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Global climate change has increased drought frequency and caused significant damage to agricultural industries worldwide; for example, soybean yield has decreased by 30% on small-scale farms. Therefore, the development of subsurface irrigation methods, which conserve both water and labor, is urgently needed for open-field cropping systems. Although water and nutrient supply through subsurface irrigation shows clear advantages, the specific design of these irrigation systems relies on empirical data rather than systematic research and practical optimization. Because of irregularities and uncertainties in field soil conditions, the theoretical approach for water diffusion in soil frequently could give a wrong range of data to an irrigation system. In this study, the water diffusion by means of a commercial subsurface irrigation system has been experimentally tested in several soil types to get practical application and understand the water diffusion performance. In the present study, to better understand the shape of the subsurface diffusion plume in an open-field soybean crop during irrigation, we supplied 4 L of water through 3 mm vertical tubes using three types of dripper, at transfer rates of 2, 4, and 8 L/h. A map of the wetted soil near the dripper showed that the optimal water supply depends on the growth stage of the soybeans. Subsurface irrigation testing was conducted on well-graded sand containing clay (SWC: sand well + sand clay) and poorly graded sand (SP: sand plastic). As both SWC and SP showed that the wetted area rises well upward to the ground surface from the dripping point with 2 L/h supply, it is considered that 2 L/h case is effective for water absorption by the roots of the crops. Besides, the size of the grown root of 30 days showed 450 mm of width and 300 mm of depth, so it was estimated that 150 mm wide and 100 mm deep after 10 days, and 300 mm wide and 200 mm deep after 20 days. Judging from the results of the root growth and water diffusion rate, it is desirable that over 8 L and 5–6 L supply after 10 days and 20 days of the bean sowing, respectively. Also, since the roots after 30 days grow 300 mm deep and reach to water supply drip line, it is appropriate to supply about 4 L. We obtained empirical vertical and horizontal diffusion data to characterize soil water transport during subsurface irrigation. In summary, in terms of water supply rate, 2 L/h was the most effective for subsurface irrigation. Additionally, depending on the growth stages of the soybeans, early, 20 days, and 30 days, 8 L, 5–6 L, and 4 L are recommended for the supplied water amount, respectively.

We studied the Leidenfrost Point (LFP) of a falling liquid droplet on various surfaces that were covered by micropillars. We used a Micro Electric Mechanical System to fabricate a bare smooth surface, and surfaces that had eight kinds of micropillar array. The pillar density was controlled by manipulating the distance between the micropillars. First, evaporation time of droplets of deionized water was measured under overheated condition (100–350 °C) to evaluate the LFP of the test samples. To analyze the LFP trend, the droplet dynamics were recorded at high speed, and their spreading diameter on impact, and the time that they took to rebound from the surface were measured. We then modelled the interaction between droplet and test samples and discussed the physical mechanism of the LFP change. We detected an optimal micropillar density at which the LFP was highest in this study. This study provides insights into the droplet impact and rebounding process on an overheated textured surface, and the resulting heat transfer.

The coupled momenta induced by thermal effects during evaporation near liquid–vapor interfaces cause complex three-dimensional flow structures, called thermal buoyant-capillary flows. In this study, we investigated the coupled flow mechanisms, observing the internal flow structure of evaporating micro-mini scale menisci in microtubes (680–1560 μm) by the micro-particle image velocimetry (PIV) technique. The horizontal and vertical motions of 1-μm fluorescent particles in volatile ethanol was visualized to obtain the shape of the flow in high-resolution (10–20 μm resolution). The evaporation rate (2–1800 ng/s) and environmental temperature (20–50 °C) were controlled using the Joule heating method. We discuss the effects of varying the thermal conditions and tube size on the position and strength of the vortices. In addition to the experimental work, a simplified numerical simulation was also carried out to estimate the thermal properties that were not measured during experiment. As a result, it was explained that the vortices near the wall of the upper tube are initialized by weak thermal effects from the wall, and higher heating condition cause them to move down the tube and enhance their vorticity by dominant buoyancy effect. In addition, vortices in tubes with large diameters are weaker and their motion is delayed due to the increase in gravitational effects.

