Ichiro Ueno

Oscillatory and chaotic thermocapillary convection in a half-zone liquid bridge

Thermocapillary-driven convection in a half-zone liquid bridge was investigated experimentally. The induced flows were categorized into several regimes mainly through the pattern of the suspended particle motion in the bridge and the surface temperature variation. Special attention was paid to the flow structures far beyond the critical condition. Chaotic and turbulent flows were realized in this configuration. They were distinguished from the periodic oscillatory flow by applying the pseudo-phase-space reconstruction from the time series of the surface temperature variation.

Flow structure and dynamic particle accumulation in thermocapillary convection in a liquid bridge

Thermocapillary convection is induced in a liquid bridge by a nonuniform surface tension distribution caused by an axial temperature difference. A toroidal vortex is formed by the thermocapillary force over the free surface. The induced flow is visualized by using fine particles as tracers. At a sufficiently high Marangoni number, three-dimensional standing and traveling oscillatory flows appear, and under certain flow conditions, the tracer particles form particle accumulation structures (PAS). In the present study, the conditions for the occurrence of PAS have been carefully investigated with focus on the spiral loop PAS (SL-PAS) that appears when the flow exhibits a traveling mode. The particles gather along a closed spiral loop that winds itself around the toroidal vortex. Observed from above, the spiral loop looks as if it is rotating azimuthally. The number of spirals corresponds with the azimuthal wave number of the traveling wave and each spiral consists of either single or double turns. The azimu...

Effect of ambient fluid flow upon onset of oscillatory thermocapillary convection in half-zone liquid bridge

Onset of the oscillatory thermocapillary convection in a half-zone liquid bridge is known to be sensitive to heat transfer at free surface of the liquid bridge and the ambient air motion. The effect of the heat transfer through the free surface upon the onset of the transition mechanism and three-dimensional nature of the oscillatory flow, however, is not yet fully understood. In the present study, the thermal fluid behavior of the ambient gas and its effect upon the criticality were investigated experimentally and numerically. In the experiment, flows in the liquid bridge and the ambient air were visualized by suspending tracer particles in both fluids. Volume of the ambient air region was adjusted by placing two partition disks perpendicular to the liquid bridge. The onset of oscillation depended on the distance between the partition disks; the critical Marangoni number increased with the decreasing distance. Three-dimensional simulation of the liquid bridge and the ambient air has been performed by the finite difference method in order to determine the onset of the oscillation. The present calculations demonstrate significant influence of the heat transfer at the free surface on the onset of oscillation. The results of calculations yield a good agreement with the experimental critical values.

Structure and dynamics of particle-accumulation in thermocapillary liquid bridges

The accumulation of small mono-disperse heavy particles in thermocapillary liquid bridges is investigated experimentally and numerically. We consider particle accumulation near the center of the toroidal vortex, the so-called toroidal core of particles (COP), and the particle-depletion zone near the axis of the liquid bridge. Based on the acceleration and deceleration of the tangential flow along the thermocapillary free surface it is argued that the interaction of the particles with the free surface is of key importance for the fast particle accumulation within a few characteristic momentum diffusion times. The experimentally determined particle-accumulation times are compared with time-scale estimates for accumulation due to either particle free-surface interaction or due to inertia of particles which are heavier than the liquid. We show that the experimental accumulation times are compatible with the accumulation times predicted by the particle–free-surface interaction (PSI) while the time-scale estimates based on the inertia of the particles are too large to explain the fast de-mixing observed in experiments. The shape of the COP resembles certain KAM tori of the incompressible flow of a hydrothermal wave. Two scenarios are proposed to explain the structure and the dynamics of the COP depending on the existence or non-existence of suitable KAM structures. The shape of the experimental particle-depletion zone agrees well with the release surface which is defined by the particle–free-surface interaction process. The favorable comparison of the dynamics and structure of experimental and numerical accumulation patterns provides strong evidence for the existence and relevance of the PSI as the most rapid physical accumulation mechanism.

Behavior of vapor bubble in subcooled pool

Behavior of growing/collapsing vapor bubble ejected to subcooled liquid bath was focused. Test fluid was purified water in the present study. Vapor bubble was produced inside the heated tube of 1.5 mm in inner diameter by supplying the test fluid to the tube by microsyringe. This system enabled the authors to observe interaction between the vapor and the liquid in the condensing process extracted from the boiling phenomenon consisting of liquid-gas-solid interactions. The bubble behavior was detected by employing a high-speed camera with up to 100,000 fps. The instability emerged over the surface of the growing and collapsing vapor is discussed as functions of the degree of subcooling, and the temperature and ejection speed of the vapor. The present study aims to understand and control the micro-bubble emission boiling known as MEB.

