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In this study, parylene-C films from plasma deposition as well as thermal deposition were pyrolyzed to prepare a carbon electrode for application in electrochemical immunoassays.

Quantum dots (QDs) are semiconducting nanoparticles that exhibit unique fluorescent characteristics when excited by an ultraviolet light source. Owing to their highly saturated emissions, display panels using QDs as pixels have been presented. However, the complications of the nanofabrication procedure limit the industrial application of QDs. This study suggests a method to arrange high-aspect-ratio QD pixels by inducing both Laplace-pressure-driven capillary flow and thermally driven Marangoni flow. The evaporation of colloidal QDs induces a capillary flow that drives the QDs toward the inner tips of V-shaped structures. Additionally, the Marangoni flow arranges the gathered QDs at the tip; thus, they could form a high dune, overcoming the limitations of the existing capillary assembly method using evaporation. Using these phenomena, clover-shaped (assembly of V-shaped edges) templates were made to gather numerous QDs, and the clover with a 30° angle afforded the highest brightness among all the angle structures. Finally, by demonstrating a 100-cm2-sized QD microarray with high uniformity (98.6%), our method shows the feasibility of large-area fabrication, which has extensive application in manufacturing QD displays, anti-counterfeiting labels, and other QD-based optical devices.

In hydrophobic mini- and microchannels, slug flow with moving contact lines is typically generated under various two-phase flow conditions. There is a significant pressure drop in this flow pattern with moving contact lines, which is closely related to the dynamic contact angles. Researchers have investigated dynamic contact angles experimentally for decades, but due to the limitations of visualization techniques, these experiments have typically been conducted in low Weber number regions (

Biphilic surfaces exhibit outstanding condensation efficiency compared to surfaces having homogeneous wettability. Especially, hydrophilic patterns on a superhydrophobic substrate significantly promote the coalescence-induced jumping of condensed droplets by increasing the nucleation rate of condensation, thus enhancing the condensation efficiency drastically. However, the application of biphilic surfaces in practical industries remains challenging because controlling the size and spacing of the hydrophilic spots on large and complex surfaces is difficult. In this study, we have achieved heterogeneous wettability using the evaporation-crystallization method, which can be applied to various surfaces as required by industries. The crystals generated using the evaporation-crystallization process drastically increased the number density of condensed droplets on a superhydrophobic surface (SHS), so the developed biphilic surface increased the cumulative volume of jumping droplets by up to 63% compared to that on a conventional superhydrophobic surface. Furthermore, the condensation dynamics on the biphilic surface were analyzed with the classical nucleation theory and the Ohnesorge number. The analysis results indicated that the generated hydrophilic crystals can reduce the nucleation energy barrier and decrease the available excessive surface energy of coalesced droplets on the biphilic surface; this implies that the size distribution of the crystals determines the condensation dynamics. In sum, this study not only introduced an effective surface tailoring approach for enhancing condensation but also provided insights into the design of optimum biphilic surfaces for various conditions, creating new opportunities to widen the applicability of biphilic surfaces in practical industries that exploit condensation.

Zirconium silicide (ZrSi2) surfaces with micro/nanoscale structures were fabricated using an anodization technique with different working solutions. The surface structures evolved depending on the reaction time, and the resulting surface structures were characterized using scanning electron microscopy. For all three solutions, the anodization conditions for producing micro/nano surface structures with the superhydrophilicity were successfully found. The etching rate and thickness were controlled depending on the type of solutions. In the organic bath-based ammonium fluoride (NH4F) solutions, the etching thickness was significantly reduced in comparison to that using a water bath-based hydrofluoric acid (HF) solution. Superior oxidation resistance of ZrSi2 was maintained for all superhydrophilic, anodized ZrSi2 surfaces at 700 °C air oxidation regardless of the type of solution. The superhydrophilic, anodized ZrSi2 surfaces significantly increased the Leidenfrost temperature by inducing liquid-solid contact between the heated surface and coolant. We expect the developed surfaces to be useful in future nuclear fuel cladding material applications for ensuring the safety of nuclear reactors.

