Both spatially-averaged kinetic energy K and influx-averaged Nusselt number Nu_influx are numerically investigated concerning the three-dimensional thermal convection in a cubic cavity heated from a bottom wall and chilled from its opposite top wall. Nu_influx represents the total influx of heat normalised by an area. Assuming incompressible fluid with a Prandtl number of 7.1 (water) in a Rayleigh-number range of 1.0×10⁴– 1.0×10⁵, the authors solve the three-dimensional Navier-Stokes equations with the Boussinesq approximation, using the finite difference method. As a result, in the Rayleigh-number range, hysteresis effects appear accompanying various steady flow structures. Hence, there can exist multiple values of K and multiple values of Nu_influx for the same Ra due to the different steady flow structures. As Rayleigh number gradually increases or decreases, there exist four stable branches. On the branches, the authors reveal the relation between K and flow structure and the relation between Nu_influx and flow structure. Besides, a steady flow structure becomes oscillatory on one branch, as Rayleigh number gradually increases.

A conventional metal–insulator nanograting has the potential to transmit near-infrared thermal radiation because an electromagnetic wave is resonated in the grating structure. Surface plasmon polaritons (SPPs) take place at the interface between the metal and the insulator with boundaries at both ends. Physicists formulated the resonance frequency of the grating from the Fabry–Pérot interference between the grating thickness and the wavelength of SPPs in a short-range coupled mode. On the other hand, engineering researchers often use a lumped-element model assuming a resonant circuit consisting of an inductance of metal and a capacitance of metal-insulator-metal grating structure. Furthermore, they have considered that the resonant circuit excites a strong magnetic field independent of SPPs. This study compares each physical model and numerical simulation results, then clearly shows that all resonance frequencies and features of the circuit resonance can be described by the Fabry–Pérot interference of the SPPs in short-range coupled mode. Moreover, the estimated resonance frequencies obviously correspond to the local maxima of the transmittance of the nanograting with the various thicknesses and pitches. In this case, a strong magnetic field can be observed in the insulator layer as if it might be an isolated magnetic quantum. However, since materials show no magnetism at near-infrared frequencies, the magnetic response appears due to the contribution of SPPs.

The gas-species dependence of permeation flow in solid oxide fuel cell anodes is experimentally and numerically investigated to clarify the Knudsen effect on the gas transport phenomena in porous media. The effective permeability of porous anodes fabricated with pore former is experimentally measured using various working gases with different mean free paths, and the effect of the working gas species on the permeation phenomena is evaluated. A linear relationship is found between the effective permeability and the mean free path of the gas species. It is clarified that the representative pore size of the porous media fabricated with the pore formers considering the permeation phenomena is larger than that quantified by the microstructural analysis. A numerical model based on the dusty-gas model is developed and three-dimensionally visualizes the flow distribution in porous anodes. The 3D simulation shows that permeation gas mainly flows through the larger pores formed by the pore formers. In addition, the molar flux distribution of the permeation flow for the gases is similar, and its absolute flow rate depends on the physical properties of the permeation gas.

A new electric environmental control system for aircraft is investigated to promote the electrification of aircraft, focusing on energy recovery from the exhaust air. In this system, by setting a recovery turbine behind the cabin, the discharged energy can be collected from the exhaust air from the cabin to the exterior. We perform a thermo-dynamic cycle analysis, where the temperature, pressure and entropy are calculated at each position of the cycle by considering the pressure ratios of the compressors as variable parameters. From the results of cycle analysis, we obtain a T − s diagram, appropriate operating conditions for pressure ratios, an energy recovery ratio ∈, a coefficient of performance(COP), and heat transfer in heat exchangers. It is found that ∈ and COP in our system have maximum values at the tip point condition where the pressure ratio γ₂₃ of air cycle machine compressor tends to unity and show better performance compared to the literature, where ∈ ～ 85% and COP ～ 0.965. In addition, it is found that the energy recovery ratio at the condition increases with decreasing altitude.

