Publishing type：Research paper (scientific journal)  

The natural convective heat transfer from a heated horizontal disc with gapped heated hollow cylinders was investigated experimentally, forming the foundation for improving the cooling performance of the heat sinks. The cooling target for this study was the central area of the heated horizontal disc where the heat dissipation was low. The heat transfer was greatly enhanced by the reversal of the flow from the gap between the heated hollow cylinders towards the heated horizontal disc. The reverse flow was promoted by the conditions of a narrow clearance between the heated horizontal disc and the heated hollow cylinder and a wide gap between the heated hollow cylinders. Conversely, for the flow along the heated horizontal disc, the heat transfer showed improvement by increasing the clearance between the heated horizontal disc and the heated hollow cylinder. The relationship between the heat transfer and flow properties was demonstrated using the Nusselt number of the heated horizontal disc and the Rayleigh number corrected for the ratio of the heated hollow cylinder height to the clearance between the heated horizontal disc and the heated hollow cylinder.

DOI： 10.1016/j.icheatmasstransfer.2023.106928

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Publishing type：Research paper (scientific journal)  

Microencapsulation of n-tetracosane, whose melting point is approximately 50 °C, in a silica shell has been performed through the sol–gel method using tetraethyl orthosilicate (TEOS) as the precursor for silica-shell formation. Additionally, two types of silane coupling agents were used to modify the surface of the microcapsules to change the wettability. The morphology of the microcapsules was observed by scanning electron microscopy. The chemical composition was characterized by Fourier transform infrared spectroscopy. The results confirmed the presence of n-tetracosane and silica in the synthesized microcapsules. Wettability analysis showed hydrophobic and hydrophilic features because of the added silane coupling agents. From the results of differential scanning calorimetry measurements, the encapsulation ratio of the microcapsules increased with decreasing TEOS/n-tetracosane ratio, and the highest encapsulation ratio was 87.1 % at a TEOS/n-tetracosane ratio of 0.25. The pH in the microcapsule solution was affected by addition of a silane coupling agent, and shifting the pH to the basic side lowered the encapsulation ratio owing to enhancement of silica condensation. After 100 differential scanning calorimetry cycles, there was no significant degradation in the phase-change temperatures and enthalpies, which confirmed the good phase-change stability and repeatability. Therefore, the microcapsules are a potential material for thermal-energy-storage systems to effectively utilize energy.

DOI： 10.1007/s10765-023-03179-1

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Publishing type：Research paper (scientific journal)  

Microencapsulated n-tetracosane as a phase change material with an inorganic calcium carbonate (CaCO3) shell has been synthesized through a self-assembly process with varying pH value of a precursor solution, and the resultant materials were systematically investigated. Spherical microcapsules with a mean diameter of less than 3 μm were seen by microscopic observation. X-ray diffraction analysis revealed the presence of three CaCO3 polymorphs such as calcite, aragonite, and vaterite in the shell, and these compositions depended on the solution pH. This fact was also confirmed by Fourier transform infrared spectra and reflected in the surface morphology of microcapsules. The characterization of the phase change behavior has shown no significant change in the phase change temperatures and more than 170 mJ/mg of phase change enthalpies with the microcapsules synthesized at pH = 1. This enthalpy corresponds to 69.2% of encapsulation efficiency which is improved from the value reported by other microcapsules with CaCO3 shells. Thermal cycling analysis exhibited good reliability and durability of phase change phenomena in the long term. These results demonstrate a promising potential of microcapsules for developing thermal energy storage materials at a mild temperature range, which can be applied for waste heat recovery applications.

DOI： 10.1021/acs.energyfuels.2c03564

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Publishing type：Research paper (scientific journal)  

A spraying system was constructed with a microencapsulated phase change material (MPCM) slurry as the working medium to weaken the adverse effects of the slurry's high viscosity and low thermal conductivity on heat transfer in a wall tube heat exchanger. The temperature distributions of pure water and MPCM slurry were first obtained through infrared thermal imaging in spraying. Furthermore, the latent heat use of MPCM slurry was investigated through a new method based on the temperature properties of the phase change fluid, in which the working and phase change temperature ranges are matched. The heat-mass transfer characteristics are compared and discussed with those of the base fluid, and the latent heat use of 10wt% MPCM slurry containing the core material n-docosane (C22H46). The experimental results indicate that the phase change enables the MPCM slurry droplets to maintain a higher temperature than a fluid with no phase change or base fluid. Different application modes (static and spraying) have correspondingly different phase change temperature ranges owing to supercooling, and the degree of matching between the working and phase change temperature ranges has a critical impact on the latent heat use of the MPCM slurry. Finally, comparing the heat transfer coefficients and Lewis numbers between the base liquid and MPCM slurry reveals that the phase change not only enhances the heat transfer but also slightly reduces water evaporation compared with pure water spraying.

