化学工学

To discuss the feasibility of the high velocity fluidized-bed (CFB) technology, brief reviews on the history of the development of CFB and on its fluidization behaviors are presented. It is also discussed the reasons why the circulating fluidized-bed boilers (CFBC) are appeared before the footlight recently. CFB is pronounced technology for the new boiler systems and also it has sufficient possibility to be applied on the new solid-gas contact systems. To support these technology, much more fundamental research works on the solid-gas multiphase system including CFB are required.

After introducing the system of coal fueled bubbling type dense fluidized bed, the relationship between coal combustion process and the characteristic phenomena occurring in the fluidized bed was described and NOx formation mechanisms during these processes were discussed. NOx reduction reactions occurring on the surfaces of fluidizing materials and burning char with the chemical compounds such as amines, cyanides formed from fuel-N and of CO, H2 were described.Finally, the combustion methods for the NOx in-furnace reduction were discussed.

The first commercial Multi-Solid Fluidized Bed (MSFB) Boiler in Japan was constructed by Mitsui Engineering and Shipbuilding Co., Ltd. and has been operated as a captive power plant for Kuraray Co., Ltd., a chemical textiles manufacturer.This paper describes the concept and features of MSFB, the operation data and the technical points which were established to have stable and reliable operating conditions in dense bed and solid circulating system, introducing a few examples of various and complicated behaviours in multiphase flow of MSFB system.

Atmospheric fluidized bed combustion (AFBC) boiler is one of the important break-through in the area of coal combustion technology, Recently a number of demonstration plants of AFBC boiler have been constcted by utility companies in the world.The coal feed system of AFBC boilers plays an important role for successful operation of AFBC boilers. Employing the unerbed feed system, we have developed a feed system for AFBC boilers and carried out experimental study using a pilot plant. The developed feed system achieved stable transportation of dense coal-air mixture and uniform distribution with the multi-outlet distributor.

Fluidized bed combustion (FBC) boilers are capable of burning high moisture, low calorie and other low quality coals. And FBC also makes it possible to employ the in-furnace desulfurization that simplifies the conventional desulfurizing systems with saving of construction costs. Because of its merits, the development of the FBC technologies has has been actively carried out in many countries.This paper describes the current status and the development of FBC technologies.

It is important to determine unburnt carbon content and temperature distribution for design and operation of coal fired boiler. A numerical simulation of combustion flows in a coal-fired boiler furnace has been done using Reynolds equations, including solid gas two-phase flow. Gas flow patterns, coal behavior and diameter variation, and distributions of the heat release rate, temperature, and oxygen concentration have been obtained as numerical simulation results. Moreover, it has been made clear that coal trajectory and its diameter effect the heat release rate and unburnt carbon content.

A bubbling-type fluidized bed boiler uses the bed slumping and activation technique for controlling steam output. Because of this unique technique incorporated into the boiler design, the prediction and assessment of boiler performance are required during the initial boiler design stage for achieving a specified boiler operation mode. The intention of this paper is to provide subscribers with current knowledge regarding a computer simulation of boiler load control based on the present author's own experience in developing commercial size bubbling-type fluidized bed boilers. Brief review of the fundamental aspect of fluidized bed boilers is followed by the description of bed slumping and activation operation using a particle mixing model. A representative result of the computed fluidized bed temperature transient during load increase is presented for a large size bubbling fluidized bed boiler. Areas for further research are presented for both bubbling-type and circulating-type fluidized bed boilers.

A numerical three-dimensional simulation model of a fluidized bed refuse incinerator, which is based upon the lateral dispersion model and two-phase model, was developed. Separating refuse into three components, moistrure, ash and combustibles, combus tion characteristic was formulated by estimating the diameter of combustibles, ignition temperature and transition temperature from reaction to diffusion in the controlling region. Temperature distribution of the emulsion phase and oxygen concentration distribution in the fluidized bed from the calculation were compared with data obtained from the experiment. The calculated results coincided well with the experimental results. The method of modeling and basic and experimental equations in this model were confirmed to be accurately applicable to the numerical calculation. Moreover, the relation between the scale of the fluidized bed and the performance of the incinerator was discussed on the basis of simulation by the mathematical model.