The effects of water accumulation at the interface of gas diffusion layer (GDL) and gas supplying channel on the water distribution in polymer electrolyte membrane fuel cells (PEMFCs) is analyzed. The amount of water at the interface and in the GDL are quantified using X-ray. Quantitative analyses show that the value of the criterion of water accumulation that can affect the water distribution is in the water saturation range of 0.22–0.24. The amount of water in the GDL increases with the water accumulated at the beginning of the water generation cycle. However, it remains constant after the water accumulation exceeds a criterion value. The result shows that the water accumulation at the interface should be investigated to understand the water distribution in PEMFC. The water distribution in PEMFC cannot be analyzed based on the steady state concept but can be analyzed based on the concept of cyclic process.

Passive cooling based on natural circulation is utilized in ex-vessel core catcher system of an advanced nuclear reactor to handle severe accident scenario. The core catcher coolant channel has a unique geometry which consists of heated downward-facing slightly inclined and vertical surfaces. A full height experimental facility with natural circulation driven flow to model ex-vessel core catcher system was designed using scaling analysis. In this study, the cooling capability and heat transfer characteristics of the ex-vessel core catcher system was carried out. Two-phase flow parameters and wall temperatures were measured under a uniform heat flux condition. Two-phase flow structures were identified by high-speed camera visualization along with measurements of two local parameters, void fraction and re-wetting time. The wall temperature and local heat transfer coefficient distribution along the cooling channel were obtained by direct measurements of the heater surface and liquid temperature. The cooling performance of the core catcher system was analyzed based on the experimental results. The results indicated that the core catcher coolant system provides adequate cooling and maintains the integrity of the core catcher plate for prototype heat flux conditions.

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.

We investigated a potential impact of the candidate materials (FeCrAl, SiC-CVD, and Zry-4) for the Accident Tolerant Fuel-cladding (ATF) in the quenching heat transfer according to the system parameters such as liquid subcooling, pressure, and flooding rates. Results obtained by water quenching experiment and system code analysis using MARS 3D-VESSEL are presented that the quenching heat transfer including the post-CHF parameters (i.e., minimum film-boiling temperature) is more influenced by the system parameters compared to change of the cladding materials. Current models of minimum film-boiling temperature in system code are also discussed for the application of ATF and its oxidation. Select option in the model library is proposed to consider the surface parameters using weighting term in thermal-hydraulic system code.

To increase the efficiency of jumping-droplet condensation, this study proposes a hierarchical superhydrophobic surface that promotes coalescence-induced jumping. Inspired by the phenomenon in which a growing droplet moves spontaneously within a superhydrophobic V structure, we fabricated nanograssed zigzag structures on the surface to induce the spontaneous motion of condensed droplets. The direction of the motion was parallel to the surface, so the condensed droplets easily coalesced on it. Compared with a conventional nanograssed superhydrophobic surface, the proposed surface increased the frequency of coalescence-induced jumping by ≥17 times and increased the cumulative volume of jumping droplets by ∼1.8 times. The proposed surface has great potential to increase the efficiency of applications such as water- and energy-harvesting and cooling systems that exploit jumping-droplet condensation.

Despite considerable research interest due to omnipresent and practical importance of interfacial phenomena (e.g., wetting and dewetting) on nanotextured surfaces in the academic and industrial fields, direct visualization of the behavior and shapes of liquid-vapor interfaces between nanoscale structures remains an arduous task because of the resolution limitations of visualization techniques. In this study, we succeeded in a first-hand visualization of the behavior and shapes of the liquid-vapor interfaces of a water droplet between nanometer-scale pillar during evaporation by introducing a synchrotron X-ray imaging technique with spatially high resolution (40 nm/a pixel). On the basis of the visualization data, we intensively analyzed and discussed the spreading and evaporation phenomena of a liquid droplet on hydrophilic nanotextured surfaces.