DOI： 10.1016/j.ijheatmasstransfer.2022.123573

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Publishing type：Research paper (scientific journal)  

Droplet transportation using a wettability gradient surface has attracted much attention owing to applications such as in microfluidic devices. A surface with a spatial structural gradient was prepared through a simple and cost-effective process even though understanding of droplet behavior on the structure was still limited. Here, we report impinging droplet motion on a gradient wrinkled surface. Surfaces were prepared through hard film deposition on soft pre-strained polydimethylsiloxane (PDMS) with a mask installed with a slit to control the amount of deposition, which is related to the wavelength of the wrinkles. Droplets were impinged with varying position with respect to the structure, and the droplet motion was observed in the direction away from the region under the slit. We found an asymmetric contact angle and alternate motion on both sides of the three-phase contact line during the motion according to the gradient of the wrinkle wavelength. These results may help not only to understand the behavior of droplet impingement on a gradient structural surface but also to further develop applications using directional droplet transfer.

DOI： 10.1039/d1ra09244h

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

Electromagnetic fields around metal-semiconductor-metal (MSM) multilayers with square island top layers were numerically simulated to elucidate the difference in physics between the circuit resonance and Fabry-Pérot interference mediated by the surface plasmon polaritons (SPP). In the current study, the top and bottom metal layers were made of gold, and the intermediate semiconductor layer was a gallium antimony (GaSb). The lumped-element and Fabry-Pérot interference models showed less accuracy when the island width of the MSM multilayer was comparatively smaller. Since the capacitor and SPP could not be supported between the top and bottom gold layers, the anti-reflection mode of the gold-GaSb bilayer mainly affected the absorptance. However, when the width of the island was sufficiently large, the time-lapse development of the electromagnetic fields at resonant wavelengths showed strong electric and magnetic responses relating to the circuit resonance. Simultaneously, the electric fields depicted the movement of the electric charge, which coupled to the short-range surface plasmon polariton (SRSP) existing at the thin GaSb layer sandwiched by two gold layers. The wavelength of the SRSP approximately corresponded to that of the Fabry-Pérot interference. It was revealed that the lumped-element and Fabry-Pérot interference models indicated the same resonant mode from two different perspectives in physics.

DOI： 10.1088/2399-6528/ac678f

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Vanadium dioxide (VO2) is a phase transition material that exhibits metallic or insulating characteristics depending upon its temperature. In this study, a multilayered film consisting of VO2, silicon dioxide (SiO2) and gold was proposed as a metamaterial that switches its absorptivity over a broad wavelength range depending on the ambient temperature as a fundamental element of a building pigment. At high temperatures, the multilayer showed a high absorptivity at mid-infrared wavelengths, promoting radiative cooling. Simultaneously, the multilayer presented a low absorptivity in the visible and near-infrared wavelengths, enhancing sunlight absorption. The daily average heat flux can possibly be suppressed in summer in comparison with a gray body whose emissivity was 0.8. Conversely, at a lower temperatures, the multilayer showed opposite absorptivity in both the mid-infrared and visible ranges, and its daily average heat flux increased in winter. The metal–insulator phase transition of VO2 caused a drastic shift of the resonant wavelength related to surface phonons and surface plasmons at an infrared wavelength, and optical interference at a visible wavelength, originating at the interface of the SiO2 layer. Thus, the radiative heat flux for both sunlight absorption and radiative cooling was simultaneously controlled depending on the temperature of VO2.