Due to the wide variety of utilization of entrained-bed coal gasification for chemicals production, hydrogen production and fuel for industry, technology has been developed worldwide. Gasification plants consist of several key technologies such as coal feeding, gasification, dust removal etc. The understanding of solid-gas flow in these fields is required in order to optimize the device configuration. This article introduces the recent R&D activities concerning two phase flow; coal feed rate and void fraction in a transportation tube, particle residence time in a swirl-flow type gasifier, high temperature dust removal technologies.

The external loop airlift bubble column facilitates the contact between gas, liquid and solid phases, since a relatively high recirculating liquid velocity is brought about by only sparging gas into liquid without any mechanical agitation. It is due to the higher recirculating liquid velocity that various properties such as gas holdup, mass transfer, etc. in the column differ from those in the conventional bubble column. The present status of knowledge of airlift column behavior is described with a brief survey on applications such as biological and chemical reactors. The areas in which further reserch is needed are also discussed.

Ultrapure water is used in the final rinse stage of silicon wafer cleaning for ULSI fabrication. Recently, advanced ultrapure water production systems have been developed. They can supply ultrapure water in which any impurities are reduced to an extremely low level. Dissolved oxygen also has been reduced to ppb level for native oxide free processing. Further, Cr2O2 stainless steel piping has been also developed for continuous ozone injection sterilization in ultrapure water systems. In addition, it is becoming possible to develop perfectly controlled ultrapure water technolgies such as ozone-added-ultrapure-water rising for the removal of organic from wafer surfaces and boiling-ultrapure-water treatment for the realization of H-terminated wafer surfaces.

The term “bioreactor” is defined as a reactor in which biochemical or biological reactions are carried out to produce useful substances from raw materials. This term can be applied to fermentors used for the cultures of microbe, animal and plant cells, and to vessels (or columns) containing immobilized biocatalysts such as enzymes, organelles and cells. The chronological aspects and future perspectives in technological development of the bioreactors are described in the present article.

A review on the classical two phase flow theory is carried out in this paper, based on the updated theories and experimental works of the 1990s. Recent developments of the emulsion phase characterization methods using powder rheology in fine powder fluidized beds are introduced and reviewed. Particularly, the physical meaning and significance of incipient bubbling points are interpreted in terms of the fracture model. The significance of flow characteristic functions of aerated fine powders, obtained theoretically and experimentally, is explained.

A review on the macroscopic flow behavior such as bed-density distribution and change in flow regime, as well as mixing and heat transfer, in risers of circulating fluidized beds is briefly made in the paper. It is usually found that formation of dense bed at the bottom or so-called S-shaped bed-density distribution in riser is usually observed in operation at relatively low gas velocity. Such density distribution, however, is easy to disappear with decreasing and/or increasing of solids circulation rate. Thus, it should be noted that the flow behavior and axial bed-density distribution in riser of circulating fluidized bed have wide variety with changing in gas velocity and solids circulation rate. For future work, it will be needed to take account of solid property for better understanding of solids and gas behavior as well as classification of flow regime.

The addition of cationic surfactants with suitable counter ion causes a remarkable drag reduction in a turbulent pipe flow. This drag-reducing technique is one of the candidates for pumping power reduction in a district heating and cooling system. Since the scale of micelles formed by surfactant additives is very small, the dynamics of rod-like micelles in a turbulent flow field can not be detected at the present stage. Also, the theoretical prediction of the movement of a rod-like micelle as a two phase turbulent flow is very difficult. The configuration of rod-like micelles and the viscoelasticity caused by the rod-like micelles are reviewed. Recent approaches to the theoretical prediction of surfactant drag reduction mechanism are also reviewed. These information reveals out the present status of drag reduction phenomenon from the stand point of multiphase flow.

The variety of multi-phase flow analysis capabilities in STAR-CD are explained here. These capabilities are classified into Lagrangian multi-phase, free surface(VOF) including cavitation models and Eulerian two-phase. Also some applications in engineering fields are introduced.

A model for simulating breakup and coalescence of bubbles and liquid particles using DEM is introduced in multi-phase flow simulation. In addition, an approximation method for simulating numerous number of particles effectively is described. Numerical examples are next shown obtained by using a commercial flow simulation software “RFLOW”, in which the present model has been implemented.

