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.

In recent years, there has been a need for cooling technology that can deal with the increase of heat generation by the miniaturization and high output of electronic devices. Fin-type heat sink with boiling cooling can be a new method that can cool the electronic devices efficiently. But, at the same time, its pressure drop will also increase. The increasing pressure drop may result in the deterioration of the heat transfer efficiency. So, it' s important to estimate the pressure drop through the heat sink. In previous studies, although most researchers have already studied the pressure drop of the fin-type heat sink, there are only a few studies treating the pin-fin as packed material in a packed bed system, which might have a possibility for general expression of pressure drop. Therefore, in this study, the pressure drop through the fin-type heat sink with the single-/two-phase flow is measured, and Ergun/Lipinski model is employed to evaluate the experimental results.

In a heat exchanger for heat pump systems, it is required to reduce the diameter of the refrigerant channel in order to improve the performance and reduce the amount of refrigerant. Since the effect of surface tension on the flow and heat transfer characteristics of gas-liquid two-phase flows becomes relatively larger with decreasing the diameter, cross-sectional shape will also affect on the phase distribution. When the cross-sectional shape of the channel is non-circular, such as rectangle and triangle, liquid film thickness will be thicker in the corner due to the surface tension. This study deals with void fraction characteristics of gas-liquid two-phase flows of FC-72 in horizontal non-circular small diameter channels, such as rectangular and triangular channels. The hydraulic diameters of the rectangular and triangular channels were 1.02 and 1.06 mm, respectively. Void fraction was measured by a capacitance sensor with flow observation. As a result, for the non-circular channel, the flow pattern transition to annular flow occurred at the lower gas Weber number, and void fractions in the non-circular channel were lower than those by the Cioncolini-Thome's correlation for circular channel because liquid film thickness at the corner was thicker due to surface tension.

Stratified flow in liquid-liquid two-phase flow of water-perfluorocarbon（PFC）system is expected to be utilized in biomedical engineering. In particular, the stratified flow is considered to be used for the research of cell sorting and transporting method for cell treatment process of transplantation. This method can transport cells without any damages and supply oxygen to cells. However, studies on liquid-liquid two-phase flows have been conducted mainly in oil-water systems, and there have been few studies under the condition where water is the upper layer. There is also little research on the shape of the interface for stratified flow, which is required for living cells sorting. In this study, we investigated the conditions for the formation of stratified flow in a water-PFC system using experimental and numerical simulation. The volume of fluid (VOF) method was employed to predict the interface shape of the stratified flow depending on the Weber number and the Eötvös number. As a result, the formation of stratified flow was observed in the range of We≥1 and in the range of We≥0.05 and Eo≥4. From the analysis, no significant difference in the interface shape was observed in this experimental condition range. Therefore, we can employ this flow system for cell treatment process to develop novel devices for cell therapy.

On the gas-liquid interface of a sodium-cooled fast reactor, gas (bubble) entrainment can be caused by free surface vortices, which may result in, e.g. disturbance in core power. Therefore, it is important to develop an evaluation model to predict accurately the entrained gas flow rate for the prevention of such undesirable phenomena. In the previous studies, simple gas entrainment experiments have been conducted, in which, typically, the gas entrainment is caused by free surface vortex in an upper-tank and the entrained gas is separated from liquid in a lower-tank which is connected with the upper-tank by a suction pipe. The entrained gas flow rate had been measured while the liquid flow parameters, e.g. liquid level or flow rate, are changed. However, the influence of the pressure difference between the upper and lower tanks had not been focused on those studies. In this study, to measure accurate the entrained gas flow rate, a simple gas entrainment experiment is conducted with focusing on the effect of the pressure difference between upper and lower tanks. The pressure difference between upper and lower tanks are controlled by changing the gas pressure in the lower-tank. As a result, it is confirmed that the entrained gas flow rate increases with increasing the pressure difference between upper and lower tanks. By the visualization of the two-phase flow in the suction pipe, it is also observed that the pressure drop in the suction pipe increases with the increase in the entrained gas flow rate when intermittent liquid plug is generated in the two-phase flow.