The lower vapor pressure of n-butane than gaseous fuels, such as hydrogen, methane, and propane, enables its fuel containers to be compact and highly portable. This study investigates the combustion characteristics and performance of a mesoscale annular combustor using highly portable n-butane. The inner and outer tubes of the combustor are fabricated using heat-resistant materials, and the combustion chamber has a volume of 1.84 cm³. The fuel-rich mixture and air are supplied to the combustion chamber through the inner and outer tubes, respectively. The experimental evaluation of the combustor was conducted in a region where the equivalence ratio is leaner than the stoichiometric ratio; this ratio is determined using the fuel and airflow rates provided to the combustor. The temperature of the burned gas at the combustor outlet can be varied from 1000 K to 1900 K. The combustor exhibited robust performance with maximum thermal efficiency, maximum thermal output, and energy density of 0.92, 251 W, and 137 W/cm³, respectively. The maximum pressure drop observed within the combustor (7.6 kPa) was sufficiently lower than the vapor pressure of n-butane, facilitating the supply of n-butane under its vapor pressure. Practically, this combustor can facilitate the development of portable power generators leveraging its efficient and mobile design and can be potentially used in emergency power supply systems and remote operations.

One effective measure to achieve carbon neutrality is to shift away from coal. Wet torrefied biomass (WTB) can be the promising solid biofuels to replace coal, when high moisture content biomass is used as feedstock. To utilize WTB for industrial use, WTB with predetermined energy properties such as higher heating value (HHV) and solid mass yield (SMY) has to be produced to meet the requirement in coal-fired power plants and industrial boilers. In this study, the analytical approach to estimate SMY and HHV of WTB for given wet torrefaction (WT) process parameters, temperature and residence time is investigated for Japanese cedar and rice straw. The Van Krevelen diagram shows that dehydration is considered as a main reaction of WTB, which is the same result as the previous studies on dry torrefied biomass (DTB). HHVs of WTB as well as DTB are well correlated with its SMY, and the experimental correlations of HHV are proposed as a function of SMY. The difference in HHV of rice straw between WTB and DTB and the difference in HHV between Japanese cedar and rice straw are closely related to the difference in ash content and the ash removal effect in WT process. The WT reaction based on volatile release is modeled as the single reaction with only one reaction rate constant of volatiles evolved. It is found that the solid-state reaction function codes F₁₂ and D₁ are the most suitable for Japanese cedar and rice straw, and kinetic parameters in WT process are determined. The estimation method of SMY and HHV is proposed based on the WT kinetic model and the HHV experimental correlation. It is found that the WT temperature and residence time to produce WTB with predetermined SMY and HHV can be provided by the proposed estimation method within an accuracy of about 10%.

In this study, characteristics of the miniature vortex combustion power system were investigated with methane, propane, n-butane and i-butane / air mixtures. The system output and CO emission were examined under various equivalence ratio conditions. The experimental results for thermal input of 600W showed that the output of methane attained 20.0W (11.89V × 1.69A) for stoichiometric condition, which is evidently higher than other fuels, 18.1W for propane, 18.6W for n-butane and 18.0W with i-butane. Furthermore, the difference of the output power among those fuels became larger with decreasing the equibalance ratio, that is, the output power of propane, n-butane and i-butane drastically decreased in fuel lean condition, Φ < 0.9. As the exhaust gas temperature showed monotonical increase with the decrease in Φ, the exhaust energy was found to be increased with the reduction of Φ. However, it was found that further energy was lost by incomplete combustion in fuel lean conditions of propane, n-butane and i-butane. For those fuels, the flame become unstable and large amount of CO was emitted for Φ < 0.9 while the output of 16.0W was maintained for methane even at Φ = 0.8. According to the flame appearance obtained by quartz combustor, it was clearly observed that the flame luminosity around the flame base was weakened for propane and butane in fuel lean conditions. As such weakening was not observed for methane, it is considered that the Lewis number effects may contribute to the weakening of propane and butane flame base, and thus, drastic decrease of the thermal input in fuel lean conditions of propane and butane.