In the present study, a novel gas-liquid separator using surface tension force is proposed and its performance is evaluated. The gas bubbles are forced out from the liquid flow at the expanding section by the minimization effect of excess gas-liquid interface free energy, while the liquid phase remains confined inside the micro grooves. Two major gas-liquid separation limits, i.e., (1) flooding limit at high liquid flow rate, and (2) entrainment limit at high gas flow rate are observed and investigated by air-water experiment. Dimensionless correlation for predicting gas-liquid separation limits is proposed. Based on the knowledge from the air-water experiment, a compact gas-liquid separator for R-410A system is developed and the basic characteristics of the separator in refrigerant cycle are investigated. The volume of the present separator is 1/7 compared to that of the conventional separators. Pressure drop reduction in an evaporator is confirmed by bypassing the gas from the evaporator. Finally, a series of 4-16kW capacity gas-liquid separators for practical use is developed.

Extraction of metal ion was studied by a countercurrent centrifugal extractor, in which oil and water were flowed countercurrently through the narrow gap between a rotated inner rotor and a static outer cylinder. Oil (dodecane) was dispersed finely by increasing the rotating speed of the inner rotor and flowed up against the downflow of water. The small drops of dispersed oil were sucked down by Taylor vortices accumulated on the inner-cylinder surface. Stable belt-shaped structures of the oil phase were observed at even intervals on the surface of the inner rotor. The extraction of Zn(II) with di-2-ethylhexylphosphoric acid dissolved in dodecane was examined. The extraction of Zn was promoted by the increase of oil hold-up and the improvement of oil dispersion, which were observed by increasing the rotating speed of the inner rotor and the flow rates of oil and water. The number of theoretical stages was evaluated as about 3 stages under the conditions that the rotating speed and the flow rates of oil and water were given as 1200 rpm and 17 ml/min, respectively. The multistage extraction was achievable successfully by the proposed centrifugal extractor with Taylor-Couette flow.

Flow measurement of spatio-temporal velocity field in a Taylor-Couette vortex flow (TVF) with a short annulus was investigated by using Ultrasonic Doppler velocity Profiler (UVP). These methods have significant advantages over conventional methods since it can obtain the instantaneous spatio-temporal information as a function of space and time even in the opaque liquid like solid-liquid multiphase fluids. Boundary effects with a short annulus of TVF called Ekman boundary layer are greatly related to the generation of the various modes in vortex structure. Parameters as an aspect ratio, Γ(=H/d) and radius ratio, η(=R1/R2) are important factors for flow bifurcation to these modes. Here, d is an annular gap width, H is a height of the test section, R1 and R2 are radii of the inner and outer cylinders respectively. In the present study, mean velocity profiles of the TVF with Γ=3 and η=0.667 are measured with the UVP. Instantaneous velocity profiles with oscillation in higher Reynolds region (Re=1140) are also measured and analyzed by using the UVP and a spectrum method. The characteristics of wavy vortex flow in the normal 2 and 4 cell modes are much different from each other. Particle behavior in TVF is much complicated. The limit cycle orbit, which has previously only been confirmed numerically in the multi-phased TVF were found experimentally. In the limit cycle, solid particles are gathered in a very limited orbit line. While this phenomenon is unique and does not occur in the general flow region, it could be useful for analyzing the anti-plugging characteristics in industrial rotating filter devices or bioreactor.

This paper demonstrates numerical simulations of single-phase and two-phase flow through complex geometry such as the packed bed and gas diffusion layer in packed bed by using of lattice Boltzmann method. In the simulations of the packed bed, Mass and heat transfer in the packed beds are explored computationally and pressure and temperature distributions are compared with existing experimental results. In the simulation of gas diffusion layer, the water infiltrations into GDL were performed and influence of porosity, fiber diameter, orientation degree of carbon fiber, contact angle of GDL and water inlet velocity on saturation and gas diffusion performance was examined.