The jet breakup length is one of important phenomenological parameters to evaluate the corium debris bed coolability during the progress of severe accident in nuclear power plant, since the parameter is related to the determination of the liquid melt fraction in debris bed. Existing jet breakup length data reveal relatively large scattering and different trends, similar to the Saito or Epstein correlation, depending on cases. The scattering and separation of the data may be due to uncontrolled factors in the melt discharge condition and also inconsistent evaluation methods used for the jet breakup length. Through melt jet breakup experiments with low melting point metal alloy (Bi-Sn alloy), the influence of the estimation methods for the jet diameter (measured or calculated value) and the jet breakup length (position- or velocity-based value) was investigated. According to the comparison between the measured and calculated jet diameters, the experimental uncertainty was reduced by producing well-controlled melt jet. The concept of the jet breakup length was revisited in detail, emphasizing the shorter breakup length by the velocity-based method. The modification of the Saito correlation constant from 2.1 to 1.7 was suggested to fit the velocity-based breakup length. The importance of a well-defined data analysis process for non-dimensional jet breakup length was highlighted for the minimization of the experimental uncertainty.

When a material crosses its critical point or a pseudocritical temperature at which the specific heat of the material at a constant pressure is maximal, the thermal and hydraulic properties vary significantly. Buoyancy, induced by a great density variation of near-wall fluid, results in asymmetric heat transfer coefficients at the top and bottom walls of a horizontal circular channel. However, flow acceleration, which occurs in the flow direction because of significant density variations, has an identical effect on heat transfer regardless of the flow direction. For this reason, only the acceleration effect can be investigated in terms of a turbulent shear stress variation for horizontal flows. Therefore, a study on the buoyancy effect for horizontal flows is required with respect to not the shear stress variation but different aspects between the top and bottom walls. In this study, semi-empirical and empirical heat transfer models were proposed based on a mixed convection. The models were evaluated using experimental data. The semi-empirical model has a mean absolute difference (MAD), the average error, of 21.73% for the top wall and 22.35% for the bottom wall. However, the empirical model has a MAD of 10.00% for the top wall and 10.44% for the bottom wall. The proposed models significantly improve the prediction accuracy of the Nusselt number at each wall, as well as for the average Nusselt number compared to the previous correlations.

Flow boiling experiments were carried out with water on a 300 mm × 120 mm area and 85 mm thick downward-facing heated copper block at 1.4 bar pressure in a channel inclined 10° upwards with respect to horizontal. Heat transfer studies were conducted on a plain and microporous coated surfaces with heat fluxes up to 550 kW/m2 for various flow rates and subcoolings. The results showed enhanced heat transfer coefficient on the microporous coated surface by a factor of two compared to the plain surface. Flow visualization study indicated that this heat transfer enhancement is due to enhanced microbubble generation on the microporous coated surface. Boiling curves were compared with the ones obtained from the small size heaters from previous researches. The local dryout or local critical heat flux was observed on the plain surface at around 450 kW/m2. When heat flux was increased beyond the critical heat flux level, transition boiling occurred instead of direct excursion to the film boiling. Visualization studies of transition and film boiling regions were also carried out and were analyzed using transient temperature data.

This study examines cooling efficiency, uniformity, and bubble dynamics on a porous surface. We use infrared (IR) thermometry to visualize results of temperature fields and liquid/vapor interfacial dynamics. Porous and non-porous micro-structured surfaces are prepared using soft-lithography and a ceramic precursor, allylhydropolycarbosilane (AHPCS). The surface cavities promote nucleation, and the heat transfer coefficient on the porous surface is approximately 30% higher than that on the non-porous surface. Additionally, the porous surface exhibits a more uniform temperature field with lower spatial and temporal variations than the non-porous surface. Bubble dynamics is visualized via an IR camera through the bottom side of the test specimens using the IR transparent characteristics of the substrate and micro-structures. The porous surface reveals higher nucleation site density and contact line density and lower equivalent bubble diameter when compared with those of the non-porous surface, and this is consistent with more effective and uniform cooling on the porous surface.