In this study, the contact angle of air bubbles on a micro-structured surface under a water pool is experimentally investigated. A previous study “Kang and Jacobi, Equilibrium Contact Angles of Liquids on Ideal Rough Surfaces, Langmuir (2011)” adopted the work of adhesion as the additional work of the process of droplet contact on a textured surface, and it explained the reason for failure of the classical theoretical model (Wenzel equation) in real wetting tests. We extended the above concept to bubble interaction with the textured surface. First, the bubble contact angles on the textured surface were modeled based on the concept of the work of adhesion in the free energy calculation. Second, the contact angle of air bubbles was measured on the bottom of test surfaces under a water pool. The test-section surfaces have micro-pillars with a size of 5–40 μm prepared by the microelectromechanical systems (MEMS) technique. The bubble contact angles were compared with both the modeled bubble shape and classical prediction (Wenzel & Cassie–Baxter equations). In addition, detail wetting features which shows wetting transition and critical wetting behavior of bubble were discussed.

The Leidenfrost point (LFP) of a single deionized water droplet on various wettability surfaces was investigated by high-speed visualization. Manipulating both the chemical and geometric properties of a silicon wafer substrate, we prepared test samples with a wide range of wettabilities, from superhydrophilic ( 165°). Nanoscale wire or needle-shaped structures were fabricated by etching process on the silicon substrate. The effects of wettability on LFP were investigated by analyzing how the droplets rebound on the overheated surface. The results show that the more hydrophilic condition, the higher LFP. As the hydrophilic surface causes vigorous nucleate boiling and large viscous dissipation during the droplet contact, higher superheated condition of the substrate is required to trigger the rebounding dynamics and stable film boiling state. Furthermore, the superhydrophilic surface, which was assisted by nanostructures, contains many cavities and produces strong capillary wicking upon droplet contact; hence, explosive transition boiling in the droplet was observed. This study provides a detailed physical understanding of droplet dynamics on overheated nanostructures and its relation on LFP evaluation.

We explored the hydrodynamic features of dynamic wetting both theoretically and experimentally. We studied the triple-line motions of glycerol-water solutions of various viscosities (85-456 mPa·s) in microglass tubes (300, 500 and 1000 μm in diameter). First, dynamic (advancing) contact angles were measured and compared with those of a well-known hydrodynamic model (O.V. Voinov, Hydrodynamics of Wetting, Fluid Dynamics (1976)). Second, the internal flow structures near moving menisci were visualized using micro-particle image velocimetry (μ-PIV). Several differences in flow shape (compared to those predicted by theory) were observed. Ultimately, we present a new method by which dynamic contact angles may be predicted, derived from analysis of wall shearing stress at the moving contact line to reflect on the liquid-solid interaction effect. Our analysis has the advantage of incorporating the effect of contact angle hysteresis on the dynamic contact angle. The modified approach yielded data in good agreement with our experimental results and other open-literature data. We thus fundamentally explored the hydrodynamic aspects of dynamic wetting.

We studied bubble growth on a microstructured surface during nucleate boiling using optical high-speed and infrared (IR) cameras. The effects of structured surfaces on bubble growth and dynamics were examined and their role analyzed with the use of simple models. A smooth, bare surface was prepared, and four microstructured test sample surfaces were fabricated with microscale gaps ranging from 5 to 80 µm. The optical high-speed camera was used to observe the bubble growth profile with high temporal resolution; the IR camera was focused on the underside of the sample for direct visualization of the boiling process. Overall, the microstructured surfaces produced more bubbles, a lower frequency and nucleation site density than the bare surface for the low heat flux range (100–300 kW/m 2 ), corresponding to the isolated bubble growth regime. The liberated bubble size was dependent on the size of the microstructure gap. Analysis of the high-speed images revealed that the liquid between the microstructures did not evaporate during bubble growth; however, during the initial growth stage, there was a brief period in which the liquid at the nucleation site evaporated. The large surface area and relatively high number of nucleation points contributed to enhanced bubble growth on the structured surfaces.

A significant increase in the Leidenfrost temperature (LFT) was observed on a micro/nano multiscale textured surface (MTS) compared with a polished surface (PS) and a micro rough surface (MRS). MTS was fabricated by anodic oxidation and has nano-scaled needles with micro roughness. It showed improved surface wetting characteristics (0° contact angle with liquid spreading). On the other hand, MRS was fabricated by mechanical polishing and it only has micro roughness. LFT on MTS and MRS increased by approximately 150 °C and 30 °C, respectively, compared with one for PS. The textures on each surface influenced the water droplet dynamics. The relationship between LFT and the dynamics of water droplet were studied by high-speed photography. The key phenomenon determining LFT was the rebound process of the droplet during a few milliseconds. On MRS and MTS, the rebound phenomenon of the droplet was disturbed by the surface-texture-induced liquid-solid contact even when the surface was initially at a high temperature over 300 °C. The precursor wetting front, observed only on MTS and the capillary wicking phenomenon are likely the responsible mechanisms that significantly increased LFT on MTS.