In slow freezing of water that includes solutes, pure ice is produced and most of the solutes are removed from the frozen phase and concentrated in an unfrozen liquid phase. At fast freezing rate, however, large part of the solutes from solution is captured into the freezing interface and then the solution is not concentrated. To prevent this capture of the solutes, the strong agitation of the freezing interface is very effective. We have been studying the applicability of ultrasonic irradiation to the agitating method, and found that the freeze concentration efficiency of solutes is improved greatly by this irradiation. In this paper, the effects of the freezing rate and the ultrasonic irradiation on the freeze concentration characteristics are reported.

From ancient times fire has been extinguished mainly by using water. But there may be some problems in large amount of water discharge against fire, such as enough water volume for fire extinguishing may not be secured in case of large-scale natural disasters, and also secondary damage may be caused by discharged water. CAFS, one of fire extinguishing method by using air foam, is the solution because it utilizes small amount of water very efficiently. This report presents an overview of fire extinguishing with air foam, how to generate it and verification experiments of fire extinguishing effect with CAFS foam in MORITA Corporation.

Moisture transport is an important phenomenon in many technological fields. We summarize a fundamental description method with use of the relative permeability and the moisture diffusion coefficient. The theoretical approach for the two-phase flow in the porous media is based on the extended Darcy model. The relative permeability and the moisture diffusion coefficient are functions of the moisture content and they express the effects of the friction loss and the capillary pressure, respectively. The model is applied to the microwave vacuum drying of food, where the vapor pressure drives the moisture from the inner to the outer because of the internal heating by the microwave irradiation. The numerical moisture-distributions are compared with the experimental results obtained by the MRI. It is shown that the microwave vacuum drying gives a desirable moisture-distribution for the drying.

Gas-liquid and liquid-liquid multiphase flows are often encountered in application of microreactors. When two immiscible fluids are introduced to a microchannel, several flow patterns may be observed depending on the fluid property, flow conditions, and the channel geometry. Slug flow is the most important flow mode among them, because the mass transfer rate across the fluid interface is greatly enhanced. The enhancement of the mass transfer was attributed to the recirculating flow within each slug, which promotes the mass transfer from the fluid interface to the interior of each slug. Thus, it is important to clarify the range of operating conditions in which slug flow can be formed. Also, to fully understand the mass transfer enhancement in slug flow regime, the fluid behavior in each slug must be studied. Many studies have been conducted to construct a general flow regime map. Also, the fluid flow within a slug has been investigated by utilizing flow visualization techniques and computational fluid dynamics simulations. This article gives an brief overview of the studies on the characteristics of multiphase flow in microchannels.

This paper demonstrates numerical simulations of droplet on/in gas diffusion layer of the polymer electrolyte fuel cell. The lattice Boltzmann method for incompressible two-phase flows at high density ratios were applied for the simulations in order to predict the shape and moving velocity of water droplet surrounding by the air in the gas channel. In addition to the simulation, droplet infiltration to the simulated gas diffusion layer were simulated. The droplet heights by the simulations were qualitatively agreed with the experimental data, while the simulation results of moving speed of droplet somewhat overestimated the experimental results.

It is expected to manufacture new materials with container-less processing by levitation techniques. However, it is indicated that the levitated droplet with levitation technique has nonlinear behavior. In this study, nonlinear dynamics of the levitated droplet is experimentally and analytically investigated. The levitation experiments with the ultrasonic levitator and electrostatic levitator are conducted under the reduced and normal gravity condition. Based on the experimental results, theoretical analyses are compared to clarify the nonlinear dynamics of the levitated droplet. Furthermore, new technique to manipulate the thermofluid process of levitated droplet and measure the viscosity in high viscosity region are proposed by levitation techniques.

In an extrusion process of rubber materials, there are a liquid-liquid phase and a gas-liquid phase. It is difficult to solve viscoelastic flow with high Weissenberg number. In this paper, interface between rubbers is solved by using the commercial software. The difficulty in the numerical simulation is specified. The solution to this problem is proposed. The proposed method is applied to the numerical solution of flow problems for a viscoelastic fluid in arbitrary computation domains. The proposed method is enabled to solve the viscoelastic flow with high Weissenberg number in comparison with the conventional method.