Liquid-film flow appears in widely industrial fields, and its characteristics can impact product reliability and efficiency. Steam turbines, for example, notorious erosion and moisture loss are significant problems due to water droplets generated on the blade surface under humid low-pressure environments. A method for directly measuring the liquid-film flow in the machine is required. However, measuring the liquid-film flow is difficult by conventional methods under high temperature and pressure. Moreover, the thin liquid-film flow is frequently accompanied by disturbance waves in such an extreme environment. This study demonstrated the thickness measurement of the thin and wavy liquid-film flow using an Optical-fiber-based Reflective Probe (ORP). This technique can measure local film thickness utilizing the relationship between the distance and the light intensity reflected at the interface. At first, we investigated the ORP signal of a pseudo liquid-film flow using a wavy-stainless steel surface. According to the results, the ORP signal tends to peak at the locally flat positions of the wave. The ORP signal processing was newly established based on this fact. After that, we performed the ORP measurement in the thin-liquid-film flow condition with its average thickness of less than 500 μm. The experiment was conducted in a blowdown wind tunnel. We flowed air (j_G=41~75 m/s) into the rectangle test section and injected tap water (Γ=1~3x10^-5 m²/s) from holes on the channel base. The ORP was fixed at this section to obtain time-series data of the liquid-film thickness. Furthermore, we visualized the liquid-film flow from the side via a high-speed camera, then estimated the film thicknesses. The results showed a good agreement, and consequently, we confirmed the ORP successfully evaluates the thickness even in the liquid-film flow with disturbance waves.

Although there exist several methodologies to measure gas-liquid two-phase flows, such as wire-mesh sensors and X-ray CT, a direct measurement of bubble coalescence and breakup is difficult due to their multidimensionality and unsteady nature. However, a comprehensive understanding of gas-liquid two-phase flow under such conditions is essential for predicting the effectiveness of scrubbing, i.e. to remove fission products during severe accidents in nuclear power plants. The objective of this study is to develop a new methodology to directly measure and analyze gas-liquid two-phase flow under high void, churn-turbulent regime in a large space through an initially stagnant pool of water. For this purpose, a new technique to measure two-phase flow using multi-view images has been developed. A total of 29 synced cameras housed in a water-proofed casing were placed inside a test section with an inner diameter of 1.5 m and a height of 4 m, to construct a multi-view camera system. The test section was filled with water up to a height of 2 m, and images were taken as air was supplied from a cylindrical nozzle at the bottom with flow rates of 500 to 2000 L min^-1. The cameras were calibrated, and an algorithm was developed to reconstruct the air flow in 3D voxel space. The results suggest that the developed technique, together with data assimilation and other techniques, could be applicable for measuring gas-liquid two-phase flow under churn-turbulent regime in a large space. In this paper, the progress of the development of the multi-view measurement and analysis technique, and the results of preliminary investigations are discussed.

Cavitation cloud formation by the backscattering of focused ultrasound from a bubble interface in gelatin has been studied experimentally. A laser-induced bubble which is generated near the geometrical focus of the focused ultrasound is used as a reflector of the incident wave to yield strong negative pressure which leads to the cavitation inception and following cavitation bubble cloud formation. The maximum size of the laser-induced bubble is about 2 mm in radius. After the laser-induced bubble collapses, a small bubble whose radius is about 0.3 mm remains around the laser focus. In the present experiment, this residual bubble is also used as a reflector bubble, and the effect of reflector bubble size on cavitation cloud formation has been evaluated. Cavitation inception and cavitation cloud formation near the reflector bubble has been observed with a high-speed camera. It is shown that the cavitation cloud grows along with the propagation axis of the incident wave. The larger the reflector bubble size becomes, the earlier the first cavitation inception near the reflector occurs. On the other hand, regardless of the reflector bubble size, the distance between the top interface of the cavitation cloud and the geometrical focus of the focused ultrasound at the maximum expansion of the cloud is almost constant. The results also show that when a residual bubble is used as a reflector, the growth rate of a cavitation cloud in the direction of the propagation axis becomes higher than that when a laser-induced bubble is used. This may be due to the difference of the interface shape of the tip of developing cavitation cloud.

This study deals with numerical simulations of the growth of bubble nuclei and the corresponding bubble cloud formation in a pressure field given by high intensity focused ultrasound (HIFU) backscattered from a laser-induced bubble interface. The imposed pressure on the nuclei is calculated with the Ghost Fluid Method (GFM). The growth of nuclei in a microscopic field that cannot be resolved under Eulerian grids in the GFM is evaluated by using the Keller equation for bubble dynamics. Once the nuclei grow so lager that they can be captured with the level-set function, the GFM with the level-set method takes over the computation in the macroscopic field. The validity of present method is confirmed by the growth of a single bubble nucleus placed on the HIFU propagation axis near the laser-induced bubble. The first growth and the following collapse of the bubble captured with the level-set function in the macroscopic field are in fairly good agreement with those predicted by the Keller equation with keeping acceptable mass conservation, although the accuracy depends on the grid resolution. It is also shown that there exists a bubble configuration where multiple bubbles grow more stably than those when each bubble grows solely: the stable multiple bubbles become a new reflector of HIFU leading to a high negative pressure field where next cavitation inception can occur. Finally, we demonstrate the cone-like bubble cloud formation observed in experiments (Ogasawara et al. (2018); Horiba et al. (2020)) using the present method. The simulated cloud formation process supports the experiments.

