In this review, inactivation and activation of microorganisms using fine bubbles are introduced. In the first chapter, we introduce domestic and overseas research trends of sterilization methods using ozone and plasma. In the second chapter, we focus on researches such as cleaning of lipstick at cosmetic application, generation of plasma-activated microbubbles for sterilization of fresh foods and verification of underwater plasma characteristics under fine bubble dispersion for improvement of chemical activity of bubbles. The third chapter is an introduction of our activity through International Symposium on Application of High-voltage, Plasma & Micro/Nano Bubble to Agriculture (ISHPMNB).

We have developed a demonstration machine of a large-capacity air purification system that purifies a large amount of polluted air using a gas-liquid mixing nozzle and a multi-layer metal fiber filter.　A gas-liquid mixing nozzle, driven by the air blower, sucks and accelerates water to form a high-speed two-phase flow. In the centrifugal force field, all the fine particles and aerosols in the bubbles are transferred to the liquid phase water, and the hypochlorite water whose disinfecting effect has been confirmed by the Product Evaluation Technology Infrastructure Organization (NITE). The demo machine can remove fine particles and viruses in the air at 100 m³ per hour with a single nozzle. We have also developed a large-capacity air purification system that can purify 2400 m³ of air per hour with 24 nozzles. In the virus removal evaluation test conducted by The Japan Textile Products Quality and Technology Center (QTEC), the sterilization performance of 99.975% or more was confirmed. We aim to put this technology to practical use at an early stage to purify the air in hospitals, schools, office buildings, etc.

In this study, nanobubble generator using a honeycomb structure is investigated towards producing a large amount of water including large nanobubble density. Our previous study showed that the pressure reduction and shear stress are involved in the honeycomb cell flow. In this study, the nanobubble generating performance is studied experimentally for honeycomb structures by varying the cell size and the flow velocity. Then, the small-scale honeycomb cell structure is studied for the broader industrial applications. Computational Fluid Dynamics analysis is also performed to simulate the experiment to find out the efficient nanobubble generation. The results show that the maximum shear stress is the main controlling factor of the nanobubble generation for small and large apparatuses.

This study has experimentally investigated fluidized powder conveying in a horizontal rectangular channel when the powder head at the powder discharge vessel and fluidizing velocity at the bottom of the discharge vessel and the horizontal channel were varied. The powder used was glass beads of the Geldart B particle, with mean particle diameter of 118μm, the particle density of 2623kg/m³ and the minimum fluidizing velocity of 12.3mm/s. As the result, steady powder conveying achieved in case of the reduced powder head in the powder discharge vessel. It is also necessary to supply the air at the bottom of the powder discharge vessel, and to give to greater than the minimum fluidizing velocity at the bottom of a horizontal channel. The profile of mass flow rate and solid loading ratio against fluidizing velocity to the bottom of the powder discharge vessel and a horizontal channel revealed the similar tendency when the powder head was changed. It was clarified that when the powder head was lowered, the total pressure drop decreased, the mass flow rate and the solid loading ratio also decreased, and then the performance of the pneumatic conveying equipment was greatly affected. In addition, it was confirmed that high dense pneumatic conveying could be realized even if the powder head was changed within the range of the present experiment. Moreover, the flow pattern in the horizontal channel during steady conveying showed the ripple flow, and the bubble flow was generated into the powder layer as the fluidizing velocity at the bottom of the channel increased. It is also inferred that the part of the pressure energy of the powder bed in the powder discharge vessel became the driving force for the horizontal conveying of the powder.

Liquid bridges between particles often appear in powder processes and are recognized to be an important phenomenon in micro scale. In the gas-liquid two-phase flow with solid particles, the fluid behavior and the force acting to particles strongly depend on such parameters as the wettability and the liquid amount. We have numerically studied the dynamics of liquid bridges by using a two-phase flow simulation based on a weakly compressible scheme. The shapes of moving particles are compromised by using the direct forcing immersed boundary method. The computational results are in good agreement with theoretical solutions and experimental results.

This paper reports preliminary experimental results of shock wave/ water droplets interaction phenomena related to shock wave pressure attenuation. Water droplets were generated by dropping from a several needle pipes, were interacted with micro-explosive-induced shock wave. The process of induced spherical shock wave/ water droplets interaction phenomena was visualized by shadowgraph method and recorded by a high-speed camera. The pressure histories of propagated shock wave interacting with water droplets were measured simultaneously by using the pressure transducer. High-speed shadowgraph sequential images show that interacted water droplets were highly deformed, and a wave front of a secondary shock wave was decayed. The simultaneously measured pressure history shows that a primary shock wave peak pressure was attenuated with interacting at water droplets, and a peak pressure ratio was decreased.

The promoting evaporation of urea-water solution by atomization is required to improve the purification efficiency of NOx for the urea SCR system. This study proposed an innovative method of atomization which processed a surface texture to the impingement walls of the spray droplet. The atomization mechanism in this method was verified by the high qualitative visualization of the impingement droplets.

