Yasuhiro Kamotani

Oscillatory thermocapillary convection in a simulated floating-zone configuration

Abstract Oscillatory thermocapillary flows in floating zones under reduced gravity conditions are investigated experimentally in a simulated configuration in which a drop of liquid is suspended vertically between two small circular rods which are maintained at different temperatures. Various zone lengths and diameters and two different fluids are tested to study their effects on the oscillatory convection. The onset of oscillations, free surface temperature distributions and flow structure are investigated. Based on the results it is explained that flexibility of the free surface is an important factor in causing the oscillation.

Experimental study of natural convection in shallow enclosures with horizontal temperature and concentration gradients

Abstract Natural convection in low aspect-ratio rectangular enclosures with combined horizontal temperature and concentration gradients is studied experimentally. An electrochemical system is employed to impose the concentration gradients. The solutal buoyancy force either opposes or augments the thermal buoyancy force. Due to a large difference between the thermal and solutal diffusion rates the flow possesses double-diffusive characteristics. Various complex flow patterns are observed with different experimental conditions. The temperature distributions and mass-transfer rates are also studied. A flow instability under certain conditions is reported.

Free surface heat loss effect on oscillatory thermocapillary flow in liquid bridges of high Prandtl number fluids

Abstract The effect of free surface heat loss on oscillatory thermocapillary flow is investigated in liquid bridges of high Prandtl number fluids. It is shown experimentally that the critical temperature difference changes by a factor of two to three by changing the air temperature relative to the cold wall temperature. In order to understand the nature and extent of the interaction between the liquid flow and the surrounding air, the heat transfer from the liquid free surface is investigated numerically for the conditions of the present experimental work. The airflow analysis shows that even when the heat loss is relatively weak (the Biot number is unity or smaller), the critical temperature difference is affected appreciably. It is shown that the heat loss effect is significant in widely conducted tests near room temperature and that the critical temperature difference is much larger than the room temperature value when the heat loss is minimized. The analysis suggests that an interaction between the surface heat loss and dynamic free surface deformation near the hot wall is responsible for the observed heat loss effect.

An experimental study of oscillatory thermocapillary convection in cylindrical containers

An experimental study of oscillatory thermocapillary convection in small cylindrical containers with a heating wire placed along the center axis is performed by investigating the flow structures and temperature distributions under various conditions. To supplement the flow visualization the surface is scanned using an infrared imager. Here 2 cS viscosity (Pr=27) silicone oil is used as the test fluid. It is observed that beyond a certain temperature difference between the container wall and the heating wire, a distinctive unsteady flow pattern appears. This unsteady phenomenon is identified as oscillatory thermocapillary. After the onset of oscillations the flow structure becomes nonaxisymmetric and wave motion is observed at the free surface. It is shown that the critical temperature difference is independent of container dimensions if the aspect ratio is fixed.

Microgravity experiments and analysis of oscillatory thermocapillary flows in cylindrical containers

Results are reported of thermocapillary flow experiments performed aboard the Spacelab. Oscillatory thermocapillary flows were investigated in open cylindrical containers filled with 2 cS kinematic viscosity (Prandtl number = 27 at 25 °C) silicone oil. The fluid was heated by a cylindrical cartridge heater placed at the symmetry axis of the container while the container sidewall was maintained at a lower temperature. Test containers with three different diameters of 1.2, 2.0 and 3.0 cm were used. The ratio of heater to test container diameter was fixed at 0.1. The liquid free-surface shape was either flat or concave. The flow and temperature fields were investigated for steady and oscillatory flows. Free-surface deformation was observed during oscillations. The conditions for the onset of oscillatory flow were determined. It is shown that the Marangoni number alone does not correlate the onset conditions. A new parameter, which represents free surface deformation, is derived for flat free surfaces and is shown to correlate the onset conditions well. Infrared images of free surface and oscillation frequencies are also presented.