An analytical model for heterogeneous bubble growth during nucleate boiling with variable wettability on an ideal smooth surface has been developed. We analyzed the growth of bubbles in terms of the motion of the triple line, and three stages of bubble growth were identified based on the triple line motion. The transition points between these stages were modelled using a free-energy analysis and force-balance of the bubble growth system. We considered two bubble-growth pathways: one with a constant angle between the liquid–vapor interface and the surface, and one with a constant base (i.e., constant triple line), and the two paths are linked during a bubble growth. It is hypothesized that the transition from a regime to another is linked to the less expansive energy path. To confirm this, a bubble growth was experimentally observed on various different wettability surfaces, and the results of these experiments were compared with the analytical model. In general, a larger contact angle (i.e., a more hydrophobic surface) resulted in bubble growth with a larger triple line, and hence a larger base diameter and a larger departing bubble diameter.

We study a heat flux partitioning analysis of nucleate pool boiling on microstructured surface through infrared visualization technique. A heat flux partitioning analysis of the nucleate pool boiling consists of three kinds of heat flux mechanisms; convective, quenching and evaporative heat flux. It is importance of understanding the dominance among those heat flux mechanisms to fundamental study of the nucleate boiling heat transfer, but it is not clearly figured out. In this study, directly measuring the boiling parameters; bubble departure size, bubble releasing frequency, nucleation site density and bubble growth time through the infrared visualization technique, a nucleate boiling heat flux portioning analysis on pool boiling has been carried out. The experimental results indicate that sum of the three heat flux partitions from the measured boiling parameters shows good agreement with the experimentally given total heat flux. In addition, the quenching heat flux and evaporative heat flux becomes dominant at high heat flux regime by numerous bubble generation and fast bubble growth. On the microstructured surface, the increased heating surface area by the roughness ratio intactly contributes the heat transfer performance enhancement, and the area increase effect have to be reflected on the heat flux partitioning calculation. Although there are still many arguments of the heat flux portioning model analysis on pool boiling heat transfer from literatures and the methodological limitation due to the chaotic boiling phenomena, this study gives good inspiration and understanding of the boiling heat transfer mechanism and the importance of each heat transfer mechanism.

The goal of this study is to investigate film boiling heat transfer (FBHT) on a completely wettable surface (CWS) with atmospheric, saturated distilled water ( T sat ∼ 97 °C). The CWS was fabricated using anodic oxidation of a zirconium rod, to achieve a contact angle of θ ∼ 0° with liquid-spreading and this was driven by capillary-wicking into the nano-scale, needle-shaped structures on the surface. To consider independently the effects of the maximum height e max of the surface roughness, we investigated a roughed zirconium surface (RZS), which was modified by polishing with sandpaper. Quenching experiments were conducted to evaluate the FBHT; the heat transfer coefficient ( h film ) during FBHT, minimum heat flux ( q ″ min ) and minimum film boiling temperature ( T min ) were all larger with the CWS than with the bare zirconium surface (BZS) or on the RZS. Through high speed visualization, we observed that intermittent wetting during FBHT resulted in an unstable liquid–vapor interface in case of the CWS. Therefore, the remarkable ability of the CWS to supply liquid when in contact with the heat transfer surface resulted in clear enhancement of the FBHT performance (i.e., increases in h film , q ″ min and T min ).

We study the effectiveness of microstructured surfaces in enhancing the boiling heat transfer (BHT) and critical heat flux (CHF). A set of experiments is designed with thirteen prepared samples: twelve with a microstructured surface, and one with a bare surface. The samples are fabricated using microelectromechanical systems (MEMS) techniques. The samples are tested using pool boiling experiments in saturated and atmospheric pressure conditions. The experimental results show that BHT increases with the surface roughness, defined as the ratio of the rough surface area to the projected area, but this enhancement gradually slows. The heat transfer coefficient of the structured surface is more than 300% that of the bare surface. The increase in the heating surface area due to the roughness ratio improves nucleate BHT due to the enhancement of convective heat transfer. The structured surface shows a 350% improvement in CHF over the bare surface. However, through analysis of the capillary flow rate on the structured surface, a critical gap size that limits the CHF is found. The critical gap size is discussed analytically and compared with experimental data. Designs for optimal boiling performance are proposed by studying the role of microstructured surfaces in both BHT and CHF.

