Atsuhide Kitagawa

Two-way coupling of Eulerian–Lagrangian model for dispersed multiphase flows using filtering functions

Abstract Eulerian–Lagrangian approaches for dispersed multiphase flows can simulate detailed flow structures with a much higher spatial resolution than the Eulerian–Eulerian approaches. However, there are still unsolved problems regarding the calculation method for accurate two-way interaction, especially on the numerical instability due to the dispersion migration through discrete computational grids. Inadequate solvers sometimes produce false velocity fluctuation which makes the simulation unstable. In this paper, a new calculation method for dispersion-to-continuous phase interaction, which is accompanied by spherical dispersion migration, is proposed. The basic principle of the method is the introduction of Lagrangian filtering functions which convert discrete dispersion volume fractions to a spatially differentiable distribution. The performance of linear, Gaussian and sinewave filtering functions is examined by simple benchmark tests and applied to the simulation of dispersion-generated fluctuation. Using the present method, three-dimensional continuous phase flow structures induced by rising spherical bubbles and/or settling solid particles are demonstrated.

Measurement of turbulence modification by microbubbles causing frictional drag reduction

In this research, in order to clarify the mechanism for drag reduction caused by microbubbles, the turbulence structure of flow field including microbubbles in a horizontal channel is experimentally investigated using particle tracking velocimetry and laser induced fluorescence (PTV/LIF) technique. First, we discuss the particle image velocimetry (PIV) and PTV results for the liquid phase velocity detection. Second, using instantaneous measurement data, we obtain the profiles of the mean velocity, turbulence intensity and Reynolds stress of the liquid phase. In order to obtain the information of both the liquid and gas phases simultaneously, furthermore, we propose a new system based on the combination of PTV, LIF and infrared shadow technique (IST).Copyright

Particle Tracking Velocimetry Measurement of Bubble-Bubble Interaction

Bubble-bubble interaction is a quite fundamental issue to understand multiphase flow dynamics and to improve mathematical models of dispersed multiphase flow for higher volume fraction of dispersion. In this study, the bubble-bubble interaction is measured using Particle Tracking Velocimetry (PTV) in various environments. First, bubbles sliding on a vertical wall are measured using 2-D PTV. Second, the free rising bubbles in an unbounded space are measured applying 3-D PTV. Third, the simultaneous measurement for gas and liquid phases in the layer of wall-sliding bubbles is carried out. The measurement data have shown that the average bubble-bubble interaction patterns in the wall-sliding bubbles and in the free rising bubbles were attractive in the vertical direction and repulsive in the horizontal direction. The relation between the carrier phase flow structure and the bubbles’ motion is detected to explain the mechanism of the bubble-bubble interaction.Copyright

Visualization of counter-current convection induced by microbubbles and small particles

We use visualization to investigate the structure of the counter-current convection induced by microbubbles and small particles. In particular, we study the effect of small particles with different specific gravities on the gas-phase flow. In our experiments, microbubbles are injected into stationary liquid from a bubble generator that is set at the bottom of a vertical channel, and small particles are injected from a particle injector that is set at the top of the channel. The mean kinetic energy of the gas-phase is significantly lower in the flow with microbubbles and small particles than in the flow with only microbubbles. This results from significant suppression of the interaction of bubble plumes by the small particles. Moreover, the mean kinetic energy of the gas-phase in the flow with microbubbles and small particles is strongly dependent on the specific gravity of the small particles.Graphical Abstract

Effects of Local Concentration on Freezing Solutions of Winter Flounder Antifreeze Protein

We have carried out experiments on the one-directional freezing of an aqueous solution of winter flounder antifreeze protein in a narrow gap between two cover glasses. The motion of the ice/solution interface has been observed with an inverted microscope. The solution has been cooled by a Peltier device. The local change in protein concentration has been estimated from the measured intensity of fluorescence from molecules tagged to the protein. It is found that highly-concentrated regions of the protein can be observed in the bottom edge of the serrated interface. These regions interact with the interface, though most of the protein diffuses due to the concentration gradient. The diffusion velocity is much lower than the interface velocity. Thus, the protein is accumulated near the interface.Copyright

Experimental Investigation of Turbulence Transition of Natural Convection Boundary Layer Along a Vertical Plate in Water With Sub-Millimeter-Bubble Injection