Analysis of velocity data taken in Surface Tension Driven Convection Experiment in microgravity

Some velocity field results from the Surface Tension Driven Convection Experiment (STDCE) that was conducted aboard the USML‐1 Spacelab in 1992 are reported. 10 cSt silicone oil was placed in an open circular container (10 cm wide×5 cm deep) and heated either by a cylindrical heater (1.11 cm diam) placed along the centerline or by a CO2 laser to induce thermocapillary flow. Tests were conducted under varieties of powers, laser beam diameters, and free‐surface shapes. The flow field was studied by flow visualization and the data were analyzed by a PIV technique. The results from the velocity measurement are presented and the effects of heating mode and free‐surface shape on the flow are discussed. The results are also compared with a numerical analysis conducted in conjunction with the experiment. Good agreement is shown.

Oscillatory thermocapillary flows

Abstract Oscillatory thermocapillary flows are experimentally investigated for half-zone, floating-zone, rectangular, and cylindrical configurations for a variety of operating conditions. The onset conditions for oscillations, free-surface temperature distributions and flow structures are determined and compared to indicate the differences among them. Silicone oils are the test fluids. In some cases numerical analyses supplement the experimental results. Indications of the factors involved in causes of the oscillations are presented.

A thermocapillary convection experiment in microgravity

Results are reported of the Surface Tension Driven Convection Experiment (STDCE) aboard the USML-1 Spacelab, which was launched on June 25, 1992. In the experiment, 10 cSt silicone oil was placed in an open 10-cm-dia circular container, which was 5 cm deep. The fluid was heated either by a cylindrical heater (1.11 cm diameter) located along the container centerline or by a CO{sub 2} laser beam to induce thermocapillary flow. Several thermistor probes were placed in the fluid to measure the temperature distribution. The temperature distribution along the liquid-free surface was measured by an infrared imager. Tests were conducted over a range of heating powers, laser-beam diameters, and free surface shapes. An extensive numerical modeling of the flow was conducted in conjunction with the experiments. Some results of the temperature measurements with flat free surfaces are presented in this paper and they are shown to agree well with the numerical predictions. 25 refs., 12 figs.

Bubble formation from wall orifice in liquid cross-flow under low gravity

Abstract Two-phase flows present a wide variety of applications for spacecraft thermal control systems design. Bubble formation and detachment is an integral part of the two-phase flow science. The objective of the present work is to experimentally investigate the effects of liquid cross-flow velocity, gas flow rate, and orifice diameter on bubble formation in a wall–bubble injection configuration. Data were taken mainly under reduced gravity conditions but some data were taken in normal gravity for comparison. The reduced gravity experiment was conducted aboard the NASA DC-9 Reduced Gravity Aircraft. The results show that the process of bubble formation and detachment depends on gravity, the orifice diameter, the gas flow rate, and the liquid cross-flow velocity. The data are analyzed based on a force balance, and two different detachment mechanisms are identified. When the gas momentum is large, the bubble detaches from the injection orifice as the gas momentum overcomes the attaching effects of liquid drag and inertia. The surface tension force is much reduced because a large part of the bubble pinning edge at the orifice is lost as the bubble axis is tilted by the liquid flow. When the gas momentum is small, the force balance in the liquid flow direction is important, and the bubble detaches when the bubble axis inclination exceeds a certain angle.

Oscillatory thermocapillary flows in open cylindrical containers induced by CO2 laser heating

Abstract Oscillatory thermocapillary flow experiments were conducted in microgravity. Silicone oil was placed in cylindrical containers and heated at the center by a CO 2 laser beam. The main objectives were to determine the onset of oscillatory thermocapillary flow and to study the important features of the oscillatory flow. Numerical and scaling analyses were also performed to understand the basic steady flows. The onset conditions show that the oscillation phenomenon cannot be explained if the fluid-free surface is assumed to be undeformable. Therefore, a parameter representing free surface deformation is derived. The oscillation patterns and frequencies are also presented.