DOI： 10.1007/s10765-021-02944-4

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Other Link： https://link.springer.com/article/10.1007/s10765-021-02944-4/fulltext.html

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier {BV}  

Microencapsulated phase change material (MPCM) slurries are energy-carrier fluids. Their latent heat utilization has been investigated by a new method based on the temperature properties of the phase-change fluid, in which match of the working and phase-change temperature ranges is considered. Based on the working-temperature range (3–15 °C) in heat-exchange circulation, the dynamic heat-transfer characteristics and utilization of the MPCM slurry with the core material n-docosane (C22H46) are discussed. The experimental results indicate that the effects of the stirring rate and flow rate on the dynamic heat-transfer characteristics are similar to those of water, and they have little influence on utilization of the 10% MPCM slurry, whereas the corresponding effect of the concentration is relatively large. When investigating the effect of the concentration, it was found that a low phase-change ratio suppressed movement of the phase-change-temperature range owing to the residual solid core material acting as a crystal nucleus, while the heat-transfer performance and latent heat utilization of the MPCM slurry suppressed each other. The 10% MPCM slurry is the most suitable energy carrier fluid, but supercooling still needs to be resolved. The experimental results provide a reference for practical efficient application of MPCM slurries as energy carrier fluids.

DOI： 10.1016/j.icheatmasstransfer.2021.105788

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Authorship：Lead author,　Corresponding author   Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Heat Transfer Society of Japan  

DOI： 10.11368/tse.30.23

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society ({ACS})  

Securing freshwater resources is a global issue for ensuring sustainable development. Fog harvesting is attracting great attention as a method to collect water without any energy input. Previous reports that were inspired by insects and plants have given insights such as the effectiveness of in-plane wettability and structural differences for droplet transport, which might enhance artificial water harvesting efficiency. However, further efforts to transfer droplets while maintaining performance are needed because droplet motion owing to these effects is limited to the in-plane direction. In this study, we report droplet transport between three-dimensional copper wire structures with nanostructured hydrophobic and superhydrophilic features. This mechanism enhanced the fog harvesting capability by more than 20% compared with the cumulative value of individual wires. In addition, the relationship between the droplet height and spacing of wires affected the performance. Our results show the importance of out-of-plane directional droplet transport from the wire surface assisted by differences in wire wettability, which minimizes limiting factors of fog harvesting including clogging and droplet shedding. Furthermore, the proposed arrangement reduces the overall system width compared with that of a two-dimensional arrangement while maintaining the amount of harvested water. These results provide a promising approach to designing large-scale and highly efficient fog harvesters.

DOI： 10.1021/acsami.1c06432

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Publishing type：Research paper (scientific journal)  

© 2020 Elsevier Ltd Near-field radiation transfer between a metal–insulator–metal (MIM)-structured emitter and receiver was investigated through numerical simulation using a finite difference time domain (FDTD) method. Both the emitter and receiver consist of a squared-island-type metal array composed of nickel (Ni), an insulator layer composed of silicon dioxide (SiO2), and a metal substrate composed of nickel (Ni). The emitter and receiver were set up with a vacuum gap of a few hundred nanometers. The results showed that the near-field radiation flux was enhanced by a factor of approximately four, when compared to that between blackbody surfaces. Simultaneously, the enhancement was spectrally controlled over frequencies ranging from 9 × 1014 to 20 × 1014 rad/s (approximating to a wavelength range of 0.9 to 2.1 μm) depending on the size of the squared island. Images of the magnetic field in both the emitter and receiver clarified that the spectrally enhanced radiation flux was caused by the magnetic polariton (MP)) generated in the insulator between the squared-island metal and the substrate metal when the frequency of the MP coincided with that of the near-field radiation. Moreover, the resonance mode between the frequencies of the MP and near-field radiation was predicted by an impedance model that considered the capacitance between the islands in the emitter and receiver, in addition to the conventional circuit of a MIM structure.

DOI： 10.1016/j.applthermaleng.2020.116041

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement. Spectral absorptance of a metal-semiconductor-metal (MSM) thin-multilayer structured thermo-photovoltaic cell was experimentally investigated. A MSM consists of a thin GaSb-semiconductor sandwiched between a top fishnet-type electrode and a flat backside electrode made of gold. A thin GaSb layer was grown on a substrate made of InAs using molecular beam epitaxy, and then all of the InAs substrate was removed using wet etching. The GaSb film was bonded on a surface of gold, which was sputtered on a Si substrate, using a van der Waals bonding method. The top fishnet-type electrode was made using electron beam lithography and a lift-off process. In the case of a 115 nm thick GaSb layer and a square fishnet aperture of a 300 nm × 310 nm size, the spectral absorptance of MSM reached a local peak (95%) at a wavelength of 1.66 µm, which is similar to spectra predicted by numerical simulation. Moreover, the equivalent resonance cavity model and LC circuit model functioned well to indicate the wavelength of several distinct peaks of absorptance.