Calculus crushing factor of TUL (or Ureteroscopy:URS) which is one of the treatments for ureteral stone disease is thermal effect of pulsed laser and collapse impact of laser-induced bubbles. These factors play an important role in crushing calculus, but can cause mechanical and thermal damage to surrounding tissues. In order to improve the safety of TUL, we observed the behavior of bubbles under several laser pulse and energy conditions, and measured temperature around bubbles using a thermocouple. As a result, the bubble behavior changes depending on length of pulse duration. Temperature around long pulse bubble is higher than that in short pulse. The measured temperature was close to one calculated by bubble collapse time based on the Rayleigh’s equation. Long pulse conditions can cause thermal damage to surrounding tissue through examination by cumulative equivalent time at 43 ℃ (CEM43℃).

Fine bubbles are attracting social attention due to their many physical characteristics. However, the stability of fine bubbles remains unclear. Controlling its stability leads to the addition of new functions to the fine bubbles. It is important to elucidate the stabilization mechanism of the fine bubbles. We studied the stabilization of fine bubbles by molecular film covering that utilized molecular adsorption characteristics of the bubble surface. The molecular film was formed by two types of molecules. One was dialkylbipyridinium which was a positively charged organic molecule with hydrophobic groups, and the other was polystyrene sulfonate which was a negatively charged polymer. These two types of molecules interacted electrostatically to form a molecular film called an ion complex film on the surface of the bubble that was insoluble in water. The fine bubbles covered with the ion complex film were observed as a spherical structure by a digital microscope. Although the number decreased, a spherical structure was observed in the solution left for about one year.

Ionic liquid exhibits outstanding performance for selective absorption of carbon dioxide(CO₂). Ionic liquid electrospray for CO₂ absorption has been proposed as a high performance CO₂ absorption system. CO₂ absorption is caused through chemical reactions at gas-liquid interface, thus generating smaller droplets leads to higher absorption efficiency. In this study, the effect of applied pulsed DC voltage on ionic liquid electrospray was experimentally clarified. By applying pulsed DC voltage, smaller droplets are ejected compared to applying continuous DC voltage. Applying pulsed voltage of 50 - 150 Hz increases the number of generated droplets more than twice of that for DC voltage. An improvement of CO₂ absorption was clearly obtained when the frequency is in the range of 50 - 150 Hz.

In this study, the wetting behavior of fluid onto the horizontal square rod array is numerically investigated. The dimension of the rod is referred to the actual stator coil of an automobile. The computational parameters are the gap of the rod and the viscosity to investigate the dependency of the structure and fluid property. The three-dimensional phase-field lattice Boltzmann method is used both for the fluid flow and phase transport. As the rod gap increases, the wetting rod number and area decrease, and the wetting in the axial direction of the rod becomes dominant. The liquid among rods supports the wetting to the spanwise direction at the narrow gap, but it disturbs the axial direction wetting at the wider gap. As for the viscosity, the higher viscosity induces less wetting area, and the capillary length among rods becomes longer. The dimensionless wetting area decreases until the threshold value of dimensionless number including both rod gap and viscosity, then it converges to the constant value. The heat removal per unit area is estimated by using an analogy.

Gas-fluidized beds are used for the combustion and gasification of solid materials such as biomass pellets and municipal solid wastes. For the enhancement of fluidization and heat transfer, inert bed materials also exist in the beds. Because the characteristics of these raw and bed materials (particle size, shape and density) largely differ from each other, the fluidization of binary systems shows diverse and complex behaviors. In this study, segregation of binary mixtures with large size difference in a gas-fluidized bed is investigated numerically. For simplicity, we consider a pseudo two-dimensional bed and the spherical shape is assumed for both large and small solids. We focus on the influence of mass fraction of large spheres For small mass fraction case, the bed is well fluidized and a segregation occurs as we expected. When the mass fraction of large spheres becomes large, on the other hand, the bed is well fluidized at first, but it gradually and partially stops with the progress of segregation which induces the decrease of local mass fraction of large spheres. Defluidization is caused by the segregation of large spheres, which partially reduced the mass fraction of large spheres in the bed, preventing the bed to fluidize. The fluidization and segregation have a mutual influence. When the local mass fraction of large spheres is large, the bed becomes easier to fluidize because the local gas velocities increase in the gap between adjacent large spheres. We also found, even in the case local mass fraction of large spheres is large, the fluidization becomes difficult if the coarse spheres are not sufficiently surrounded by small particles. Although the local mass fraction of the large spheres has a great influence on the fluidization, the packing fraction of the smaller particles around the large spheres is also very important in the fluidization.