In this study, based on the CO conversion rate obtained with a monolith honeycomb catalyst and adsorbed surface species identified with in-situ FTIR for powdered catalyst, detailed CO oxidation surface reaction mechanisms on Pd/Al₂O₃ have been developed. As a result, it is found that CO absorbs onto Pd by linear and bridge regime, which is not detected for Pt and Rh in previous work. Based on those results, thermodynamically consistent detailed surface reaction mechanism for CO/O₂ reaction on Pd is developed. Further, combining the CO / O₂ surface reaction mechanism for Pd with previously developed mechanisms for Pt and Rh, CO conversion rate on bimetal catalysts of Pt/Rh, and furthermore, tri-metal catalyst of Pt/Pd/Rh are simulated and compared with experimental results for several PGM ratios. Results of numerical simulation for various Pt/Pd/Rh ratios as well as CO/O₂ ratios quantitatively agreed with experimental results.

This study investigated the influences of a small obstacle upon an unstable detonation front using the smoked-foil technique. In the present experiments, a stoichiometric propane–oxygen diluted with nitrogen mixture at initial pressure of 30 or 70 kPa was used as an explosive gas mixture and four types of obstacles were used: forward-facing steps and slopes and backward-facing steps and slopes. We used the propane-fuel gas mixture, because its non-dimensional activation energy is larger than that of the hydrogen-fuel gas mixture used in our previous study. In the cases of forward-facing steps and slopes, the detonation front structure was not significantly affected similarly to the previous study of hydrogen–oxygen diluted with argon mixture. The shock-tube model shows that the reflected shock wave from the forward-facing step does not drive a transverse wave obviously stronger than the intrinsic transverse wave. On the other hand, the detonation re-activation phenomena were observed in the vicinity of the sidewall downstream of the backward-facing steps and slopes. Although the cellular pattern was similar to the case of hydrogen gas mixture as a whole, the transverse waves were more attenuated behind the diffracted shock wave in the propane gas mixture. Moreover, the detonation re-activation also occurred in a far region from the sidewall in this study. However, noteworthy was that the distance between the backward-facing step and the re-activation position z_ra was expressed by the height of the step |h| and the CJ detonation cell width λ_CJ as z_ra/λ_CJ = 2.9(|h|/λ_CJ)^0.84 and this empirical formula well describes also the results of hydrogen gas mixture and many other mixtures, showing that the non-dimensional activation energy has little effect on the position where the detonation re-activation occurs. This implies that the detonation re-activation on the sidewall is predominantly governed by the transition from the regular reflection to the Mach reflection on the sidewall of the diffracted shock wave decoupled with chemical reaction through the slip-line formation, where the degree of the decay of the diffracted shock wave is influenced not by the stability of the detonation-front cellular structure but by λ_CJ, representing the induction-zone length between the leading shock wave and the subsequent exothermic reaction.

The shape, size, and performance of microfluidic chip heaters are designed to fit the housing of microfluidic chips and channels. During the heating process, some of the heat is transferred to the surrounding environment where the microfluidic chip is located, which is typically undesirable. In order to better control the heating of the fluid within the microfluidic channel, this study examines a heating wire placed coaxially inside the channel. The fluid flows through the annular space between the channel and the heating wire, which maintaining a constant heat flux and minimizing heat transfer to the environment. To provide a basis for comparison, an analysis of fluid flow within a straight channel chip with a heat source of constant temperature is also conducted. The methodology used combines analytical modeling with experimental testing. Thermal entropy and entransy flow rate during fluid flow within the microfluidic channel are studied in relation to changes in volumetric flow rate, the temperature of the microfluidic chip housing, and the heat transfer from the heating wire. Additionally, a modified irreversibility ratio is used to further describe the relationship between thermal entropy and entransy flow rate. The results show that the fluid heated in the annular microfluidic channel exhibits lower thermal entropy and higher entransy flow rate compared to the straight channel chip.

In this study, a microscopic visualization of the pyrolysis of woody biomass is conducted using the BL20B2 beamline at SPring-8, a large synchrotron radiation facility. Changes in the shape and internal structure of the woody biomass are visualized using ultra-high-speed X-ray computed tomography (CT). The specimens are Japanese cypress, ramin, and bamboo with a height of 5 mm and diameter of 5 mm. Infrared halogen heater is used as the heat source to achieve a high heat flux. The specimens expand under a high heat flux. Compared with the experimental results under a low heat flux obtained in a previous study (Daitoku, 2017), a completely different aspect is observed. In a nitrogen atmosphere, the internal structure of the specimens during transient pyrolysis are visualized using ultra-high-speed X-ray CT.