For developing a novel micro-fabrication process of flexible and large-area thin-film display MEMS device, liquid-liquid two-phase slug droplets formation in T-junction microchannel with square cross section is investigated through computational fluid dynamics (CFD) simulation by using a diffuse-interface tracking method based on lattice-Boltzmann model numerical solution scheme plus conservation-modified Allen-Cahn advection equation. The volumetric flow rate ratio is fixed at 1.0 within the range of low capillary, Reynolds and Weber numbers for silicone oil-water system with hydraulic diameter of 100 μm, kinematic viscosity of 1.0 cSt. and interfacial tension of 41.6 mN/m. The major findings are as follows: (1) the continuous and dispersed-phase slugs become shorter at nearly-constant length difference between them as their flow rates are increased; (2) their lengths in the simulation agree well with experimental data; (3) the dispersed-phase volume fraction are well predicted in comparison with experimental and one-dimensional two-fluid model CFD simulation results.

In a recent article (R. Seto, R. Mari, J. F. Morris, and M. M. Denn, Phys. Rev. Lett., 111:218301, 2013) we found frictional contact forces to be essential for reproducing the shear thickening behavior of non-Brownian suspensions. Although the introduction of frictional contact to a Stokesian Dynamics simulation is speculative and requires the existence of particle-particle contact despite fluid lubrication, the simulation results are in good agreement with experimental data. The model also provides physical insight into the relation between shear thickening and a jamming transition. This article describes the transition from a fluid mechanics perspective to a granular physics perspective.

Solid-liquid mixing in a stirred vessel is widely used in chemical industrial processes, such as crystallization, leaching and chemical reaction by using solid catalysis. In these processes, control of sedimentation of particles on bottom is important to maximize efficiency of the chemical reaction and to prevent particle agglomeration. And particle collision with impeller blades often induces attrition and breakage of particles and erosion of the impeller blade. The mechanism of particle suspension from the vessel bottom and particle collision with impeller blade are not sufficiently understood, mainly because of the difficulties in experiments for characterizing detailed particle behavior.  In this study, the relation between particle rising behavior from a vessel bottom and liquid flow around bottom was clarified by using Computational Fluid Dynamics (CFD) coupled with the Lagrangian simulation of each particle motion. The results show that a common relation of four steps exists between particle rising behavior and fluid flow around bottom among different type of stirred vessels. That is (i) formation of stagnant region on bottom, (ii) sweeping of particles until the stagnant region, (iii) inducement of upward flow above the stagnant region, (iv) particles are caught up in the upward flow. And the particle collision behaviors with impeller blades, such as collision velocity and collision point distribution, were quantified. The results show that, on the front (loading) face of impeller blade, particle collision with large collision velocity concentrates along the blade edge. On the back face, particle collisions mainly occur around the center of blade face.

The present review provides a brief introduction to the capillary bridge force, which frequently appears and plays an important role in multiphase flows containing solid bodies. We explain the fundamentals and formulation of the capillary bridge force as well as its importance over other forces such as the van der Waals force and the electrostatic force. Also, we give some examples of the capillary bridge forces that are hardly recognized but appear in the real systems.

The requirement of energy saving on sewage and wastewater treatment is increasing for GHG (Green-House Gas) reduction. Aeration tanks of sewage treatment have larger surface area and smaller water depth relatively than bubble columns on the mass transfer in chemical engineering. We understood gradually that the oxygen transfer on aeration tanks of sewage treatment were strongly affected by the flow pattern and the state of surface. This report showed some studies about gas transfer at free surface.

It is very difficult or impossible to apply optical measuring methods to multiphase fluids, such as solid-liquid, gas-liquid and liquid-liquid systems, in order to clarify their flow behavior, mixing process, and dispersion state in a stirred vessel, due to the opacity of such systems. Computational fluid dynamics (CFD) and the electrical resistance tomography method (ERT), which are introduced in this article, are very powerful and useful for analyzing such systems. Investigation results using both methods are also discussed.

One of the mysteries in ultrafine bubbles is their long lifetime in spite of the fact that the gas diffusion theory predicts the very short lifetime of 20 μs － 80 μs for ultrafine bubbles of 100 nm － 200 nm in diameter in liquid water saturated with air. In the present review, 8 proposed models for the stability of ultrafine bubbles are discussed. In addition, we propose the dynamic equilibrium model of an ultrafine bubble partly covered with hydrophobic material.

