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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. [ABSTRACT FROM AUTHOR]

• Functional multilayer with VO 2 was proposed for passive emissivity control. • Suitable geometric parameters were determined using Bayesian optimization. • Bayesian optimization saved computational cost less than a few percent. • Radiative cooling was promoted with two isolated metallic VO 2 slabs. • Insulating VO 2 suppressed radiative cooling performance at cold environment. A vanadium dioxide (VO 2) film grown on a titanium oxide crystal shows a metal–insulator transition at room temperature with drastically changed optical properties. A multilayered slab with a sub-micron scale VO 2 film was proposed to utilize its unique properties for passive intensity control of sunlight absorption and radiative cooling. Its optimal geometries were numerically explored using the Bayesian optimization (BO) method. BO was applied for three types of multilayered slabs, those having one, two, or three isolated slabs of different widths. For each type of multilayered slab, BO could optimize geometric variables with practical calculation times considering the total number of possible combinations of variables, which is subsequently referred to as the total number of candidates. Optimization results revealed that two isolated slabs had the most suitable spectral absorptivity in both hot and cold environments. The infrared absorptivity of the double slab was kept low in cold conditions to suppress radiative cooling. However, the double slab exhibited good radiative cooling performance under hot conditions. Electromagnetic energy density surrounding the slab illustrated that metallic VO 2 and gold placed in a parallel manner excited the coupled mode of surface plasmon polaritons to enhance absorptivity. Radiative cooling faded for the triple slab because each slab could couple with radiation propagating only across a portion of the cross-sectional area. Through three BO trials, improvement of the VO 2 visible reflectivity was recognized as a future issue for further development of passive sunlight absorption control. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Highlights • Electromagnetic fields around metal–semiconductor–metal multilayers were evaluated. • Spectral absorptances of multilayers were modulated by fishnet-structured layer. • The spectral absorptance reach unity at the wavelength of active range of GaSb. • High reflectance was maintained for mid- or far-infrared radiation. • The fishnet Au layer affected the optical interference phenomena inside GaSb layer. Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We experimentally investigate the water/ice mixture- and freezing-front behavior in a water liquid bridge under isothermal and non-isothermal conditions. We find rapid propagation, temporary suspension, and regression of the water/ice mixture front, and finally, it merges with the freezing front when part of the liquid bridge is higher than the freezing temperature. However, freezing-front propagation follows dendritic ice formation, and a protrusion forms at the middle of the liquid bridge as long as the whole liquid bridge is lower than the freezing temperature. We explain those phenomena by quasi-stationary heat-transfer considerations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A concept is proposed for spray-heat exchangers with the aim of addressing the mismatch of heat supply-demand within and outside a water droplet. The concept involves using a microencapsulated phase-change-material (MPCM) slurry as the working medium, enabling two-phase-change heat transfer with air. The temperature of the water droplet rapidly decreases owing to continuous evaporation, leading to a rapid decrease in its heat-transfer capacity. The phase change of the MPCM particles not only provides sufficient heat for evaporation but also maintains a high surface temperature, thus sustaining effective evaporation. A numerical model was developed to theoretically investigate the effect of the MPCM phase change on external convection. The heat-mass transfer of a droplet with a radius of 0.25 mm composed of water and 20 vol% MPCM was simulated. The results indicate that the MPCM phase change effectively reduces the temperature decrease rate. However, when the overall surface temperature of the MPCM droplet exceeds that of water, the total heat-transfer coefficient is not necessarily higher. Further local analysis reveals that maintaining both an overall high surface temperature and a reasonable local temperature distribution is crucial to achieve a higher total heat-transfer coefficient. The MPCM phase change significantly influences the local surface-temperature distribution. • A two-phase-change concept of heat transfer for a droplet and air is proposed. • A heat and mass transfer numerical model for a single droplet in air was developed. • The effect of the phase change on external convection was analyzed. • The surface-temperature distribution affects the local Nu and Sh downstream. • The phase change has a crucial effect on the surface-temperature distribution. [ABSTRACT FROM AUTHOR]

• MPCM slurry viscosity and thermal conductivity affect its heat transfer. • Spraying MPCM slurry weakens the above-mentioned adverse effects. • The working and phase change temperature ranges of the used MPCM slurry are matched. • Faster cooling yields more supercooling and a wider phase change temperature range. • Heat–mass transfer is better for MPCM slurry than for sprayed pure water. 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 (C 22 H 46). 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. [ABSTRACT FROM AUTHOR]

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 (C 22 H 46) 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. • Matching analysis of the working and phase-change temperature ranges used to evaluate MPCM slurry utilization. • Apparent specific heat capacity of the MPCM slurry measured in static and dynamic tests. • Heat-exchange circulation used to obtain the dynamic heat-transfer characteristics. • MPCM slurry's latent-heat utilization and heat-transfer coefficient are mutually restricted. • Residual solid core material is conducive to eliminating supercooling. [ABSTRACT FROM AUTHOR]