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 sodium-cooled fast reactor (SFR) is a promising next generation reactor type. Existing prototype generation IV nuclear reactors use a steam Rankine cycle for power conversion. However, the reaction between sodium and water presents a major safety issue in the development of SFR. In this aspect, the nitrogen Brayton cycle can be a suitable alternative to the conventional steam Rankine cycle for SFR. Compared to the steam Rankine cycle, the nitrogen Brayton cycle is relatively simple; further, nitrogen does not react with sodium due to its chemically inert feature. Therefore, a nitrogen Brayton cycle coupled with SFR can be a good combination. In this work, the nitrogen Brayton cycle for KALIMER-600 has been studied. In addition, sensitivity analysis, optimization of the cycle, and the preliminary design of components were performed. Mathematical models for conducting sensitivity analysis of the cycles were developed using FORTRAN. Sensitivity studies were carried out by varying the cycle maximum temperature, turbine inlet pressure, and pressure ratio. The optimized results showed that the cycle efficiency is 38.26%. The preliminary design of components was performed to assess the viability of the proposed system. In order to improve cycle thermal efficiency, sensitivity studies of heat exchanger was performed to understand the effects of printed circuit heat exchanger (PCHE) channel shape. The analysis results show that a significant improvement in cycle efficiency (improve 1.3%) was obtained by changing the PCHE channel shape.

This paper presents the effects of surface structure and coating on the heat transfer deflection point (HTDP) when the rate of increase of the heat transfer coefficient (HTC) changes during the early stage of nucleate boiling. Two types of micro-cylinder arrays were fabricated on a Si substrate using microelectromechanical system techniques. A SiO2 thin film was deposited on the microstructured surfaces to reveal how the surface coating affects the boiling behavior. The HTDP differs from the minimum nucleation criteria indicating that the HTDP occurs at a slightly higher heat flux than the minimum nucleation criteria. A deflection occurs at a specific value of the HTC on each surface, namely, ∼3.27 kW/(m2·K) with a SiO2 layer and ∼4.67 kW/(m2·K) without a SiO2 layer, regardless of the surface structures. In the after-deflection regime, instantaneous slopes of the boiling curves are governed by the surface structure. Regardless of the surface coating, the final slopes increase as the surface roughness increases, namely, 31.2, 63.5, and 87.0 kW/(m2·K) for flat, coarse, and dense surface structures, respectively. These results will contribute to a better understanding of the behavior of the HTDP during boiling.

This paper presents optimization of a supercritical carbon dioxide Brayton cycle for three types of 300-MWth small modular reactors (SMRs); a pressurized water reactor (PWR), a sodium-cooled fast reactor (SFR) and a high-temperature gas-cooled reactor (HTGR). The parameters of the pressure ratio and the flow split fraction were examined for sensitivity analysis and optimization of cycle. The optimized cycle efficiencies of PWR, SFR, and HTGR were 30.6%, 46.38%, and 50.04%, respectively. Key components, i.e. turbomachinery and heat exchangers for the SMRs were designed to develop the optimized cycles. The cycle thermal efficiency was improved by using investigating the effects of the channel shape (zigzag, s-shape, airfoil fin) of the printed circuit heat exchangers (PCHEs) on the pressure drop. The study indicated that using airfoil fin type PCHE may increase the cycle thermal efficiency by about 1.0% in comparison with zigzag type PCHE. The effect of turbomachinery efficiencies on the cycle thermal efficiency were investigated.

To understand the Leidenfrost phenomenon, which is the results of formation of a thin vapor layer, the progression of the vapor should be analyzed. However, due to the limitation of measuring techniques, the empirical measurement of the vapor layer under a dynamic Leidenfrost drop as a function of time has not been reported because the vapor is only tens of micrometers thick and forms within a tenth of a millisecond. Therefore, this paper presents a synchrotron X-ray imaging with the precise resolution to overcome the limitation of previous measurement technique. The liquid–vapor interfacial behavior of a drop of ethanol that is being levitated above a flat SiO2 surface by the Leidenfrost phenomenon is analyzed depending on surface temperature. Measurements suggest that a thin (

The adhesion force of water droplet on the gas diffusion layer (GDL) is modeled based on the droplet deformation. The deformed droplet is represented as an ovoid shape. The adhesion force is calculated based on it and verified by the surface tilting experiment. The model predicts the shape of deformed droplet and adhesion force within 30% error, whereas previous models predict adhesion force with error larger than 30%. The modified model is used to compare the adhesion force among 3 types of GDL having pore gradient. The comparison result is well matched with the water distribution in polymer electrolyte membrane fuel cell (PEMFC) and water detachment phenomena at the GDL. High adhesion force makes more water accumulation at the interface of GDL and gas supplying channel. This makes different boundary condition and changes the water distribution in PEMFC.