It can be emphasized that miniaturization of bubble‐size is one of the superiority with regard to the non‐uniform creation of reaction fields within liquid phase. Owing to the combination of energy supply such as ultrasonic or electromagnetic waves with reaction fields, physicochemical phenomena occurring in these reaction fields are significantly observed. In this paper, the classification of the reaction field presented in the gas‐liquid‐solid system under fine‐bubble supply was carried out. Then, the engineering applications for environmental conservation and material manufacturing were described.

Scientific knowledge about ultrafine bubbles obtained so far has been explained for supporting the promotion of international standardization of fine bubble technology (ISO/TC281). The mechanism of the ultrafine bubble generation, the long-time duration of its existence, various useful effects has been explained based on the obtained scientific experimental and analytical results, which would not be enough so far. The expected industrial applications and the important fundamental research topics of ultrafine bubbles have been described based on the several characteristic features of ultrafine bubbles.

A lot of steel engineers researched various means to decrease reducing agents at a blast furnace in order to reduce CO2 emissions. For example, injection of H2, waste plastics and carbon neutral materials such as biomass into a blast furnace is better alternative. When iron oxide is reduced by H2 gas, CO2 don' t form. We have studied optimal reduction conditions of iron oxide. In this work, we focused on H2-CO gas mixtures reduction of iron oxide. In order to prepare samples, reagent Fe2O3 was mixed with SiO2 and CaO in the mass ratio of Fe2O3：SiO2：CaO = 80：7.14：12.86. Then the mixture was pelletized by handrolling and sintered at 1220℃ (Sinter-A, a porous sample) or 1250℃ (Sinter-B, a relatively porous sample). The formed pellets were reduced from Fe2O3 to FeO at 800℃ and from FeO to Fe 1100℃. From weight loss curves of samples, chemical reaction rate content (kc) and effective diffusion coefficient (De) in a product layer were calculated by using an unreacted core model. The values of kc and De in H2 reduction were much higher than those values in CO reduction. The particle size of the reduced Fe in H2 reduction was smaller than that one in CO reduction.

In most previous works, liquid hold-ups were studied by using a fixed bed soaked prior to experiments. In the present study, the characteristics of liquid hold-ups and liquid flow were investigated with initially unsoaked bed. Contact angles (θ) for these particle/liquid (tap water) systems were about 70° and 10° for fluorine-coated particles and non-coated particles. Under bad wettability condition (θ≒70°), total and static hold-ups for initially unsoaked bed packed with small balls are remarkably smaller than those for initially soaked bed. In initially unsoaked bed, total and static hold-ups under bad wettability condition (θ≒70°) indicate maximum values at about Dp =10 mm and decrease abruptly in proportion to a decrease in particle size, despite an increase in the specific surface area. Under good wettability condition (θ≒10°), total and static hold-ups are smaller than those of bad wettability condition (θ≒70°), because only restricted liquid droplets and/or liquid rivulets are formed within the packed bed for initially unsoaked bed, nevertheless liquid is easy to spread out on the solid surface under good wettability condition.

The understanding of non-Newtonian two-phase flows is a far from easy issue because two-phase flows dynamically and intricately behave through boundary interfaces with the local variation in rheological properties. A key factor to explore structures and mechanisms of non-Newtonian two-phase flows is to perceive a local profile of non-Newtonian properties corresponding to the shear-rate. In this article, I review current situation and challenges about the study of non-Newtonian two-phase flows through the author' s studies on the dynamic motion of bubbles and drops in non-Newtonian fluids.

A stirred vessel is a device for mixing fluids. It is widely-used in industry and plays an important role in the chemical process. In the stirred vessels, liquid is stirred by the rotational motion of impeller, and often, there exists large deformation of free surface. The moving particle semi-implicit (MPS) method is a meshfree particle method suitable for such complex physical phenomena because moving boundaries can be handled with relative ease. The devolatilization process is one of the applications of stirred vessel, where dissolved volatile components are removed by inducing the nucleation of bubbles in the tank. This paper explains numerical algorithms for the stirred vessel analysis with the MPS method. Furthermore, numerical simulations of the devolatilization process using the MPS method with incorporated bubble models are presented.