DOI： 10.1364/OE.410828

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

© 2019 Elsevier Ltd The electromagnetic field around a thermophotovoltaic (TPV) cell made of a metal–semiconductor–metal (MSM) multilayer with a square-grid electrode layer, referred to as a fishnet layer, was numerically calculated using a three-dimensional finite difference time domain method to obtain the absorptance of the MSM multilayer. In this study, gold and gallium antimony assumed to be materials of the metal and semiconductor layer, respectively. Even if there was no top metal layer, several peaks of absorptance were observed because of the optical interference inside the gallium antimony thin film. Here, the peak wavelength shifted to a shorter wavelength region by placing a fishnet layer on the gallium antimony layer. Moreover, the wavelength of the first peak of the optical interference could be adjusted at the active range of wavelength from 0.8 um to 1.8 um for the TPV cell made of gallium antimony for a practical power generation system. As a result, even in a several hundred nanometers thickness of semiconductor, a 60% of radiant energy absorbed could be concentrated in the active range of wavelength that is defined as a spectral efficiency. From the simulation results with an equivalent waveguide model, the shift of the first peak using the fishnet layer could be described using a dispersion relation of the waveguide, which was fulfilled with gallium antimony. Moreover, the superposition of different resonant modes originated from the waveguide-like structure and strip-wired structure affected the spectral absorption of the MSM multilayer when the wall of the fishnet structure was expanded.

DOI： 10.1016/j.ijheatmasstransfer.2019.01.087

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

© Begell House Inc. 2020. Electromagnetic fields around metal-semiconductor-metal (MSM) multilayers with a squared island top layer were calculated numerically, and their spectral absorptances were evaluated. The top and bottom metal layers were set to be gold; however, the intermediate semiconductor layer was set to be gallium antimony (GaSb). With a squared island top gold layer, the first peak shifted to a longer wavelength range. Uncharted second peaks emerged from 1.0 to 2.0 μm when the island width was longer than 300 nm. By observing the distributions of the z-directional electric fields at the wavelength of the absorptance peak, it was clarified that the second peak of absorptance was generated by horizontal directional Fabry-Perot interference inside GaSb layer depending on the width of the island. Moreover, the first peak could be described using the LC equivalent circuit model in the condition with wide distance between two islands. However, it was indicated that the LC circuit model needed a correction about capacitance between the upper and ground layer in the condition with narrow distance between two islands.

DOI： 10.1615/RAD-19.410

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (scientific journal)  

© 2017 Elsevier Ltd This study shows that near-field radiation transfer is spectrally enhanced using nanometer-sized square pillar array structured surfaces, which are faced toward each other with a vacuum gap of a few hundred nanometers. In this case, the emitter exhibits a temperature of 1000 K, while the receiver exhibits a temperature of 300 K. Moreover, the pillar array structured surfaces made of aluminum-doped zinc oxide (AZO) were assumed. The electromagnetic fields inside AZO and in the vacuum gap were calculated using the finite difference time domain numerical simulation method, which uses a spherical emission source with a sinusoidal modulated Gaussian pulse distributed inside the AZO. As a result, there are local maximum radiation fluxes at the fundamental frequency originating from the Fabry–Pèrot interference between the fundamental wavelength and the pillar height, at its second and third harmonic frequencies, and at the asymptote frequency of the surface wave relating to the plasma frequency. The most striking feature is that the radiation flux at the fundamental frequency becomes more than 30 times higher than that for the far-field blackbody radiation transfer by using a pillar width and a channel width of 80 nm, because the electromagnetic energy emitted spherically is collimated in the direction from the emitter to the receiver.

DOI： 10.1016/j.ijheatmasstransfer.2017.07.075

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Event date： 2016.10.2 - 2016.10.6

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