Thermocapillary Convection In Liquid Bridge Research Articles

The Payload Development and the Experiments for Studying Thermocapillary Convection in TG-2 Liquid Bridge

The development of space experiment payload for studying thermocapillary convection in the liquid bridge with large Pr number on TG-2 space laboratory as well as the experiments are presented in detail in this paper, and the objectives of the space experiments are confirmed. The functions of the payload are analyzed, and the technical and engineering specifications are determined. Detailed designs and experimental verifications are performed on the structure of liquid bridge columns, the method of bubble removing in the liquid, the bridge cleaning system, the accurate control of aspect ratio and volume ratio, and the high-sensitivity measurement of fluid temperature. Matching experiments on the ground according to space experiment properties are carried out, 5cSt silicone oil is selected as the fluid medium in space experiments. And the states of liquid bridge and temperature oscillation signals obtained from space experiments are presented at the end of this paper. Specific summarizations and discussions to the experiment project on fluid science in space are conducted in this paper, which will provide a useful reference for scientists participating space experimental research in the future.

A new cognition on oscillatory thermocapillary convection for high Prandtl number fluids

A direct numerical simulations on the oscillatory thermocapillary convection in a non-axisymmetric liquid bridge of high Prandtl number fluids under normal gravity has been conducted by using a new method of mass conserving level set method for capturing any micro-scale migrations of free surface. Against the former studies, the oscillatory behaviors of surface flow (the perturbation of velocity, temperature, and free surface) and flow pattern have been quantitatively investigated simultaneously for the first time. The present results show that the instability of thermocapillary convection originates from the oscillations of velocity, temperature, and free surface at the hot corner. The velocity oscillation responds slowly to the temperature oscillation, which are opposite in transfer direction for each other, resulting in the free surface oscillation. The oscillatory thermocapillary convection in the liquid bridge is eventually ex-cited by the coupling effects of these three kinds of oscillations, which discloses clearly the oscillatory mechanism of thermocapillary convection for high Prandtl number fluids.

Numerical study on the effects of convective and radiative heat transfer on thermocapillary convection in a high-Prandtl-number liquid bridge in weightlessness

This paper reports the effects of interfacial heat transfer due to convection and radiation on thermocapillary convection in a liquid bridge of high-Prandtl-number fluids (i.e., Pr = 67) in weightlessness. A liquid bridge is placed inside a cylindrical chamber filled with argon gas, and the two-dimensional (i.e., axisymmetric) time-independent flow and temperature fields in the liquid bridge and in the ambient gas are numerically simulated. The effect of interfacial heat transfer is investigated by providing forced ambient gas flow and by changing the temperature of the chamber wall. It is confirmed that convective heat transfer is more sensitive to the ambient gas motion than to the chamber wall temperature, whereas radiative heat transfer shows the opposite tendency. The primary component of the heat transfer on the liquid bridge free surface is determined to be convection for many cases. However, when the directions of gas flow and thermocapillary flow coincide, the radiative component can exceed the convective component even under conditions with strong ambient gas flow. The interfacial heat transfer strongly affects the flow and temperature fields of thermocapillary convection in a liquid bridge not only in the vicinity of the free surface but also in the interior region. The present study is expected to contribute to the upcoming microgravity experiment called JEREMI, which focuses on the effect of interfacial heat transfer on thermocapillary convection in a high-Pr liquid bridge.

Space experimental study on wave modes under instability of thermocapillary convection in liquid bridges on Tiangong-2

We carried out space experiments on thermocapillary convection in liquid bridges with large Prandtl number in the Tiangong-2 space laboratory, studying the influence of geometrical parameters, including aspect ratio (Ar, height to diameter) and volume ratio (Vr). It is found that there are two modes of temperature oscillation in thermocapillary convection in liquid bridges: low- and high-frequency oscillation, corresponding to different critical temperature difference and geometrical parameters, respectively. In this paper, the geometrical effect on the wave pattern and its transformation in the oscillating thermocapillary flow is studied in detail. Temperature signals from thermocouples on five measuring spots are analyzed to distinguish different oscillation modes corresponding to characteristics of azimuthal waves, such as wave number, traveling wave, and standing wave. Under a small volume ratio, the low-frequency traveling wave mode transits to the high-frequency standing wave mode; under a large volume ratio, the high-frequency standing wave has abundant mode transformations, such as standing wave → traveling wave, standing wave → traveling wave → standing wave, and so on, and the transformation process of the wave mode is sensitive to the aspect ratio.

The effects of geometry and heating rate on thermocapillary convection in the liquid bridge

The experimental study on thermocapillary convection in liquid bridges of large Prandtl number has been carried out on Tiangong-2 in space. The purpose of these experiments is to study the oscillation instability of thermocapillary convection, and to discover and recognize the mechanism of destabilization of thermocapillary convection in the microgravity environment in space. In this paper, the geometry of a half-floating-zone liquid bridge is featured by the aspect ratioArand volume ratioVr, and its influence on critical conditions of oscillatory thermocapillary convection is studied. More than 700 sets of space experiments have been finished. The critical conditions and oscillation characteristics of thermocapillary convection instability in theAr–Vrparameter space have been fully obtained under microgravity conditions for the first time. It is found that theAr–Vrparameter space can be divided into two regions of different critical conditions and oscillation characteristics: the region of low frequency oscillation, and the region of high frequency oscillation. More importantly, we obtain the complete configuration of these two stability neutral curves, and find that the low frequency mode is a ‘’ type curve. Based on this, we discuss the influence of heating rate on the oscillation mode. It is found that the heating rate affects the selection of critical mode, which results in a jump change of critical temperature difference. The findings of this study are helpful to better understand the critical modes and transition processes of thermocapillary convection in liquid bridges with different configurations.