We have recently developed a machine learning method capable of inferring the constitutive equation for the stress of a fluid with memory (e.g., polymeric fluid) from microscopic simulations. For this, we used a Gaussian Process regression scheme to learn the constitutive relation, given as the time-derivative of the stress, as a function of the local stress and velocity strain tensors. Crucially, no assumptions are made regarding the functional form of this relation. We have applied our method to the Hookean dumbbell model, for which the exact analytical constitutive relation (Maxwell equation) is known, in order to validate the approach. Our results are in excellent agreement with the analytical solution, showing that we are able to capture the history dependence of the flow, as well as the elastic effects in the fluid. Compared to full multi-scale simulations, in which the micro and macro degrees of freedom are directly coupled, our method provides a similar degree of accuracy, at a small fraction of the cost. In addition, the method can be easily generalized to more complex and realistic polymer models. (As part of this article, we include a Japanese summary of our main results, published in [7], CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/).)

As ultrafine bubble (UFB) water becomes widespread in society, it is extremely important to see if the UFB water can be treated like a popular aqueous solution. The most fundamental handling includes storage, transportation, concentration, dilution and removal. Although the number concentration of UFBs gradually decreased even in an environment where it was allowed to stand at room temperature, it maintained about a few hundred million/mL for several months. There was also no significant decrease in number concentration due to the international transportation between Japan and Germany. The UFBs in water could be concentrated by vacuum evaporation method or very slow forward partial freezing method. Furthermore, UFB water could be diluted with ultrapure water. The UFB number concentration distribution hardly changed in these various concentration adjustment operations. The UFBs were removed from UFB water by ultrasonic irradiation in the MHz band.

Ultrasound was irradiated to ultrapure water. Ultrafine bubbles with a diameter of 90 - 100 nm were generated. Number concentration of ultrafine bubbles increased with time and approached asymptotically to an equilibrium value. Ultrafine bubble concentration increased with increasing ultrasonic power and decreasing frequency. For comparison, highly concentrated ultrafine bubble water was prepared by pressurized dissolution method, and ultrasound was also irradiated to the ultrafine bubble water. Reduction of ultrafine bubbles was observed and ultrafine bubble concentration decreased to the equilibrium value. Ultrafine bubble concentration decreased with increasing ultrasonic power and frequency. Generation and reduction of ultrafine bubbles by ultrasound was modeled to analyze experimental data. The results calculated by our model were in good agreement with the experimental data.

Bubbles with a diameter less than 1μm are called ultrafine bubbles (UFB). And UFB is expected to be applied in the environment, agriculture and medical treatment. In this study, we will examine the possibility by using ultrasonic waves and ultrapure water (UPW), also the attenuation characteristics of UFB water, to analyze the properties of UFB. The attenuation characteristics of UFB water in broadband ultrasound waves up to 32.5 MHz were investigated. In the 1.5-2.5 MHz ultrasonic band, the attenuation coefficient in the near-field was relatively close to the theoretical value calculated from the bubble density measurement results. For the 32.5 MHz ultrasound, there was a clear difference in the attenuation coefficient for the bubble density distribution between the UPW and the UFB water. In addition, the change in the number density distribution of bubbles and the change in the attenuation coefficient were observed depending on the elapsed days after UFB generation. It is possible that the change is related to the increase in the bubble diameter of the UFB.

In the present study, two experimental examples as the applications by ultrafine bubble for railway fields are introduced. The first one was confirmed the cleaning effects of ultrafine bubble water on the brush cleaning machine used in the station buildings. It was found that the removal rate was increased by using ultrafine bubble in the brush cleaning machine compared to the tap water. However, it was also shown that the removal rate increased or decreased with respect to the number of rotations of the brush and the pressing force, and there were some conditions in which the effect of the ultrafine bubble could not be confirmed. In this experiment, it was shown that unevenness on the floor surface and dirt, and microbubbles grew from ultrafine bubble may be important for cleaning effect and understand of the mechanism. The second one was shown an interesting effect that the friction reduction effect by microcloth with ultrafine bubble water. In this experiment, the static frictional coefficient and dynamic frictional coefficient of the microcloth with tap water and ultrafine bubble water were measured on the glass plate. As the results, it was found that both frictional coefficients were decreased by ultrafine bubbles but when the glass plate was not dirty, both frictional coefficients did not decrease.