The present study models the flow configuration in a tube downstream of multi-branching in a catalytic converter. Convective heat transfer in a thin circular straight pipe is analyzed by direct numerical simulation (DNS). Forced transverse vibration is applied to the pipe itself to enhance the heat transfer. The main computational domain is sufficiently long to capture the spatial development of the vibration effect with the inflow-outflow boundary condition. The representative Reynolds number is set to be less than that in a driver domain which generates the turbulent inflow of the main domain. Profiles of the Nusselt number exhibit that the positive effect of the vibration on the heat transfer becomes apparent at certain downstream locations. The distance to its location is short with the high vibration frequency keeping the amplitude constant. In the region where the heat transfer is enhanced, the main flow with high temperature is shifted to one side and the other side of the pipe wall in the vibration direction alternatingly with the appearance of the organized streamwise vortices. The flow structure in the present spatially developing configuration is governed mainly by the vibration frequency although the terminal heat transfer performance is approximately a function of the velocity amplitude of the vibration.

Direct numerical simulations of turbulent Taylor-Couette flows are performed to investigate the effect of a traveling wave control on torque and heat transfer. In the Taylor-Couette flow, inner and outer cylinders are rotating and immobile, respectively, and the temperature difference between cylinder walls is maintained as constant. The ratio between the inner and outer cylinder is 0.882, and the Reynolds number is set as 84,000. A traveling wave-like blowing and suction is imposed on an inner cylinder wall. A parametric study shows the effect of control parameters on torque and heat transfer. We focused on three characteristic parameter sets: heat transfer enhancement, relaminarization phenomenon, and simultaneous achievement of torque reduction and heat transfer enhancement. We employed identity equations by using three-component decomposition to clarify contributions from advection, turbulence, and diffusion on torque and Stanton number. The results indicated that the traveling wave control affects the turbulence and advection contributions.

The study of microbubble emission boiling (MEB) is gaining popularity because it violently emits several microbubbles during subcooled pool boiling, for which the heat flux is greater than the critical heat flux (CHF). Although the occurrence of MEB and the heat transfer mechanisms have been analyzed, several aspects of this phenomenon are unknown. In this study, the behavior of coalesced bubbles that form above the primary bubbles on a horizontal heat transfer surface are focused. Experimental observations of boiling behavior, including the transition process from nucleate boiling to MEB, were conducted under the subcooling condition of 40 °C. The transition from nucleate boiling to MEB was found to occur below the CHF when the boiling space was limited to a height lower than that of the coalescent bubbles. Furthermore, the following two types of transitions were observed, depending on the heat flux considering the restriction of the boiling space: from nucleate boiling to MEB, or from nucleate boiling to film boiling. The restriction of boiling space reduces the condensation rate of coalesced bubbles and increases partial dryout. The surface dryout has two possibilities: promoting the transition to film boiling and reducing the vapor supply to the coalesced bubbles. The latter results in a shrinkage of the coalesced bubbles which induces the direct contact of subcooled liquid to the heat transfer surface. The high heat transfer rate in MEB may be attributed to the direct contact between the solid and liquid, along with microlayer evaporation.

Boiling heat transfer has a combination of sensible heat transfer of liquid and latent heat transfer due to vaporization. To examine the sensible heat transfer in boiling, thermometry of liquid in liquid-vapor multiphase flow must play a significant role. Although there are several optical methods proposed for the thermometry of boiling phenomena, it is challenging to directly measure the temperature field of boiling at relatively high heat flux due to many boiling bubbles' interruption of the illumination and observation. This study proposes a novel thermometry method using a confined space, a sandwiched space between two transparency plates, and two-color laser induced fluorescence thermometry to measure the liquid temperature distribution around multiple boiling bubbles. The confined space restricted the fluid motion to make it possible to illuminate and observe the almost whole area of interest. The intensity ratio of the two kinds of fluorescent dye exhibits the local and temporal temperature without any invasion of physical probes. We successfully observed the scavenging of superheated liquid from the heat transfer surface to demonstrate this method's utility. The temporal temperature changes at several positions extracted from experimental data with this method were consistent with the boiling bubble behavior. We also discussed remained issues on the method.