One of effective paths to achieve net zero energy-related CO₂ emissions by 2050 is to shift away from coal. A reforming technology that combines torrefaction treatment with densification molding is considered to be the promising production method for solid biofuels to replace coal and coal coke. In the study, an optimum combination condition of wet torrefaction (WT) and densification molding is investigated to improve energy properties of solid biofuels such as higher heating value (HHV) and char yield (CY). HHV of wet torrefied samples is correlated with mass ratio of biomass to water (B/W) as well as solid mass yield (SMY), and HHV for B/W =1/20 is 3 to 4% lower than that for B/W =1/5 for the same SMY. From the proposed model to estimate HHV of wet torrefied sample, it is found that the change in fixed carbon content due to B/W results in the change in HHV. Particle density of densified biofuel with wet torrefied sample (WTB-fuel) in the SMY range above 0.6 is equal or higher than that with raw sample. But the advantage of densification molding of wet torrefied sample is not observed in the SMY range lower 0.6. For the same WT condition, the increase in CY due to densification molding is closely related to the suppression of volatile matter generation, which is confirmed by the increase in activation energy. From the investigation results on both effects of wet torrefaction and densification molding on improvement of char yield of WTB-fuel, it is found that enhancement factor of fixed carbon (EFC) for any molding temperature reaches a maximum at SMY of around 0.8, and the maximum EFC is obtained at molding temperature of 200 ℃. Therefore, it is concluded that the optimum combination conditions to produce WTB-fuel are wet torrefaction in the SMY range between 0.7 and 0.8 at higher B/W and densification molding at molding temperature of 200 ℃.

A significant portion of industrial energy is lost as low-grade thermal waste, typically at temperatures below 230°C. Efficient recovery of this waste heat remains a major challenge, and solid-state technologies such as thermomagnetic motors have emerged as promising solutions for energy harvesting. These motors operate based on the temperature-dependent magnetic phase transition of magnetocaloric materials near their Curie temperature (T_C), allowing mechanical motion to be generated from thermal gradients. This study investigates the dynamics of a thermomagnetic motor composed of a pair of magnetocaloric heat exchangers (MHEs) operating in opposite phases. Computational simulation, supported by experimental force and temperature data, was developed to analyze the influence of three key parameters: the distance between MHEs (d_MHE), the half-cycle period (p), and the suspended weight (W). A maximum net power output of P = 1.43 W was obtained for the configuration d_MHE = 31 mm, W = 18 N, and p = 2.6 s. The results provide insights into the impact of these parameters on the motion and power output of the motor, revealing the interplay between magnetic forces, thermal gradients, and mechanical response. Understanding these effects is crucial for optimizing thermomagnetic energy harvesting systems, enhancing their efficiency and applicability in waste heat recovery. The findings contribute to the ongoing development of thermomagnetic motors for converting low-grade thermal energy into mechanical or electrical power.

With the increasing power density of modern electronic devices, efficient thermal management has become a critical challenge. This study investigates the highly efficient heat transfer phenomenon observed when the working fluid filling ratio in a pulsating heat pipe (PHP) is extremely low. The authors previously introduced this phenomenon as a novel heat pipe concept, termed the meander-shaped low-fill heat pipe (MLFHP). The device comprises a flat aluminum tube, 400 mm in length, with 28 straight channels, each having a square cross-section of 1.26 mm per side. In this study, the heat transfer characteristics of the heating and cooling sections were evaluated to understand this unique phenomenon. The working principle of the MLFHP differs from that of the conventional PHP, in that the MLFHP relies predominantly on latent heat transfer through stable phase change, with minimal oscillatory motion, especially in the cooling section. This distinction is essential in interpreting the observed thermal performance. Additionally, experiments were conducted using working fluids other than water, including alcohol-based fluids and refrigerants, to investigate the effect of different working fluids on the device performance. The results confirmed that the MLFHP operated effectively with these alternative fluids, achieving a minimum thermal resistance of 0.041 K/W in the 400-mm device. These findings suggest that MLFHPs offer a promising solution for next-generation thermal management systems for compact and high-performance electronic applications.