Since hydrogen does not emit carbon dioxide when it is burned, burning hydrogen directly is one option. However, as the hydrogen ratio in the total fuel increases, the visible flame emission becomes weaker. Therefore, when using fuels with hydrogen mixed with city gas, it is necessary to have a system that monitors incomplete combustion to ensure safe use of combustion equipment. In this study, we measured the C₂ and CH chemiluminescence intensities of a laminar premixed flame of hydrogen and methane. These chemiluminescence intensities were evaluated using the ICCD camera, compared with flame images taken by the commercially available digital camera. Especially, we proposed a method to determine the C₂ and CH chemiluminescence intensities simultaneously in terms of the signals composed of three primary colors of red (R), green (G), and blue (B) obtained by the digital camera images. The G signal of the flame obtained by the digital camera is almost proportional to the C₂ chemiluminescence intensity. The B signal also shows a good linear relationship when data are divided into two groups according to the presence or the absence of the outer flame. We concluded that if the C₂ and CH chemiluminescence intensities are predicted from the G signal and the B signal.

Recently, the thermal management of electronic devices has become paramount because the power density of semiconductor elements is rapidly increasing with the rapid miniaturisation of electronic devices. Boiling heat transfer (BHT) is an effective technique to achieve high cooling systems, and micro-structures are expected to enhance the BHT. In this study, copper, a hydrophobic surface, was used as a heat transfer surface, and microstructure was applied to the surface to investigate the effect of hydrophobic microstructured surfaces on the enhancement of BHT, and the effects of changing flow velocity from laminar (Reynolds number = 1,220) to the turbulent (Reynolds number = 6,640) were investigated. Comparing the boiling curves of smooth and microstructured surfaces, we confirmed that the heat flux on the microstructured surface was higher than that on the smooth surface at the same superheating, attributable to the increase in the number of bubble points caused by the cavities of the microstructured surface, which facilitated heat transfer. For the effect of changing the flow velocity, the critical heat flux (CHF) increased with the flow velocity for the smooth surface, but no significant improvement was observed for the microstructured surface. The prediction of the relationship between heat flux and bubble population density was found to be reasonable because the difference between the experimental values and the theoretical values obtained from a well-known prediction equation was small. Using this equation to predict the bubble population density on the microstructured surfaces, it was estimated that a small fraction of the number of cavities was activated, even near the CHF condition. In the future, it will be necessary to investigate the cause of the small number of activated cavities of the microstructured surfaces in detail.

Thermal management using solid–solid phase change materials (PCMs) is gaining attention as a viable technology for improving the reliability of smart devices, such as smartphones and tablets. This technology relies on the latent heat of PCMs to level temperature fluctuations in electronic chips, does not require additional power, and can be miniaturized. The temperature-leveling performance of thermal management devices based on solid–solid PCMs depends on the thermophysical properties and thickness of the PCM and the generated heat density of the heat source. However, these factors complicate the comparison of PCM performances. Therefore, clarifying the relationship between these factors and temperature-leveling performance of solid–solid PCMs and defining a performance factor are necessary for material development. In this study, we defined an evaluation index of PCMs suitable for thin smart devices. The temperature-leveling performances of VO₂ and NiTi alloys, which are typical inorganic solid–solid PCMs, were compared to define the performance factor. Thermal simulations were performed to define and calculate the optimum PCM thickness that maximized the temperature-leveling performance and cost-effectiveness under various heat densities generated by a heat source. We defined the maximum effective energy capacity (E_eff) as the temperature-leveling performance factor, calculated using the optimal thickness and volumetric latent heat. PCMs with high E_eff are promising for thin smart devices. The simulation results indicated that E_eff of NiTi was 1.46–1.50 times higher than that of VO₂ owing to the high thermal conductivity of NiTi. The simulation and experimental results were compared to validate the proposed thermal simulation model.