Space experiment on the instability of Marangoni convection in large liquid bridge - MEIS-4: effect of Prandtl number -

Microgravity experiments on the thermocapillary convection in liquid bridge, called Marangoni Experiment in Space (MEIS), are carried out in KIBO of ISS. Three series of experiments, MEIS-1, 2, and 4, have been conducted so far. This paper reports the results obtained from MEIS-4, in which 20cSt silicone oil (Pr = 207) is used to generate large liquid bridges. They are suspended between coaxial disks that are 50mm in diameter, with their maximum length equal to 62.5mm. MEIS-4 aims at (1) determining the critical temperature difference for the onset of oscillatory flow; (2) realizing high Marangoni number conditions for high Pr fluid; (3) clarifying the effects of volume ratio, heating rate, hysteresis, and cooled disk temperature; and (4) observing whether the hydrothermal wave with azimuthal mode number m = 0 appears or not. The main results are presented and compared with those obtained in MEIS-1 and 2, which utilized liquid bridges of 5cSt silicone oil (Pr = 67).

Instability and associated roll structure of Marangoni convection in high Prandtl number liquid bridge with large aspect ratio

This paper reports the experimental results on the instability and associated roll structures (RSs) of Marangoni convection in liquid bridges formed under the microgravity environment on the International Space Station. The geometry of interest is high aspect ratio (AR = height/diameter ≥ 1.0) liquid bridges of high Prandtl number fluids (Pr = 67 and 207) suspended between coaxial disks heated differentially. The unsteady flow field and associated RSs were revealed with the three-dimensional particle tracking velocimetry. It is found that the flow field after the onset of instability exhibits oscillations with azimuthal mode number m = 1 and associated RSs traveling in the axial direction. The RSs travel in the same direction as the surface flow (co-flow direction) for 1.00 ≤ AR ≤ 1.25 while they travel in the opposite direction (counter-flow direction) for AR ≥ 1.50, thus showing the change of traveling directions with AR. This traveling direction for AR ≥ 1.50 is reversed to the co-flow direction when t...

Effect of shape of HZ liquid bridge on particle accumulation structure (PAS)

We focus on the dynamic particle accumulation structure (PAS) due to thermocapillary effect in a half-zone liquid bridge. Effects of shape of liquid bridge upon shape of the PAS itself and motion of particle on the PAS are discussed in the present study by tracking particles in the liquid bridge and by measuring temperature over the free surface. It is found that the variation of the shape of the liquid bridge leads to a significant variation of the temperature gradient on the free surface, which results in difference of the shape of the PAS. The variation of the PAS shape is mainly explained by drastic change of the axial velocity of the particle and less change of its azimuthal velocity near the free surface.

Thermal-Fluid Phenomena Induced by Nanosecond-Pulse Heating of Materials in Water

Thermal-hydraulic phenomena adjacent to the liquid metal-water and solid material-water interfaces induced by nanosecond pulsed Nd:YAG laser (wavelength: 532 nm. FWHM: ∼13 ns) heating with the fluence F of 5.0×10 1 ∼1.0×10 3 mJ/cm 2 were experimentally investigated. By applying the high-speed photography with a frame speed up to 2.0×10 7 fps, the aspects of the bubble formation, shock wave generation and propagation were observed. The bubble formation on the heated materials surface of about 80 nm in diameter was detected in Si-water system from the time-resolved reflection (TRR) signal by applying the pump and probe method

Space Experiment of Marangoni Convection on International Space Station

This paper reports some important results obtained from a series of microgravity experiments on the Marangoni convection that takes place in liquid bridges. This project, called Marangoni Experiment in Space (MEIS), started from August 22, 2008 as the first science experiment on the Japanese Experimental Module “KIBO” at the ISS. Two series of experiments, MEIS-1 and 2, were conducted in 2008 and 2009, respectively. The experimental methods used are explained in some detail. The maximum size of the liquid bridge that could be realized during these experiments was 30 mm in diameter and 60 mm in length, giving an aspect ratio of 2.0. The results are obtained for a wide range of aspect ratios of the liquid bridges, including the values that cannot be reached in 1g experiments, and therefore, they provide indispensable amount of data for the study of instability mechanisms of the Marangoni convection.Copyright