Frost formation on cold surfaces significantly impacts heat exchanger performance by increasing thermal resistance and reducing airflow efficiency. Understanding the microstructure of the frost layer during its growth phase is essential for predicting and mitigating its effects. This study investigates the frost layer formation process using X-ray µCT at SPring-8. Experiments were conducted on a 6 mm-diameter aluminum cooling surface, capturing the frost microstructure every two minutes over a 90 minute period. In the early stage of the frost formation, the crystal morphology was influenced by the size of ice droplets: larger droplets predominantly generated columnar crystals, while smaller droplets formed plate-like crystals. During the frost growth stage, the frost layer developed into three distinct layers: an ice droplet layer, an inner layer composed of plate-like crystals, and ice crystals in outer layer connected through columnar crystals. The heat transfer load in the outer layer caused the sublimation of columnar crystals connecting the outer layer to the ice droplet layer, thereby altering the frost layer structure. The frost density distribution in the thickness direction was evaluated. The density increase in the outer layer was related to frost layer growth, while that in the inner layer contributed to an increase in frost density. The respective mass fluxes were successfully quantified.

This study investigates the thermo-hydraulic performance of shell-and-tube heat exchangers with advanced combined and twisted tube bundle configurations. Numerical simulations were conducted using COMSOL Multiphysics. employing a k-ε turbulence model to analyze heat transfer and fluid flow at various mass flow rates. The numerical results were validated against experimental data from the literature, confirming the accuracy of the model. The twisted configuration, featuring elliptical tubes on the periphery and circular tubes at the core with a 45° twist angle, demonstrated enhanced performance by intensifying turbulence and promoting secondary flow structures. This design achieved an improvement of 12.68% to 16.59% in the overall heat transfer coefficient compared to conventional circular tube bundles, outperforming both smooth combined tube designs and configurations proposed by Saffarian et al. (2019). Although this twisted design resulted in a 31.40% increase in pressure drop compared to the smooth combined configuration, it still maintained a 22.08% lower pressure drop relative to the Saffarian et al. setup, indicating a favorable balance between thermal performance and hydraulic penalties. The STHE_TC configuration exhibited significant heat transfer enhancement by increasing fluid residence time and convection, while the STHE_SC configuration provided a balanced compromise between thermal and hydraulic efficiency. Both designs have the potential to reduce heat exchanger size while maintaining equivalent thermal capacities, promoting compactness and lowering installation costs. This work underscores the promise of combined and twisted tube bundles as compact energy-efficient solutions for industrial applications and paves the way for future exploration of advanced geometries and materials in thermal system optimization.

A new efficient numerical approach for Reynold-averaged Navier–Stokes (RANS) simulation coupled with the discrete element method (DEM) is developed for the calcination of limestone flowing down in a real kiln, considering the combustion reaction of practical fuel. The validity of the method is assessed by comparison with experiments. In addition, the calcination process in a real kiln is investigated in detail by injecting the limestone with a particle size distribution that reflects actual conditions, while also considering chemical reactions occurring inside the kiln, including the combustion flow field formed by two spray burners and surface reactions on limestone particles. The results show that the calcination of limestone in a real kiln is successfully reproduced by the RANS/DEM, which is developed under the assumption that air turbulence does not significantly affect the movement of limestone, and therefore enables the analysis of the long residence time in the real kiln. The limestone with a small diameter of 5 mm is fully calcinated more than 10 m upstream from the kiln outlet, while larger limestones with a diameter of 35 mm are discharged without complete calcination, even upon reaching the outlet. This suggests that the proper control of the limestone size leads to the optimal design of real kilns.

This review paper summarizes the research for which the author has received the Nukiyama Memorial Award in 2022. The key essence of the research is “materials informatics (MI) for heat transfer” entailing the combination of nanoscale thermal science and data science. Various MI studies carried out at the Thermal Energy Engineering Lab with the collaborators on nanostructure design for heat conduction and thermal radiation are introduced. The MI method coupling heat transfer calculations and black-box optimization exhibits high efficiency to optimize nanostructures for target heat transfer properties. The method enabled the computational design and experimental realization of aperiodic superlattices that optimally impede coherent thermal transport and multilayer metamaterials with wavelength-selective thermal radiation.