We investigated the effect of Bond number of sphere Bos and surface super-hydrophilicity at minimum film-boiling temperature TMFB and minimum heat flux q″min using quenching experiment at atmospheric pressure and the saturation temperature of water. In particular, we focused on the vapor-releasing dynamics in film boiling and evaluated the main parameters such as vapor-bubble releasing frequency fb and vapor-bubble departure diameter Db. We selected two sizes of quench sphere (sphere diameter Ds = 15 mm and 25 mm) based on critical Bond number BoC to evaluate the vapor-releasing dynamics depending on the Bos. The super-hydrophilic surface was prepared by the anodic oxidation on zirconium (Zr-702) sphere. High speed visualization and inverse heat transfer calculation facilitate a qualitative and quantitative analysis of film boiling heat transfer. The surface super-hydrophilicity of the quench sphere increases TMFB and q″min: 12% and 366% increase for Ds = 15 mm and 20% and 305% increase for Ds = 25 mm, respectively. Db strongly depends on Ds and exhibits a relatively weak dependency to the surface super-hydrophilicity. fb is affected by the Ds and the surface super-hydrophilicity. The increase in TMFB is explained by the liquid-solid contact in film boiling. The D25-CWS exhibits the large area fraction of liquid-solid contact versus total heat transfer surface compared to the D15-CWS. The increase in q″min is related to minimum frequency of vapor-bubble releasing to sustain the stable liquid-vapor interface fb,min because the large fb,min indicates the fast destabilization of the liquid-vapor interface in film boiling during quenching.

Adequate equivalent diameters for non-spherical particles in predicting the pressure drop of fluid flow through particle beds were pursued. A series of experiments to measure the pressure drop of single-phase air flow through particle beds were performed using three kinds of spherical particles (2, 3.5 and 5 mm diameter) and four kinds of cylindrical particles. The experimental data were utilized for evaluating previous models and for proposing a new model, a modified Wu et al. (2008) model with an alternative Ergun constant for the viscous energy loss term. The proposed model agreed well with the pressure drop of single-phase flow through spherical particle beds within 10% in average. The experimental data for cylindrical particle beds also agreed well with the proposed model within 16% in average by applying the equivalent diameter proposed by Li and Ma (2011) as a characteristic size of non-spherical particles.

A hydrophobic surface promotes bubble nucleation due to hydrophobicity. Thus, a hydrophobic surface has a higher boiling heat transfer coefficient (BHTC) than a hydrophilic surface. In contrast, a hydrophilic surface supplies liquid to a heating surface. This mechanism enhances the critical heat flux (CHF). In this respect, there is a trade-off between a hydrophobic and a hydrophilic surface. In this study, we examined the effect of heterogeneous wetting surfaces on flow boiling performance. We designed four types of hydrophobic stripes; there are two directions, parallel and crossed to the flow, and the width (the pitch) of the hydrophobic stripes in each direction is 3 or 1 mm. In the macro-channel, the flow boiling performance on the surfaces depended on the patterns. The parallel striped surfaces had higher CHFs than the crossed striped surfaces. In addition, the narrow patterns in each direction had higher CHFs than the wide patterns. The difference in BHTC among the parallel striped surfaces was not large, but the difference in BHTC among the crossed striped surfaces was considerable. A visualization technique revealed that the merging and confinement of bubbles were key factors in explaining the boiling characteristics. Considering the drag coefficient and bubble breakup, we suggest appropriate designs of the hydrophobic pattern for the improvement of BHTC and CHF in the flow boiling performance.

