Hisato Minagawa

Enhancement of microbubble generation in a pressurized dissolution process by packing the nozzle with porous ceramics

The pressurized dissolution method is often used for microbubble generation. However, the main disadvantage of this method is that a large amount of energy (more than 0.3 MPa) is required to generate many microbubbles, each of which have a diameter of several dozen μm. To overcome this problem, we investigated the effectiveness of porous ceramic when used as the packing material in the pressurized dissolution method. The results showed that when compared with the control (no porous ceramics), use of porous ceramics resulted in a 39% increase in the number of microbubbles. Furthermore, when this system was used for the flotation separation of artificial suspended solids and activated sludge, the level of separation achieved with porous ceramics at 0.15 MPa was the same as that achieved using no porous ceramics at 0.25 MPa. It was estimated that the use of porous ceramics led to a 40% reduction in the energy required for the dissolved air flotation, with subsequent decreases in the operating cost.

Effect of collision and velocity model of lattice Boltzmann model on three-dimensional turbulent flow simulation

ABSTRACT Various collision and velocity models of the lattice Boltzmann model (LBM) were compared to determine their effects on the efficiency of a three-dimensional homogeneous isotropic decaying turbulent flow simulation. We determined that a decrease in the number of velocities, in particular, 13-velocities, which can be used in the quasi-equilibrium lattice Boltzmann and in the multiple-relaxation time models (MRT), could considerably decrease the computational effort. However, decreasing the number of velocities deteriorates the stability and the accuracy of the results. By comparing the collision models, we also determined that the stability of the entropic lattice Boltzmann model (ELBM), and 19- and 27- velocity MRT is much higher than in other models. However, the numerical viscosity introduced by the ELBM underestimates the enstrophy, and the computational effort increases because of the calculation overhead required to solve the additional equations if special care is not given to the calculation.

The Effect of Fine Particles Added Into Water on the Motion of Bubbles Rising Helically

LDV, PIV and some methods using ultrasonic sound have been often employed to measure multiphase flows. Fine particles are usually added into flows as tracer or scattering particles. The effect of particles added in on the flow characteristics is, however, not examined in detail. Because multiphase flows, especially gas-liquid systems, have gas-liquid interfaces, where impurities are known to aggregate, fine particles may aggregate in gas-liquid interfaces, and may affect the flow situation. Therefore, we measured the movement of helically rising bubbles to investigate the effect of fine particles mixed into the liquid phase. Polyethylene particles of 160μm and 10.6μm medium diameters are used. The reductions of helical sizes and rising and moving velocities are recognized by adding particles. The effect of particle size is also discussed.Copyright

Measurement of Liquid Flow Field Using Micro Bubbles Instead of Solid Particles

The authors have studied for the effect of supplying oxygen into water with micro bubbles produced by the shearing method, and found that the micro bubbles are very effective for the oxygen supplying compared with general size bubbles. We also generate micro bubbles which make water cloudy through the pressurizing dissolution method. The average diameter of micro bubbles generated with this method is about 40 micro meters in our laboratory. They are much smaller than those produced by the shearing method, and are the similar size to the tracer or the reflector used to measure the flow field by the PIV or the UVP (Ultrasonic Velocity Profile monitor). Moreover, they are very economical and ecological compared with solid tracer particles. Therefore, we examine to apply micro bubbles generated with the pressurizing dissolution method to the measurement of liquid flow field using the PIV or the UVP. A flow in a square cavity at Re = 1000 was measured using the PIV and the UVP. We could recognize a clear vortex. A flow around a circular cylinder was also measured using the PIV. We can recognize the possibility of micro bubbles for these applications.Copyright

Study on the Flow Pattern of Gas-Liquid Two-Phase Flow in Horizontal and Hilly-Terrain Pipes. 2nd Report. Flow Pattern in Horizontal-Upward Inclined Pipes.

The flow patterns of the air-water two-phase flow in horizontal-upward inclined pipes were studied experimentally in order to obtain basic information on the flow characteristics of gas-liquid two-phase flows in hilly-terrain pipelines. Flow regime maps in horizontal-upward inclined pipes were expressed by the combination of a basic flow pattern and some auxiliary flow patterns. In addition to the effect of inclination angle on the flow pattern, their mutual relation between one in horizontal and others in inclined part was described. Comparisons of the flow regime maps with those obtained in single horizontal or upward inclined pipes without bending part were carried out.

Flow Pattern of Gas-Liquid Two-Phase Flow in a U Bend Pipeline with Upper and Lower Horizontal Pipes.

For next-generation nuclear reactors, hybrid safety systems which consist of active and passive safety systems have been planned. Steam generators using U bend pipelines with upper and lower horizontal pipes will be used as one of the passive safety systems. Liquid is generated due to condensation of steam in pipelines if these pipelines are used as condensers. It is supposed that flow patterns in the horizontal parts of these pipelines are different from those in horizontal pipes without U bend. An experimental study was carried out using a U bend pipeline with upper and lower horizontal pipe which has a lower tank and a water supply section at midway of lower horizontal pipe. Flow patterns were observed at four parts, and their maps were compared with those in the previous studies.

Application of a Two-Phase Flow Model based on Local Relative Velocity to Gas-Liquid-Solid Three-Phase Flow.

In the present study, a two-phase flow model based on local relative velocity, which had been previously proposed by the authors, was extended to a model for gas-liquid-solid three-phase flows. The extension was carried out by utilizing a hypothetical two-phase flow, which is conceived by removing one phase of the three phases. In order to examine the usefulness of the extended model, the measured area-averaged volumetric fractions of gas-liquid-solid three-phase bubbly or slug flows in vertical pipes were correlated based on the basic equations of the model. The accuracy of the obtained correlation was compared with the drift-flux correlation, the correlation based on a multiplier method and the correlation based on a gas-liquid-solid three-phase slug flow model. Consequently, it was confirmed that the extended model gives simpler and more accurate correlations for the area-averaged volumetric fractions of the gas-liquid-solid three-phase flows.

Examination of constitutive equations required for the analyses of solid-liquid two-phase flow in vertical pipes with the two-fluid model.

In order to make an accurate prediction of two-phase flow with a two-fluid model, it is necessary to establish a set of correct constitutive equations. The constitutive equations for the analyses of the solid-liquid two-phase flow in vertical pipes were examined in this study. Seven kinds of equations for interfacial momentum transfer were derived from the constitutive equations used in the gas-liquid two-phase flow analyses. It was confirmed that the most accurate prediction is achieved when Andersens momentum transfer equation is adopted with correlations for the distribution parameter and suspension velocity of solid particles proposed by the authors. The constitutive equation for momentum transfer between a wall and each phase was deduced from the experimental correlation of frictional pressure drop. Calculated results agree well with experimental data with the assumption that there is no momentum transfer between the solid phase and the wall.