This paper presents an experimental investigation of the turbulence transition of the natural convection boundary layer along a vertical plate in water with sub-millimeter-bubble injection. In this study, we focus on the relationship between the bubble injection position L and the turbulence transition of the boundary layer. Temperature and velocity measurements show that sub-millimeter-bubble injection for L = 1.6 mm suppresses the turbulence transition of the natural convection boundary layer, while that for L = 3.6 mm enhances the turbulence transition of the boundary layer. For L = 1.6 mm, the appearance region of the bubble-induced liquid velocity fluctuation at the upstream unheated section is restricted near the wall, though the peak value of the liquid velocity fluctuation is high. In contrast, in the case of L = 3.6 mm, the relatively large liquid velocity fluctuation induced by bubbles at the upstream unheated section distributes widely over the laminar boundary layer width. Therefore, we expect that the turbulence transition of the natural convection boundary layer for the case with bubble injection depends on the magnitude and appearance region of the bubble-induced liquid velocity fluctuation at the upstream unheated section.© 2011 ASME

Heat Transfer Enhancement of Natural Convection Along a Vertical Heated Plate by Microbubble Injection

This paper describes the heat transfer enhancement of natural convection along a vertical heated plate due to injection of microbubbles. Thermocouples are used for the temperature measurement and an image processing technique is used for obtaining the bubble diameter and the bubble layer thickness. The working fluid used is tap water, and hydrogen bubbles generated by electrolysis of the water are used as the microbubbles. The mean bubble diameter dm ranges from 26 to 57 μm. For each of the laminar and transition regions, the significant heat transfer enhancement is caused by the microbubble injection. Under a constant bubble flow rate (Q = 42 mm3 /s), in the laminar region, the heat transfer coefficient for dm = 39 μm is higher than that for dm = 57 μm, but it is vice versa at x = 770 mm (transition region). Under a constant bubble size (dm = 39 μm), at each measurement position, the heat transfer coefficient for Q = 42 mm3 /s is higher than that for Q = 30 mm3 /s. These are deeply related to the fluctuation of the bubble layer thickness and small-scale eddy motions inherent in the flow. Moreover, in the case of dm = 39 μm and Q = 30 mm3 /s, the heat transfer gain (which is the ratio of the heat transfer rate obtained with the microbubble injection to the power consumption of the mirobubble generation) is approximately 33. Therefore, microbubble injection is a very highly efficient technique for enhancing the natural convection heat transfer of water along a vertical flat plate.Copyright

Motion of Descending Solid Particles and Local Flow Around the Particles in Downward Turbulent Water Duct Flow (Keynote)

The Stokes number, the ratio of the particle time scale to flow time scale, is a promising quantity for estimating changes in statistics of turbulence due to particles. First, we explored the Stokes numbers in some recent studies. Secondly, we discussed the results of our direct numerical simulation for turbulent flow with a high-density particle in a vertical duct. In the discussion, we defined the particle Reynolds number from the mean fluid velocity in the near-particle region at any time. We evaluated a new local Stokes number for the particle. It is found that the Stokes number is effective for the prediction of the distance between the particle center and one wall. Finally, we carried out experiments for turbulent water flow with aluminum balls of 1 mm in diameter in a vertical channel. The motions of aluminum balls and tracer particles in the flow were captured with a high-speed video camera. We found that the experimental results for the time changes in the wall-normal distance of the ball and the particle Reynolds number for the ball are similar to the predicted results.Copyright

Effects of Bubble Size on Heat Transfer Enhancement for Laminar Natural Convection by Sub-Millimeter Bubbles

Sub-millimeter-bubble injection is one of the most promising techniques for enhancing heat transfer for the natural convection of liquids. So far, we have investigated experimentally the heat transfer enhancement by sub-millimeter bubbles. However, the effects of the bubble diameter on the heat transfer have not yet been understood. The purpose of this study is to clarify the effects of bubble diameter on the heat transfer enhancement by sub-millimeter bubbles for the laminar natural convection of water along a vertical heated plate. We conduct temperature measurements using thermocouples and velocity measurements using a Particle Tracking Velocimetry (PTV) technique. We also carry out two-dimensional numerical simulations to comprehensively understand the effects of bubble injection on the flow near the heated wall. Temperature measurements show that the heat transfer coefficient increases with a decrease in the bubble diameter. This is due to a significant increase in the bubble advection effect.

PTV measurement using non-tracer particle and the inverse analysis

In ordinary PTV (Particle Tracking Velocimetry), tracer particles with the good traceability have been used. However, in the field of multiphase flow, there are needs which want to analysis non-tracer particles, such as bubbles and particles with specific gravity differenced from working fluid and a large diameter. Therefore, it is necessary to discuss on the traceability of non-tracer particle. In this study, the traceability for these non-tracer particles is estimated by using Lagrangian method, which directly uses the translation momentum equation of a non-tracer particle. Also velocity vectors of working fluid for the non-tracer particle are obtained by inverse analysis, these force components acted on the non-tracer particle are shown.