Heat transfer in a horizontal tube represents different behaviors at top and bottom walls. Buoyancy and bulk flow acceleration, which are induced by drastic variation of properties, especially density, are major reasons for such a difference. In this study, in order to investigate the effect of buoyancy on heat transfer of supercritical CO2, the validities of buoyancy parameters including Bu C and Bu J which is a modified form of Bu C were experimentally examined in the horizontal tube. The test section had an inner diameter of 7.75 mm and a heated length of 0.91 m. The experimental variables were mass flux (64.1–250.5 kg/(m2 s)) and heat flux (3.1–25.9 kW/m2) with constant inlet temperature (30°C) and pressures (75.87–77.35 bar). As a result, the heat transfer transition from enhancement to deterioration occurs earlier at the top wall than at the bottom wall because hot and low-density fluid moves toward the top wall by buoyancy. The previously suggested thresholds of 10−3 for Bu C and 10 for Bu J are merely applicable to determine the existence of buoyancy. We concludes that Bu J shows the improved prediction compared to Bu C . While, bulk flow acceleration is closely related to the heat transfer behavior along the flow direction rather than buoyancy.

To develop a cycle operation strategy for a supercritical carbon dioxide (S-CO 2 ) Brayton cycle, a series of MARS code simulation results were compared to experimental data from an SCIEL compressor test, which was collected at both component and whole-loop level. The simulation results show reasonable agreement with the experimental results. The MARS code was used to simulate behavioral responses at varying scenarios, the valve control for cycle operation, a power swing to simulate load following behavior, and an effect of the heat sink reduction to simulate failure. The electric power output of the turbine decreased to 50% of that during normal operation through valve control. After power swing, the system reverted to the initial steady-state conditions. The fluid behavior in precooler and the eventual change in the system were identified through the scenario of the heat sink reduction. The simulations provide a notable insight into the responsiveness of an SCIEL to these variable scenarios. Even though the MARS code was specifically developed for analysis of water reactor transients, its field of application may be extended to analyze the S-CO 2 Brayton cycle, and may thus assist with the development of control strategies for SCIEL and the S-CO 2 Brayton cycle.

In the late phases of severe accidents in nuclear power plants, in order to ensure the long-term coolability of the debris bed on a reactor containment floor, it is crucial to cool down and stabilize molten core debris, which has an internal heat generation by decay heat. Therefore, it is of key importance to continuously supply water into a debris bed, which is affected by the pressure drop depending on the characteristics of debris bed such as bed porosity, particle morphology, particle size distribution, etc. Thus, in the present work, the influence of particle size distribution and the adequacy of mean diameters for predicting pressure gradients in particle beds were evaluated. Experimental data were obtained on the pressure gradients of air flow in packed beds composed of either spherical or cylindrical stainless steel particles having a size distribution of 1–10 mm. The results were compared with the values calculated by a proposed model in our previous work. When the area mean diameter was adopted as the effective particle diameter, the measured pressure gradients of air flow through each spherical particle bed with a particular size distribution agreed with the calculated values within a mean absolute percentage deviation of 9%. For a cylindrical particle bed with a particular size distribution, the measured pressure gradients agreed with the calculated values within a mean absolute percentage deviation of 12% when adopting the area mean diameter calculated using the equivalent diameter, the product of the Sauter diameter and particle shape factor as the effective diameter for non-spherical particles. This selection of mean and effective diameters produced the best fit among several candidates. Thus, we propose the area mean diameter (calculated using the equivalent diameter) as the effective diameter for determining the hydraulic diameter affecting fluid resistance in porous beds composed of non-spherical particles with a particular size distribution.

