures resulted in high heat transfer performance without critical heat flux deterioration.

Enhanced heat transfer is dependent on thickness of graphene films: the heat dissipation during boiling

Boiling heat transfer (BHT) is a particularly efficient heat transport method because of the latent heat associated with the process. However, the efficiency of BHT decreases significantly with increasing wall temperature when the critical heat flux (CHF) is reached. Graphene has received much recent research attention for applications in thermal engineering due to its large thermal conductivity. In this study, graphene films of various thicknesses were deposited on a heated surface, and enhancements of BHT and CHF were investigated via pool-boiling experiments. In contrast to the well-known surface effects, including improved wettability and liquid spreading due to micron- and nanometer-scale structures, nanometer-scale folded edges of graphene films provided a clue of BHT improvement and only the thermal conductivity of the graphene layer could explain the dependence of the CHF on the thickness. The large thermal conductivity of the graphene films inhibited the formation of hot spots, thereby increasing the CHF. Finally, the provided empirical model could be suitable for prediction of CHF.

Loss of superhydrophobicity of hydrophobic micro/nano structures during condensation

Condensed liquid behavior on hydrophobic micro/nano-structured surfaces is a subject with multiple practical applications, but remains poorly understood. In particular, the loss of superhydrophobicity of hydrophobic micro/nanostructures during condensation, even when the same surface shows water-repellant characteristics when exposed to air, requires intensive investigation to improve and apply our understanding of the fundamental physics of condensation. Here, we postulate the criterion required for condensation to form from inside the surface structures by examining the grand potentials of a condensation system, including the properties of the condensed liquid and the conditions required for condensation. The results imply that the same hydrophobic micro/nano-structured surface could exhibit different liquid droplet behavior depending on the conditions. Our findings are supported by the observed phenomena: the initiation of a condensed droplet from inside a hydrophobic cavity, the apparent wetted state changes, and the presence of sticky condensed droplets on the hydrophobic micro/nano-structured surface.

Experimental Investigation on the Pool Boiling Critical Heat Flux of Water-Based Alumina and Titania Nanofluids on a Flat Plate Heater

Pool boiling heat transfer and critical heat flux (CHF) of water-based nanofluids with alumina and titania nanoparticles of 0.01% by volume were investigated on a disk heater at saturated and atmospheric conditions. The experimental results showed that the boiling in nanofluids caused the considerable increase in CHF on the flat surface heater. It was revealed by visualization of the heater surface subsequent to the boiling experiments that a major amount of nanoparticles deposited on the surface during the boiling process. Pool boiling of pure water on the surface modified by such nanoparticle deposition resulted in the same CHF increases as what boiling nanofluids, thus suggesting the CHF enhancement in nanofluids was an effect of the surface modification through the nanoparticle deposition during nanofluid boiling. Possible reasons for CHF enhancement in pool boiling of nanofluids are discussed with surface property changes caused by the nanoparticle deposition.

Impact of corrosion on the emissivity of advanced reactor structural alloys

Abstract Under standard operating conditions, the emissivity of structural alloys used for various components of nuclear reactors may evolve, affecting the heat transfer of the systems. In this study, mid-infrared emissivities of several reactor structural alloys were measured before and after exposure to environments relevant to next-generation reactors. We evaluated nickel-based alloys Haynes 230 and Inconel 617 exposed to helium gas at 1000 °C, nickel-based Hastelloy N and iron-based 316 stainless steel exposed to molten salts at 750–850 °C, 316 stainless steel exposed to liquid sodium at 650 °C, and 316 stainless steel and Haynes 230 exposed to supercritical CO2 at 650 °C. Emissivity was measured via emissive and reflective techniques using a Fourier transform infrared (FTIR) spectrometer. Large increases in emissivity are observed for alloys exposed to oxidizing environments, while only minor differences were observed in other exposure conditions.

MHF (Minimum Heat Flux) Point on Anodized Zirconium Surface During Quenching

Quenching experiment for the evaluation of the Minimum Heat Flux (MHF) point on an anodized zirconium surface was conducted. The anodized zirconium surface showed complete wetting (Contact Angle, ∼ 0°) due to the capillary wicking force by nano- and micro-scaled structures in contrast to bare zirconium surface (∼ 54.3±2°). The cylindrical test sections (bare and anodized zirconium surface) heated up to 800 [°C] by radiation furnace was rapidly immersed into saturated distilled water. The temperature history of the test section showed the enhancement of the MHF point noticeably from 324 [°C] for the bare to 497 [°C] for the anodized zirconium surface. High speed visualization focusing on the interfacial dynamics at the film boiling showed stable wavy motion at the bare zirconium surface. On the contrary, vigorous fluctuation of the liquid-vapor interface on the anodized zirconium surface occurred. The visual observation suggested that it was caused by completely wetting features at the anodized zirconium surface. Therefore, it triggered the enhancement of the MHF point.© 2014 ASME

Potential Recovery Actions from a Severe Accident in a PWR: MELCOR Analysis of a Station Blackout Scenario

