Research topics I have been working for the last 27 years related to rising bubbles are reviewed with the emphasis of surfactant effect on the multiscale structure of bubbly flows. Small amounts of surfactant can drastically change single bubble behaviors. For example, a bubble in aqueous surfactant solution rises much slower than one in purified water. This phenomenon is explained by the so-called Marangoni effect caused by a non-uniform concentration distribution of surfactant along the bubble surface. More interestingly, this Marangoni effect influences not only the rising velocity, but also the lateral migration in the presence of mean shear. Furthermore, these phenomena influence the multiscale nature of bubbly flows and cause a drastic change in the bubbly flow structure. In this article, these interesting phenomena are reviewed for the celebration of 30th anniversary of the Japan Society of Multiphase Flow

The development of numerical simulation models and methods in gas-solid two-phase flows by the author' s group in the last three decades is briefly reviewed. Eulerian-Lagrangian coupling simulations have been largely developed, especially DEM-CFD coupling simulation has become popular, and widely used in many operations of powder technology field. The establishment of the Japan society for multiphase flow played a very important role in the development of DEM-CFD coupling model.

This paper presents mainly a review of measuring techniques for multiphase flows in the past 30 years and an outlook of future progress on distributed measuring systems. Laser based measuring system had started by point measurement such as a probe, laser Doppler, and Phase Doppler anemometry, and then extended to Particle Imaging techniques to obtain 2D plane information. Recently, high speed digital cameras allowed us to get 3D3C temporal velocity information. The possibility of IoT based distributed sensing system is discussed with an application of multiphase flow analysis for future developments.

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.

This study was performed to verify the possibility that microbubbles with larger size (100 μm more or less in diameter) are involved in “retention of DO saturation”. The results revealed that all bubbles measured were contraction type, contributing to O₂ dissolution in water: in particular, microbubbles with larger size were long-life. In addition, lifetime of the bubbles was highly sensitive to seawater salinity, and among the salinities examined, 35‰ (normal seawater salinity) was the most long-lived. These results strongly suggest that microbubbles with larger size are closely related retention of DO saturation.

Minute bubbles have various uses. One of which is water purification. In water purification, additional pumps are necessary to diffuse microbubbles widely throughout the water besides a microbubble generator. Due to this, the consumption of electricity is high. In the previous study, a bladeless mixer was focused on as a new minute bubbles generator for water purification. The bladeless mixer has a very simple shape which is a half-sphere with some holes through it. The mixing is carried out by a jet from each outlet by centrifugal force. Then, as air is sucked from an aeration hole on each a flow channel, small bubbles can be generated. But the distribution and average diameter of the generated bubbles have not been clarified. In the present study, the distribution and average diameter of generated bubbles were investigated. Also, the effect of the shape of an aeration hole and the outlet channel on the diameter of generated bubbles was investigated for fragmentation of generated bubbles. In the experiment, the distribution and average diameter of generated bubbles were measured for 4 aeration types which have different diameter and numbers and for 7 outlet channel types which have step, change of flow path and so on. The conclusions are as follows: (1) The average diameter of bubbles generated by a bladeless mixer is about 140~200 μm and the percentage of microbubbles in the water tank is around 40 %. (2) The minute bubbles generated by high shear force on the outlet caused by rotation. (3) The average diameter of bubbles became small using a smaller aeration hole but there is no effect caused by changing number of aeration holes. (4) The shape of the outlet channel influenced the diameter of the generated bubbles. Especially, the average diameter of bubbles decreased about by 10 % with the wall type and edge wall type which have a wall near the outlet causing an impact on the air column.

In the present study, the characteristics of the floatation thickening of activated sludge by the method of sludge supply after decompression of activated sludge were investigated. In the experiment, the process of floatation thickening of activated sludge was observed in the range of absolute pressure P_v = 11 ~71 kPa in the test tank and the range of MLSS=2000, 2500, 3000 and 3500 ppm. And the estimated equations and performance index were suggested. The conclusions are as follows: (1) The absolute pressure range which cloud be complete flotation thickening is P_v=21~51 kPa. (2) The height of supernatant liquor increased (thickness of concentrated sludge decreased) with time progress, dramatically, and closed to the certain constant height (final height of supernatant liquor and final thickness of concentrated sludge). (3) The dimensionless final height of supernatant liquor increases as the P_v is lower and MLSS is bigger. (4) Completion time of thickening is shorter as the P_v is lower and the influence of the MLSS is very small. (5) Inverse number of concentrated phase density (dimensionless final thickness of concentrated sludge per unit MLSS) and completion time of thickening is approximately quadratically increase with P_v / P₀ (P₀: atmospheric pressure). (6) Performance index of floatation thickening by decompression is suggested, the performance is higher as P_v is lower.

We confirmed the effect of organic material adhesion as ultrafine bubble(UFB) stabilization mechanism. Organic material was added to generate UFB,and the state of UFB particles was investigated by dynamic light scattering method, transmission electron microscope (TEM), resonance mass measurement method. Disappearance of UFB was suppressed by the addition of organic material, and it was confirmed that the organic material adhered to the surface of UFB. We got the conclusion that adhesion of organic material inhibit gas dissolution from bubbles.