In this study, we observed the behavior of water droplets near the Leidenfrost point (LFP) on zirconium alloy surfaces with anodizing treatment and investigated the droplet cooling performance. The anodized zirconium surface, which consists of bundles of nanotubes (∼10–100 nm) or micro-mountain-like structures, improved the wetting characteristics of the surface. A deionized water droplet (6 μL) was dropped onto test surfaces heated to temperatures ranging from 250 °C to the LFP. The droplet dynamics were investigated through high-speed visualization, and the cooling performance was discussed in terms of the droplet evaporation time. The modified surface provided vigorous, intensive nucleate boiling in comparison with a clean, bare surface. Additionally, we observed that the structured surface had a delayed LFP due to the high wetting condition induced by strong capillary wicking forces on the structured surface.

Our experiments and analyses of the bubble incipient on ideally-smooth, horizontal heated surface confirm the observed low superheat and the weak surface-wettability dependence. These are contrary to the previous heterogeneous nucleation predictions, so to clarify this difference and the experimental results not explained by previous nucleation theories we adapt a new model based the thermal boundary layer. The model includes the kinetic dynamics of the superheated liquid and the thermodynamic stability of the generated vapors. The fluid particle transfer rate is estimated with the Smoluchowski equation. Consequently, the incipient bubble nucleation on the ideally-smooth horizontal surface, with different wettability, is described and the predictions match the experiments fairly well.

The Leidenfrost temperature TLFP of falling water droplet was studied on sintered porous wick surfaces. Various surface factors were analyzed to identify those that significantly contribute to increasing the TLFP. To decouple the effects of capillary wicking on porous wick surfaces, the results obtained using ethanol as a working fluid were compared to the results obtained using water. When ethanol was used, the capillary wicking did not differ significantly between the porous wick surfaces. The evaporation time of the droplets was measured at high temperatures (100–600 °C) to evaluate the TLFP. The effect of surface permeability on the absorption of the vapor layer through a porous wick surface had a negligible influence on the TLFP. Within the range of low thermal effusivity of the heating surface, an analysis of the interface temperature shows that in liquid ethanol, the thermal properties dominate the TLFP as well as the overall boiling regime. Similarly, in water (for which the capillary wicking effect cannot be ignored), the TLFP and film boiling regime were determined by the thermal effusivity. In both liquids, the thermal effusivity was the dominant determinant of the TLFP, regardless of the capillary wicking rate. However, the capillary wicking significantly affected the boiling heat transfer in water, until it reached the transition boiling regime.

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 ...

Numerical simulation code such as commercial CFD one, ANSYS-CFX was studied and validated with the previous experimental data for magnetohydrodynamics (MHD) analysis in the liquid metal breeder flow of the Korean Test Blanket Module (TBM) considering the ITER operation conditions. And, a system code for safety analysis was developed with the MARS-FR (Multi-dimensional Analysis Reactor Safety for Fusion Reactor) in this study, whose base code was MARS and it was developed for commercial pressurized water reactor and gas cooled reactor in Korea. First, with the selected benchmarking problem with previous experimental studies and analytic solution, ANSYS-CFX was validated and evaluated. It shows a good agreement with the pressure loss prediction in MHD flow. Second, the system code was developed to predict the MHD effect for the one dimensional flow analysis based on a MHD solution which Miyazaki derived and has semi-implicit scheme to a flow direction. This study shows good agreement between ANSYS-CFX MHD prediction and experimental results as a validation, and the developed system code presents good performance of prediction in MHD pressure loss.

To develop a Korean 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 Electro-Magnetic (EM) module for analyzing the Magnetohydrodynamic (MHD) effect of the liquid breeder under a high magnetic field was 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 Korean 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 0.008337 Pa 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 (ELLI), a preliminary analysis was performed using the installed test section and magnet, which could produce a magnetic field of up to 2.2 T.

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.

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.

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.