Abstract Accident-tolerant fuel (ATF) cladding materials have been a focus of recent work to provide a greater resistance to fuel degradation, oxidation, and melting in light water reactors for beyond-design accident scenarios such as a station blackout (SBO). In a previous study, researchers at The University of Wisconsin–Madison used the Surry Nuclear Plant as the pilot plant to examine the effect of ATF substitute clad materials with the short-term SBO as the postulated accident, examining the effect of a loss of auxiliary feedwater (AFW) with the MELCOR systems code. In this work, we examine the effect of recovery actions for an SBO in Surry as a follow-on topic. Specifically, we selected two kinds of core cladding materials (Zircaloy and FeCrAl), and then conducted comparative analysis of the effect of water injection; first with a delay in water injection start times into the reactor pressure vessel (RPV) and then with steam generator (SG) steam-side AFW end times. We find that alternative cladding materials (FeCrAl) can effectively delay fuel degradation and system failures for both water injection strategies. One finds that RPV water injection can prevent such severe accident effects if restored in a few hours into the SBO. Conversely, SG steam-side AFW flow with alternative cladding materials (FeCrAl) can delay the fuel degradation and system failure processes by hours. We mainly focus on analyzing the severe accident progression by different quantitative signals, such as the onset of rapid hydrogen production, hot-leg creep rupture failure, and core slump. Analyses are now underway to consider the effects of proposed coating materials on Zircaloy cladding and if such coatings can afford similar benefits.

Long-term Prediction of Emissivity of Structural materials for High Temperature Reactor Systems

The Reactor Pressure Vessel (RPV) and the internal components rely partially on radiation from their outer surface for cooling. In the event of an unexpected high temperature excursion, the dependence on radiation for the expulsion of heat from the system becomes all the more important because of the fourth power temperature dependence of radiated heat. The key material parameter that dictates the extent of heat radiated from the surface is emissivity, which is defined as the ratio of emissive power of the materials’ surface to that of an ideal black body. Emissivity is a surface phenomenon and is dictated by the materials’ surface chemical composition as well as the physical nature of the surface such as roughness, porosity, and texture. Since oxidation or some type of surface corrosion will inevitably at high temperatures, it is important that these surface effects be taken into account in emissivity evaluations. The research involves a study of emissivity of a range of materials of relevance to a variety of reactor concepts. We will measure hemispherical emissivity, which provides an integrated measurement, as well as spectral emissivity from which the integrated quantity can be obtained. Hemispherical emissivity is needed for MELCOR, RELAP, and FLUENT type calculations, while spectral emissivity is needed for a detailed Monte Carlo Transport theory computation of heat transfer. In recent years, both universities have conducted short-tem emissivity studies of materials and we will use this data in conjunction with reaction kinetics and transport calculations on oxide/corrosion product film formation and spallation, to develop and validate long-term models for emissivity. While the focus of research will be on the RPV and related materials such as A533B and A508 steels, we will also extend this study to a broader range of reactor concepts, materials, and environments. We will study Fe-9Cr-1Mo ferritic steel which is being considered as an RPV material for high temperature reactors, 316 stainless steels which has broad usage in the fluoride salt-cooled high temperature reactor (FHR) and sodium fast reactor (SFR), Alloy 800H and IN 617 for high temperature gas-cooled reactor (HTGR), and Hasteloy-N for the FHR. The differences in material chemistries among these alloys, temperature of operation, and the fundamentally different environments will result in the evolution of vastly different surface corrosion product films and consequently different emissivity behaviors. The alloys will be exposed to their respective environments to understand emissivity changes as a function of chemistry, morphology, and growth trends of corrosion product films that form on these alloys, that can be then used to develop models for long-term emissivities. The emissivity data and models developed will also be used to assess changes in core heat removal from the Reactor Cavity Cooling System (RCCS) where emissivity data for the absorbing riser ducts intended to remove heat from the simulated reactor pressure vessel is crucially important.

Effects of Surface Roughness, Oxidation, and Temperature on the Emissivity of Reactor Pressure Vessel Alloys

Abstract Thermal radiation will be an important mode of heat transfer in future high-temperature reactors and in off-normal high-temperature scenarios in present reactors. In this work, spectral directional emissivities of two reactor pressure vessel (RPV) candidate materials were measured at room temperature after exposure to high-temperature air. In the case of SA508 steel, significant increases in emissivity were observed due to oxidation. In the case of Grade 91 steel, only very small increases were observed under the tested conditions. Effects of roughness were also investigated. To study the effects of roughening, unexposed samples of SA508 and Grade 91 steel were roughened via one of either grinding or shot-peening before being measured. Significant increases were observed only in samples having roughness exceeding the roughness expected of RPV surfaces. While the emissivity increases for SA508 from oxidation were indeed significant, the measured emissivity coefficients were below that of values commonly used in heat transfer models. Based on the observed experimental data, recommendations for emissivity inputs for heat transfer simulations are provided.