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.

Diesel-ignited methanol direct injection engine can achieve high methanol substitution rate and low emissions. The effects of pilot diesel injector specifications, including hole number and installation location, on the combustion and emissions of a diesel-methanol dual-fuel direct injection engine were investigated in this study. The results show that increasing the number of diesel injection holes reduces the proportion of methanol premixed combustion, which is beneficial to reducing ringing intensity (RI) and reducing pollutant emissions. Different pilot injector types and installation positions have a significant impact on the combustion mode of methanol, but have little impact on fuel economy. The equivalent indicated specific fuel consumption (EISFC) is below 165g/kWh in all cases. When the diesel ignition area deviates from the center of the cylinder, methanol mainly undergoes premixed combustion, and NO_X, HC, and CO emissions increase significantly. When the diesel combustion area is located in the center of the cylinder, methanol mainly undergoes diffusion combustion, with lower RI and less pollutant emissions.

In recent years, environmental problems such as global warming and air pollution have been recognized more seriously. Soot in the combustion products is one of the causes of air pollution. Hydrogen is now attracting attention as an alternative fuel to fossil fuels because it can be carbon-neutral fuel synthesized from renewable energy sources. If hydrogen is mixed with hydrocarbon fuels, it is possible to reduce CO₂ emission. In this study, we investigated the effect of hydrogen addition on a coaxial jet diffusion flame. We used methane and propane as main fuels, and soot region was visualized using a laser induced incandescence (LII) method. In experiments, two-dimensional distribution of the soot concentration was obtained, and the maximum soot volume fraction and the total soot volume were evaluated from the LII signal. In order to clarify the effect of hydrogen addition, nitrogen was also added to the main fuel, and both results were compared.

One-dimensional numerical simulations of an unstretched propagating hydrogen/air premixed flame and two-dimensional numerical simulations of outwardly propagating hydrogen/air laminar premixed flames are performed using a flamelet-generated manifold (FGM) method. The effects of considering preferential diffusion and flame stretch in the FGM method are investigated in detail. The results of the one-dimensional numerical simulations show that the flame behavior can be qualitatively reproduced without considering both these two phenomena, but it can be predicted more precisely by considering preferential diffusion. On the other hand, the results of the two-dimensional numerical simulations show that considering both these two phenomena is essential to accurately predict the behavior of stretched hydrogen/air premixed flames at an equivalence ratio of less or more than 0.7. The results also show that the consideration of preferential diffusion and flame stretch is less important at an equivalence ratio of around 0.7. In addition, the validity of an assumption that the Lewis numbers of each species are constant, which is used in most previous studies, is investigated. The results show that assuming constant Lewis numbers worsens the prediction accuracy. The computational cost of the FGM method considering both preferential diffusion and flame stretch is about 1/15 that of the detailed calculation for the two-dimensional numerical simulations, which indicates the efficiency of the extended FGM method.

The present study focuses on the computation of the distribution of local solid–liquid interfacial thermal resistances (ITRs) at a solid–liquid interface with a nanostructured surface, at a spatial resolution of 1.96 10^-1nm based on the non-equilibrium molecular dynamics method. As a calculation parameter, three different interaction strengths between the solid atoms and the liquid molecules were employed to reproduce the hydrophobic and hydrophilic conditions. In our calculation system, liquid molecules occupy the gap between the sidewalls of the nanostructure. We showed that the combined interfacial thermal resistance calculated from the local ITRs agrees with the overall ITR. We investigated the spatial distribution of the local ITRs via spectral analysis. The results showed that the local ITRs increased at the bottom corners and decreased at the top corners of the nanostructure. When the interaction parameter between the solid atoms and the liquid molecules is large, we find evidence of adsorption of liquid molecules on the solid, which causes fluctuations of the local ITR. The local vibrational states of the solid atoms and liquid molecules varied at each local interface. The local ITRs were negatively correlated with the overlaps of these vibrational densities of states, implying that each local vibrational state is one of the factors that determines the corresponding local ITR. In addition, the peak frequencies of the local spectral heat flux agreed with those of the vibrational density of states. These results indicate that the vibrational states of the solid atoms are dominant factors in the vibrational properties of the thermal transport across the local solid–liquid interfaces.