The freezing of water droplets deposited or impinged on cold surfaces causes many problems in traffic lights, power transmission wires and heat exchangers. Thus, the suppression of the freezing of these droplets is very important. In this study, we carried out two-dimensional numerical simulation on the freezing of the water droplets on a horizontal cold surface using a Phase-field method. A new equation was developed to predict an increase in the total volume of the droplets due to the freezing. The changes in the physical properties of supercooled water with temperature were taken into account for an impinged droplet. The computational results for deposited droplets showed that a projection was formed on the cap of a frozen droplet and that the volume of ice was 8.8% higher than the volume of a deposited water droplet in its unfrozen state. However, the change in the mass due to the freezing was less than 0.001%. Also, a concave shape of the freezing front was predicted after an ice layer was formed. This was consistent with the results for freezing droplets observed by other researchers. Moreover, similar results were obtained in the case of impinged droplets. With these, a projection was formed. The volume of ice was 8.1% higher than the volume of the impinging droplet in the air, while the change in the mass was less than 0.0023%. The predicted freezing fronts were similar to that observed in previous studies.

The increasing demands of computational power have accelerated the development of 3D circuits in the semiconductor industry. To resolve the accompanying thermal issues, two-phase microchannel heat exchangers using have emerged as one of the promising solutions for cooling purposes. However, the direct boiling in microchannels and rapid bubble growth give rise to highly unstable heat flux on the channel walls. In this regard, it is hence desired to control the supply of vapor bubbles for the elimination of the instability. In this research, we investigate a controllable bubble generation technique, which is capable of periodically producing bubble seeds at the sub-micron scale. These nanobubbles were generated in a solid-state nanopore filled with a highly concentrated electrolyte solution. As an external electric field was applied, the localized Joule heating inside the nanopore initiated the homogeneous bubble nucleation. The bubble dynamics was analyzed by measuring the ionic current variation through the nanopore during the bubble nucleation and growth. Meanwhile, we theoretically examined the bubble growth and collapse inside the nanopore by a moving boundary model. In both approaches, we demonstrated that by altering the pore size, the available sensible heat for the bubble growth can be manipulated, thereby offering the controllability of the bubble size. This unique characteristic renders nanopores suitable as a nanobubble emitter for microchannel heat exchangers, paving the way for the next generation microelectronic cooling applications.

In film cooling approaches, a film of liquid fuel is formed on the chamber wall of bipropellant thrusters to protect the chamber wall from high-temperature combustion gases. To optimize the amount of liquid fuel required to sufficiently cool a chamber wall in this manner without degrading the performance of bipropellant thrusters, the formation process of the liquid film needs to be understood. To this end, in this study, factors affecting the spread of liquid film were experimentally investigated. In particular, experimental apparatus that could reproduce the state of a liquid jet being injected onto a wall to form a liquid film was developed. Water was used as the test liquid because hydrazine-derivative fuels, which are generally used in bipropellant thrusters, are toxic, and the density and surface tension of water are similar to those of hydrazine. The liquid film formation processes were visualized and analyzed by using a still camera. Results indicated that the liquid jet velocity, nozzle diameter, and impingement angle were the key factors affecting the film width, and the maximum film width exhibited a linear relation to the liquid jet velocity component perpendicular to the wall. Considering these results, a general relationship between the key factors and maximum film width was identified, and it was noted that the dimensionless maximum film width could be defined as the product of the Weber number and sine value of the impingement angle. In this manner, the maximum film width can be predicted when deciding injection conditions, which can assist thruster designers during the design process.

Silicon carbide (SiC) is a power semiconductor used to supply and control the electric power source. Top-Seeded Solution Growth (TSSG) method is a promising technique for producing high-quality SiC single crystals. In order to achieve a high- and uniform-growth rate in this growth technique, however, the complex fluid flow developing in the growth melt/solution, mainly induced by the electromagnetic field of the induction-heating coils, free surface tension gradient, and buoyancy, must be well-controlled. Our previous studies have shown that the applications of a static magnetic field and seed rotation are effective in controlling the components of this melt flow and the associated control parameters were optimized effectively using the Bayesian optimization. In this study, we analyze the optimal state determined by the Bayesian optimization in more detail and it is found that the separation of the Marangoni flow near the seed edge leads to a non-uniform growth rate. In addition, the most sensitive region of the melt flow is determined by using an explainable machine learning technique based on a convolutional neural network and the sensitivity map obtained by SmoothGrad. This machine learning technique automatically predicts the preferred melt flow pattern that would lead to high-quality crystal growth. The interpretations by the explainable machine learning technique used in the present study are consistent with those of previous studies carried out on the optimization of the TSSG method.

