Seol Ha Kim

Dynamics of water droplet on a heated nanotubes surface

This study investigated an effect of nanotubes on a heated surface onto Leidenfrost droplet through high speed visualization and momentum balance analysis. Delayed cutback phenomena and Leidenfrost Point (LFP) by dramatically high heating level were observed, and it is elucidated through wettable and spreadable features induced by nanotubes. As much delayed LFP, transient boiling regime with explosion-like dynamics of a water droplet on the nanotubes was observed. Furthermore, nanotubes required higher wall temperature to maintain non wetting cushion, due to the induced slip condition by porous features.

STUDY OF LEIDENFROST MECHANISM IN DROPLET IMPACTING ON HYDROPHILIC AND HYDROPHOBIC SURFACES

Water droplets, 2 mm in diameter, were allowed to fall freely onto hydrophobic and hydrophilic heated surfaces, and their impacts were imaged using high-speed cameras to investigate the droplet dynamics and heat transfer. As the heating power increased, the water droplets evaporated faster, eventually hovering over the surface due to the formation of a boiling film when the Leidenfrost point (LFP) was reached. The heat transfer from the surface into the droplet was evaluated, and LFP transition phenomena were investigated using time-resolved imaging of both side and bottom views. The hydrophilic surface showed a higher heat transfer rate and a higher LFP than the hydrophobic surface did. Furthermore, the droplet dynamics revealed very different shapes depending on the surface wettability; vigorous bubble nucleation and growth was observable for the hydrophilic surface, but not the hydrophobic surface. The rebound behavior of the droplets was analyzed based on the droplet free energy, including kinetic, potential, and surface energy terms.

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

Status and Plan of the System Codes Development for ITER Test Blanket Module and Fusion Breeding Blanket in Korea

Abstract The design scheme and system codes for fusion application have been developed for the ITER Test Blanket Module (TBM) program in Korea in parallel with the breeding blanket development, which were based on the developed system codes in Gen. IV reactor development projects such as MARS (Multi-dimensional Analysis of Reactor Safety) and GAMMA (GAs Multi-component Mixture Analysis). Considering the unique and common features with both the Fusion and Gen. IV reactors, four approaches have been carried out: (1) modifying the heat transfer model and suggesting a 3D analysis for considering the one-sided heating with extreme temperature differences, (2) implementing a tritium permeation model for a simulation of its behavior and amount simulation in a fusion coolant system, (3) developing a physical properties generation model for PbLi and Li considering the liquid metal breeders in these codes, and (4) implementing the magnetohydrodynamics (MHD) model by Miyazaki et.al. To integrate these separate codes into single ones, called MARS-FR (Fusion Reactor) and GAMMA-FR, their environments were carefully handled during their development procedure.

Nonwettable Hierarchical Structure Effect on Droplet Impact and Spreading Dynamics

In this study, the nano/micro hierarchical structure effect of a nonwettable surface on droplet impact was investigated by high-speed visualization. A dual-scale structure of a superhydrophobic surface was designed for manipulating a wide range of capillary pressures (103-106 Pa), and it was supposed to trigger a hierarchical effect on the droplet dynamics. Distilled water droplets of various sizes and initial velocity were subjected to the prepared samples, and the impact behavior, the spreading diameter, and contacted time, were quantitatively measured. The apparent maximum spreading and contact time of the low Weber number ( We#) condition was less dependent on the microscaled design factor of the multiscale-fabricated surface. However, in the high We# condition, the wavy formation shape and the fragmented criteria of the droplet impact were affected by the configuration of the surface morphology. The hierarchical effect from the dual-scale structure on droplet spreading dynamics has been discussed through a balance between capillary pressure induced by the structure and the dynamic pressure of droplet impact.

MHD Analysis and Preparation of an Experiment for Developing the Korean Test Blanket Module

To develop a Korean (KO) test blanket module (TBM) for an International Thermonuclear Experimental Reactor (ITER), particularly for a liquid breeder type, a currently developed conventional code, CFX, with an electromagnetic module for analyzing the magnetohydrodynamics (MHD) effect of the liquid breeder under a high magnetic field is numerically validated using a theoretical equation and previous experimental data. From a comparison with the previous data, the code shows its capability to simulate the MHD effect very well. The KO TBM was then analyzed using the code for comparing the velocities and pressure drops when there was a high magnetic field of the ITER condition (5 T) and when there was not. From the analysis, the pressure drop increased from 0.008337 to 486.8 Pa, and the flow became more uniform in the upward region but concentrated in a corner or inlet manifold, which provided the idea to modify the manifold design. Since the pressure drop by the MHD effect will be tested with the constructed experimental loop for the liquid breeder, a preliminary analysis was performed using the installed test section and magnet, which can produce a magnetic field of up to 2.2 T.

MHD analysis and preparation of an experiment for developing the Korean Test Blanket Module

To develop a Korean (KO) test blanket module (TBM) for an International Thermonuclear Experimental Reactor (ITER), particularly for a liquid breeder type, a currently developed conventional code, CFX, with an electromagnetic module for analyzing the magnetohydrodynamics (MHD) effect of the liquid breeder under a high magnetic field is numerically validated using a theoretical equation and previous experimental data. From a comparison with the previous data, the code shows its capability to simulate the MHD effect very well. The KO TBM was then analyzed using the code for comparing the velocities and pressure drops when there was a high magnetic field of the ITER condition (5 T) and when there was not. From the analysis, the pressure drop increased from 0.008337 to 486.8 Pa, and the flow became more uniform in the upward region but concentrated in a corner or inlet manifold, which provided the idea to modify the manifold design. Since the pressure drop by the MHD effect will be tested with the constructed experimental loop for the liquid breeder, a preliminary analysis was performed using the installed test section and magnet, which can produce a magnetic field of up to 2.2 T.