Toshiyuki Ogasawara

The effects of surfactant on the multiscale structure of bubbly flows

It is well known that a bubble in contaminated water rises much slower than one in purified water, and the rising velocity in a contaminated system can be less than half that in a purified system. This phenomenon is explained by the so-called Marangoni effect caused by surfactant adsorption on the bubble surface. In other words, while a bubble is rising, there exists a surface concentration distribution of surfactant along the bubble surface because the adsorbed surfactant is swept off from the front part and accumulates in the rear part by advection. Owing to this surfactant accumulation in the rear part, a variation of surface tension appears along the surface and this causes a tangential shear stress on the bubble surface. This shear stress results in the decrease in the rising velocity of the bubble in contaminated liquid. More interestingly, this Marangoni effect influences not only the bubbles rising velocity but also its lateral migration in the presence of mean shear. Together, these influences cause a drastic change of the whole bubbly flow structures. In this paper, we discuss some experimental results related to this drastic change in bubbly flow structure. We show that bubble clustering phenomena are observed in an upward bubbly channel flow under certain conditions of surfactant concentrations. This cluster disappears with an increase in the concentration. We explain this phenomenon by reference to the lift force acting on a bubble in aqueous surfactant solutions. It is shown that the shear-induced lift force acting on a contaminated bubble of 1 mm size can be much smaller than that on a clean bubble.

Surfactant effect on the bubble motions and bubbly flow structures in a vertical channel

It is well known that a small amount of surfactant can drastically change the motion of a single bubble and this causes a dramatic change of the whole bubbly flow structure. In our previous studies using upward vertical channel flows, it was shown that surfactant influences the shear-induced lift and the lateral migration of a bubble, which causes bubble accumulation and clustering near the wall. In this paper, the dependence of surfactant concentration on the motions of a 1 mm bubble rising through the laminar shear flow is investigated using 1-, 3-Pentanol and Triton X-100. The results are compared with the numerical ones, which show quantitative agreement on the lift and drag forces. Furthermore, we analyze the experimental data for the condition of bubble clustering in upward channel flows with the consideration of contaminant level in tap water. The results indicate that lower contaminant level and higher shear rate cause the significant bubble migration toward the wall, which leads to the formation of bubble clusters.

Surfactant effects on single bubble motion and bubbly flow structure

Surfactant effects on single bubble motion in quiescent water and upward bubbly flow in a rectangular channel are investigated. Generally, path instability of the single bubble and the bubble motion in inhomogeneous flow are sensitive to the contamination of water. Addition of surfactant in gas‐water system yields immobilization of the bubble surface due to Marangoni effect. Single bubble 3D trajectories in dilute surfactant solution are measured by two high‐speed cameras. All measured trajectories are plotted on two‐dimensional field of bubble Reynolds number Re and instantaneous boundary slip condition. In free‐slip and no‐slip condition, bubble motions are dependent on Re. However, in half‐slip condition, bubble motions are spiral and almost independent from Re. Bubbles in certain condition move along trajectories changing from spiral to zigzag. These interesting motions are caused by changing slip condition. Bubble motion in upward channel flow is also observed. The local void fraction distribution ch...

Influence of local void fraction distribution on turbulent structure of upward bubbly flow in vertical channel

Turbulent structure of upward dilute bubbly flow with 1 mm bubbles in a vertical channel is investigated experimentally. Small amount of surfactant is added to water to avoid bubble coalescence and to control local void fraction distribution. Liquid phase velocity is measured using two-dimensional laser Doppler Velocimetry. In 1-Pentanol solution of 20 ppm, bubbles have half-slip surface and migrate strongly toward the channel wall due to the shear-induced lift force which leads to wall-peaked distribution of local void fraction. On the other hand, in Triton X-100 solution of 2 ppm, bubbles become fully-contaminated and do not migrate toward the wall or the channel centre due to near-zero lift force, causing uniform distribution of local void fraction in the wall-normal direction. Once bubbles accumulate near the wall, transport of turbulent energy produced by the wall shear towards the channel centre is blocked. Then turbulence induced by the bubble motion becomes dominant in a wide core region (so-called pseudo turbulence). By contrast, in the case of the uniform distribution of bubbles, a mechanism of a turbulent energy transport which is the same as that of a single-phase turbulence still exists and furthermore the bubble-induced turbulence is added on it.

Experimental and numerical investigations of the bubble collapse at the center between rigid walls

The collapsing behavior of a bubble generated at the center of the rigid walls and its effect on the wall are investigated experimentally and numerically. The bubble collapse drastically depends on the ratio of the gap width to the maximum bubble radius w*. In case of 2.66 < w* < 1.17, by the decrease in w* to 2.5, the splitting collapse occurs; the bubble splits into two at the center of the gap during collapse and each bubble collapses near the wall with the translation toward nearer wall. Further decrease in w* to 1.4 causes neutral collapse where the bubble collapses at the center of the gap without splitting and translation. These collapsing behaviors are successfully simulated by considering the bubble shape at the maximum bubble volume. The peak pressure on the wall decreases by the transition of the collapsing behavior from the splitting collapse to the neutral collapse due to the decrease in w*, which indicates the wall damage reduction due to the neutral bubble collapse in thinner gap.

Observation of the growth of cavitation bubble cloud by the backscattering of focused ultrasound from a laser-induced bubble

The present study is concerned with the cavitation inception and the growth of a cavitation bubble cloud by the backscattering of focused ultrasound from a laser-induced bubble. The cavitation inception close to the interface of a laser-induced bubble has been observed by a high-speed video camera with the frame rate of up to 1.25 Mfps, and its location and the successive cavitation cloud growth are discussed for various ultrasound conditions. It is shown that the normalized distance between the cavitation inception location and the bubble interface by the wavelength of ultrasound is an increasing function of η = t0 / ts where the time t0 is the characteristic time for cavitation bubble collapse and the time ts the period of ultrasound. Also the magnitude of the dimensionless distance is about 0.05-0.3 times of the wavelength of ultrasound, and the positive pressure threshold of ultrasound for a cavitation inception is about 35 MPa. It is also shown that the cavitation bubble cloud by the backscattering of the incident ultrasound grows conically along the propagation axis of the focused ultrasound. As the incident focused ultrasound pressure at the focus becomes stronger or the duration of the focused ultrasound becomes longer, the cavitation bubble cloud grows larger. However, even though the ultrasound duration becomes longer, this growth ends up and reaches a limited value when the cavitation bubble cloud grows out of the focal region of the focused ultrasound.

Cavitation inception by the backscattering of pressure waves from a bubble interface

The secondary cavitation that occurs by the backscattering of focused ultrasound from a primary cavitation bubble caused by the negative pressure part of the ultrasound (Maxwell, et al., 2011) might be useful for the energy exchange due to bubble oscillations in High Intensity Focused Ultrasound (HIFU). The present study is concerned with the cavitation inception by the backscattering of ultrasound from a bubble. In the present experiment, a laser-induced bubble which is generated by a pulsed focused laser beam with high intensity is utilized as a primary cavitation bubble. After generating the bubble, focused ultrasound is emitted to the bubble. The acoustic field and the bubble motion are observed with a high-speed video camera. It is confirmed that the secondary cavitation bubble clouds are generated by the backscattering from the laser-induced bubble. The growth of cavitation bubble clouds is analyzed with the image processing method. The experimental results show that the height and width of the bubb...