To yield an effective control to impede the progress of combustion oscillation and to lead to the development of completely stable combustors, it is necessary to reveal the mechanism of the destabilization. For revealing the destabilization characteristics of combustion oscillation, methane-air turbulent lean premixed flames in a swirl-stabilized combustor were investigated by high-speed simultaneous measurements of stereoscopic particle image velocimetry (SPIV) arranged side by side, OH planar laser induced fluorescence (OH PLIF), OH chemiluminescence and pressure fluctuation. The transition process from stable state to unstable state of combustion oscillation is defined based on the root-mean-square (rms) values of pressure fluctuation p’_rms. Experimental condition was set as the swirl number of 1.14, equivalence ratio of 0.69 and total flow rate of 350 L/min where the transition process is observed. In the transition process, magnitudes of fluctuating properties gradually gain. Pressure fluctuation phase-based analyses clarified that, as the transition process advances, intermittent large-scale vortical motion in the outside of sheer layer expands and approach the inlet, which has a close affinity with the growth of oscillation with making burnt regions involve unburnt regions. The transition process holds well for the Rayleigh criteria in that heat release fluctuates approximately the same phase of pressure fluctuation. On the other hand, in-flow velocity fluctuates in the antiphase of pressure fluctuation.

In the pulverized coal combustion, coal particles cross over a steep temperature gradient formed by a diffusion flame. This temperature gradient affects the particle temperature. This study has experimentally investigated effects of field temperature and residence time in high-temperature regions on the flame structure of single coal particles, since the substances of the devolatilization process varied due to the particle heating rate. The inlet velocity and the oxygen concentration of a laminar couterflow vary to control the residence time and the temperature gradient, respectively. A magnified two-color pyrometry was carried out to understand flame structure and the time series of flame and particle temperature. The results showed that the increase of oxygen concentration raises the volatile matter combustion temperature and flame diameter, and prolongs duration of the volatile matter combustion. The char combustion temperature decreases as the flow velocity increases. The maximum effective flame diameter increases linearly with increasing volatile matter combustion temperature regardless of particle size. This suggested an increase in flame interference distance. The maximum flame diameter increases monotonically with increasing volatile matter combustion temperature.

Herein, novel simultaneous in situ measurements of electrode slurries used to fabricate polymer electrolyte fuel cell electrodes during the drying process were conducted and analyzed via numerical simulation to understand mass transfer during drying and structure formation mechanism of the electrodes. The structure of the porous electrodes that affects cell performance is formed during drying of the electrode slurries. However, directly observing the state of the opaque electrode slurries is difficult, and thus their drying behavior has not been well understood. This study focuses on the variation in coated film thickness and the variation of electrical resistance of the electrode slurries during drying due to solvent evaporation and solid-phase agglomeration. A novel measurement technique of the electrode slurries’ electrical resistance is developed using a microelectrode-terminal chip. Time-dependent variations of electrical resistance and film thickness during drying are simultaneously measured. The results are analyzed and compared with the numerical simulations results. Electrode material distribution in the slurry varied according to the type of carbon materials. The electrode slurry containing platinum-supported carbon particles as a catalyst formed a sedimentary layer and a surface accumulation layer of the particles. Such drying behavior affected the resultant porous structure in the form of cracks and creating inhomogeneous large voids.

In the present study, solid oxide cells (SOCs) were operated in the electricity generation (solid oxide fuel cell, SOFC) mode and the hydrogen production (solid oxide electrolysis cell, SOEC) mode. The fuel electrodes fabricated with nickel-gadolinia doped ceria (Ni-GDC) and nickel-yttria stabilized zirconia (Ni-YSZ) composites were investigated. The correlations between changes in the microstructure and degradation of electrochemical performance are discussed. The degradation mechanisms correlated with Ni phase were found to be similar for Ni-GDC and Ni-YSZ electrodes. On the other hand, the stability of the ceramic phase differs significantly between the two electrode materials.