The goal of this work is (i) to evaluate the cooling rate on a super-hydrophilic surface as a function of the subcooled degree ΔTsub of the liquid coolant, (ii) to analyze the contact heat transfer q″c of the liquid-solid contact, and (iii) to investigate the mechanism of microbubble emission boiling (MEB). We fabricated a super-hydrophilic surface by anodic oxidation of a zirconium vertical rod, so called completely wettable surface (CWS), which had surface microstructures with super-hydrophilicity. The CWS results in a decrease of the cooling time tcool as compared with the Bare Zirconium surface (BZS) results under small ΔTsub (tcool ∼ 50% decrease for ΔTsub = 0, 15, and 40 K, respectively). However, its surface effect is limited in the case of large ΔTsub (tcool ∼ within 5% for ΔTsub = 60 and 75 K). The fast quench on the CWS under ΔTsub, explained by the increase in minimum film-boiling temperature TMFB and rewetting velocity U, is due to the liquid-solid contact. We evaluate the contact area Ac and volumetric absorption rate of the liquid dV/dt by conducting liquid absorption experiments. The increase in Ac and dV/dt contribute to an increase in q″c, by forming the liquid film at the liquid-solid contact spot. The orders of the time scale between capillary-wicking and liquid-solid contact are comparable. Destabilization of the large vapor bubble is caused by an increase in q″c, which is a major reason for MEB generation, and this mechanism enables the q″ to be significantly high on the CWS under subcooled quenching.

The effect of through plane pore gradient of gas diffusion layer (GDL) on the performance of Polymer Electrolyte Membrane Fuel Cell is investigated experimentally. The performance with GDLs having no, medium and high pore gradient are compared at 2 different relative humidity (RH) conditions. The medium pore gradient GDL shows generally the best performance in both RH conditions. The performance difference is analyzed based on the water distribution. The water distribution is measured through the X-ray visualization. The amount of water is reduced with the pore gradient GDL. This change reduces the concentration over-potential, and thereby increases the performance at high RH condition. However, the reduction of liquid water results in dehydration of the membrane at low RH condition. This makes lower performance with high pore gradient. The highest performance is not matched with the highest pore gradient. The effect of pore gradient is distinct when water exists sufficiently.

The effect of operating pressure on the water distribution and its effect on performance of proton exchange membrane fuel cell (PEMFC) was investigated. The experiment was conducted by changing the relative humidity (25%, 100%) and the back pressure value (0.2, 0.4 bar). The PEMFC was either not pressurization, or pressurization on both sides, or on the anode side or on the cathode side. In these conditions, the PEMFC showed higher performance when the cathode side was pressurization than when the anode side was pressurization. The performance difference was analyzed based on the water distribution utilizing X-ray visualization technique. Increase of water saturation in membrane was measured when the cathode side was pressurization at water deficient condition. And the saturation was getting increased as the value of pressure increased. This change caused significant increase in proton conductivity, and performance eventually. At water abundant condition, this phenomenon became weaken and performance difference became narrow. However, increase in pressure on the cathode side increased the air penetration through water and reduced the concentration over-potential. Precedent researches concentrated on the reaction kinetics to analyze the effect of pressure, but the change of water distribution should be considered simultaneously to understand the effect of pressure.

The enhancement of the condensation efficiency is required for saving energy and resources in industries exploiting condensation. In this regard, the Laplace pressure-driven spontaneous movement of condensed droplets on hierarchical superhydrophobic surfaces has been exploited to increase the condensation efficiency via the fast removal of the condensates from the surfaces. Although the moving droplets enabled the nucleation sites to be exposed to vapor before the droplets were removed from the surface, nucleation after the exposure was delayed by vapor depletion around the moving droplets. Since the heterogeneous nucleation energy barrier depends on the surface wettability, we investigated the effect of the wettability of the nucleation sites on the nucleation after the nucleation sites were exposed owing to the moving droplets. We arranged hydrophilic crystals inside the inner tips of a zigzag-structured surface and adjusted the crystal size with capillary flow-induced self-arrangement. The crystal size was highly related to the nucleation period at the inner tips because the nucleation energy barrier decreased with increasing crystal size. Consequently, the nucleation period reduced by the adequately sized crystals promoted droplet jumping, thereby increasing the frequency and cumulative volume of the jumping droplets by up to 410% and 130%, respectively. Furthermore, we verified that the increased crystal size did not affect the jumping probability on the biphilic-zigzag surfaces; this is because the wetting transition induced by the V-shapes minimized the work of adhesion. These results provide insight into the relationship between the wettability and nucleation period on hierarchically structured surfaces and highlight the synergetic effect between the hierarchical structures and heterogeneous wettability, which promotes droplet jumping.