Cryogenic heat exchangers are important for vaporizing liquefied natural gas or liquefied hydrogen and utilizing their cryogenic energy, but frost formation causes severe problems that deteriorate performance. Frost formation on a cryogenic surface is characterized by simultaneous mist formation. However, the detailed frosting mechanisms, especially the interaction between frost and mist formation under forced convection, have not been revealed. In this study, we experimentally observed the frost and mist formation on a flat plate under forced convection over a wide range of cooling surface temperatures (T_w), from general-low to cryogenic. The frost and mist layer height was quantitatively measured with a white LED and a laser sheet light source in addition to the frosting amount. As a result, the frosting style was classified into three groups depending on the cooling surface temperature under the air temperature of 27°C, air humidity of 12 g/m³, and air velocity of 2.0 m/s. When T_w ≥ −75°C, the mist was hardly generated, and needle-shape or dendric frost formed by desublimation almost uniformly on the entire plate surface. On the other hand, frost is not uniformly formed in the flow direction at T_w ≤ −100°C. Frost hill is formed by desublimation at the leading edge and protrudes higher as the cooling surface temperature is lower. The high frost hill leads to the flow detachment behind it and the persistent mist generation, affecting the frost growth on the rear. Thus, under forced convection, the frost hill shape and the flow detachment behind it are important factors determining the frosting style.

The backdraft dynamics are numerically investigated using a proposed combustion model that attempts to accurately simulate the multi-combustion regime with a fast flame propagation in the inhomogeneous thermal condition. This scenario is often found in the limited-ventilation fires. The modeling strategy includes the flame-front tracking by the level-set function and the solution of non-adiabatic flamelet and a stratification parameter. In a comparison to the previous works, the present approach shows a better agreement with experiments. The results also underline the important effect of gravity current that promotes the rapid flame evolution in the case with less-dense initial mixture. Four different phases are identified based on the transient characteristics of backdraft, and they are (a) initial ignition, (b) free propagation, (c) limited-front propagation and (d) accumulation & flame-ejection. The dominant driving forces of buoyancy convection and turbulence effect on burning velocity are revealed. The backdraft process is found to encompass the initial premixed flame propagation and the diffusion burning outside the chamber following the formation of fireball. This work gains the valuable insights on the backdraft dynamics and provides useful information to design effective protection measures.

CO₂ biomass gasification is an attractive option for reducing CO₂ emission which produces gas mixtures with higher CO concentrations than typical syngas compositions. However, carbon deposition and catalyst degradation may occur when CO-rich fuel gas is supplied to a Ni-Yttria-stabilized Zirconia solid oxide-based fuel cell. In this study, the electrochemical performance of an SOFC (anode: NiO-YSZ/NiO-GDC) with various CO-rich gas compositions was experimentally investigated. Four distinct gas mixtures were tested using electrochemical experiments at 1073 K. Impedance and steady-state polarization measurements were taken, and electric power generation was assessed at 100 mA·cm^-2 under the 4 gas mixtures. Gas 1, a mixture of H₂ (38%), CO (19%), CO₂ (9.5%), and H₂O (5%) was prepared as a reference gas to model a typical syngas mixture obtained from biomass gasifier. It generated the highest voltage without any degradation of the anode over 8 h. Gas 2, containing CO (19%), CO₂ (9.5%), and H₂O (5%), caused serious anode degradation compared with Gas 1. Gas 3, containing CO (19%) and H₂O (5%) showed good performance with a relatively small polarization resistance and good stability in long-term electric power generation test. Gas 4, consisting of CO (20%) and CO₂ (10%), exhibited the largest charge-transfer resistance at low overpotential and a significant diffusion resistance at high overpotential. Additionally, Gas 4 had the worst stability in the electric power generation test. Based on these results, we conclude that biomass gasification gas is a promising fuel for use in SOFCs. CO direct oxidation can provide electric power generation, although it appears to be unstable and shows high resistance at both low and high overpotentials. H₂O addition can efficiently improve the stability of the CO direct oxidation process.