Recently, many embankments have often collapsed by overtopping an embankment because of heavy rain fall in natural disaster. Variation in water saturation in the embankment influences the collapse of the embankment in the precipitation. Then Hibi and Tomigashi developed a numerical simulation method coupling groundwater with atmosphere and surface (ASGMF method). Purpose of this study is to confirm that ASGMF could reproduce infiltration into the embankment and overtopping the embankment by two-dimensional tank experiments and numerical simulations. As a result ASGMF could reproduce distribution of water saturation in the embankment obtained by the infiltration experiment and the overtopping experiment. On the other hand ASGMF was successfully able to reproduce raise of water level on a side of overflow in the overtopping experiment. However the raise of water level obtained by ASGMF was not almost consistent with that obtained by the infiltration experiment. As the whole this study concluded that ASGMF could almost reproduce the infiltration of water into the embankment when water in a river overflowed the embankment, but we should estimate the ability of ASGMF for the reproduce of the raise of water level in the storage pond by some experiments without a slope of the embankment in the future.

In recent years, a highly efficient cooling system is eagerly desired because of high heat flux associated with miniaturization of electronic devices and large amount of heat with upsizing of space equipment. The heat removal performance of cooling system using flow boiling, it is necessary to deepen the knowledge on the forced flow phenomenon affected by dissolved gas in the working fluid. The purpose of this study is to clarify the effect of dissolved air on boiling heat transfer and flow behavior in forced flow boiling. Therefore, we conducted a visualization experiment of boiling two-phase flow and measurement heat transfer coefficient. As a visualization result, the frequency of bubble formation due to boiling increased with the increase in the concentration of dissolved air. In addition, the precipitated non condensable gas was observed to form a boiling nucleus.

After the accident at the Fukushima nuclear power plant, filtered containment venting systems (FCVSs) have garnered increasing attention as an effective facility for nuclear accident management. FCVSs with a wet-type scrubber, a droplet separator, and several other stages for fine aerosol filtration are typical used of many countries. Most of these FCVSs comprise a self-priming venturi scrubber nozzle for air injection. An optimized design of the VS (in terms of the throat length, throat size, nozzle diameter, etc.,) may significantly improve the filtration efficiency of the nozzle and wet-scrubber filter. Furthermore, adding a baffle plate on top of the VS also can improve the efficiency of the wet-scrubber filter. In this study, to understand the effects of the baffle plate on the efficiency of the wet-scrubber filter, two VS nozzle configurations were used. The difference between these two configurations is that one of them involved a baffle plate installed above the outlet. The decontamination factors of the wet scrubbers using these two nozzle configurations were measured and compared. The two-phase flow pattern in the pool was observed using a high-speed camera. It is discovered that the baffle plate contributed to increase the efficiency of the wet-pool scrubber. Moreover, the aerated surface exhibits better stabilization when the baffle plate is used.

Weakly nonlinear focusing of quasi-planar ultrasound in a liquid nonuniformly containing many spherical microbubbles is theoretically investigated with a special focus on a thermal conduction at the bubble-liquid interface toward medical applications such as tumor coagulation by HIFU. Based on the previously derived Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation by our group (Kanagawa, J. Acoust. Soc. Am., 137 (2015), 2642), we derived a KZK equation newly incorporating the viscosity of bubbly liquids and the thermal conduction at the bubble-liquid interface by utilizing the energy equation inside bubble. As a result, two types of dissipation term were discovered in the resultant KZK equation: one is the second-order partial derivative term owing to the viscosity of bubbly liquids and the liquid compressibility and the other is a term without differentiation owing to the thermal conductivity. We found that the thermal conduction strongly contributes the dissipation effect.

Weakly nonlinear (i.e., finite but small amplitude) propagation of plane progressive pressure waves in compressible water flow uniformly containing many spherical gas bubbles is numerically investigated with a special attention to a drag force acting bubbles and translation of bubbles. The gas and liquid phases are flowing with initially independent velocities. Drag force and virtual mass force are introduced as interfacial momentum transports. Translation and spherically symmetric oscillations are considered as bubble dynamics. In this paper, under these assumptions, we numerically solve the KdVB (Korteweg-de Vries-Burgers) equation previously derived by ourselves (Yatabe et al., Phys. Fluids, 33 (2021), 033315) from basic equations based on a two-fluid model. The main results are summarized as follows: (i) The drag force acting on bubbles increases a dissipation effect of waves and drastically changes the phase and amplitude of waves. (ii) Although the translation of bubbles increases the nonlinear effect of waves, its contribution to waveform is quantitatively small. (iii) The effect of the drag force decreases with decreasing the initial void fraction and with increasing the initial bubble radius. That of the translation decreases with decreasing the initial void fraction, and is almost independent of the initial bubble radius. (iv) The spatiotemporal evolution of two type of dissipation effects (i.e., dissipation terms) due to the acoustic radiation and to the drag force is different tendency.