Effect of interfacial heat transfer on basic flow and instability in a high-Prandtl-number thermocapillary liquid bridge

The effects of interfacial heat transfer on the basic flow and instability of thermocapillary convection in high-Prandtl-number (Pr) liquid bridges are investigated experimentally and computationally. Liquid bridges of silicone oil (Pr = 28) are formed in a gap between the lower cooled rod and the upper heated rod, both with a diameter of 5 mm, where the rods are surrounded by a cylindrical enclosure made from an acrylic block. The instability data are collected experimentally for AR = 0.30–0.50 and a wide range of (TC-Ta), where AR is the aspect ratio (= height/diameter) of the liquid bridge, TC is the cooled rod temperature, and Ta is the ambient gas temperature. The data indicate the appreciable effect of interfacial heat transfer on the instability and basic flow pattern of thermocapillary convection in liquid bridges. Each instability curve for AR = 0.35–0.50 shows a local peak of the critical temperature difference and the oscillation frequency at a certain (TC-Ta), where such a peak is associated with the transition of the azimuthal oscillation mode. The heat transfer ratio QLB/QHR is evaluated from the numerical simulation to discuss the effect of interfacial heat transfer, where QLB and QHR are the heat transfer rates at the liquid bridge surface and at the heated rod surface, respectively. It is found that the instability conditions for different AR values are well correlated with QLB/QHR. This correlation is consistent with the effect of QLB/QHR on the basic flow and temperature fields, both in the liquid bridge and in the ambient gas. The resultant change in the basic flow and temperature field inside the liquid bridge leads to the change in the onset conditions of oscillatory thermocapillary convection.

Effect of Ambient Conditions on Flow and Heat Transfer in a Liquid Bridge

The present study focuses on thermocapillary convection in liquid bridges of high-Prandtl-number fluid (5 cSt silicone oil) under microgravity environment and varying ambient air conditions. The geometry has a cylindrical liquid bridge surrounded by ambient air and is coaxially placed between two differentially heated rods of the same diameter. The commercial software ANSYS Fluent 16.0, based on the finite volume method, is used to solve the governing equations in both the liquid and air domains. An axisymmetric computation is carried out in the liquid bridge by varying control parameters such as the temperature difference between the ends, the ambient air temperature, and the velocity of the ambient air. For passive ambient air around a liquid bridge, the effect of varying the air temperature is considered on the surface tension gradient along the interface, causing thermocapillary flow from the hot to the cold end. To account for the effect of ambient air motion, the velocity values of the axial airstream for codirected and counterdirected flows are chosen to be less than the Marangoni velocity. The study confirms that the percentage change in the maximum local surface velocity near hot corner is found to be around 20% due to the effect of the ambient air velocity as compared to that of stationary air.

Effect of Marangoni number on thermocapillary convection and free-surface deformation in liquid bridges

Floating zone technique is a crucible-free process for growth of high quality single crystals. Unstable thermocapillary convection is a typical phenomenon during the process under microgravity. Therefore, it is very important to investigate the instability of thermocapillary convection in liquid bridges with deformable free-surface under microgravity. In this works, the Volume of Fluid (VOF) method is employed to track the free-surface movement. The results are presented as the behavior of flow structure and temperature distribution of the molten zone. The impact of Marangoni number (Ma) is also investigated on free-surface deformation as well as the instability of thermocapillary convection. The free-surface exhibits a noticeable axisymmetric (but it is non-centrosymmetric) and elliptical shape along the circumferential direction. This specific surface shape presents a typical narrow ‘neck-shaped’ structure with convex at two ends of the zone and concave at the mid-plane along the axial direction. At both θ = 0° and θ = 90°, the deformation ratio ξ increases rapidly with Ma at first, and then increases slowly. Moreover, the hydrothermal wave number m and the instability of thermocapillary convection increase with Ma.

Numerical study on thermocapillary convection in liquid bridge under surrounding shear gas stream

In this paper, a numerical simulation on the flow of liquid bridge and gas flow channel has been conducted to analyze the effect of airflow velocity on the isothermal liquid bridge free surface and the flow pattern in liquid bridge. The results show that the different pressure makes the lower part of liquid bridge shrink but makes the upper expand for the silicone oil liquid bridge of volume ratio V = 1, aspect ratio τ =1. It will cause volatility in surface when shear gas stream just acting on the free surface of liquid bridge. When the shear gas stream acting on the free surface lasts 0.14s, the fluctuation of free surface reaches a steady state, and the fluctuation of surface is less than 0.5μm. In the process of free surface wave, the maximum value of deformation usually appears in the bumps or the pits, and the deformation of the point near z = d / 2 is always zero.

The effect of aspect ratio and axial magnetic field on thermocapillary convection in liquid bridges with a deformable free-surface

ABSTRACTThree-dimensional numerical simulations are performed to analyze the effect of the aspect ratio Ar and axial magnetic field on thermocapillary convection in liquid bridges with deformable free-surface under microgravity, in which the volume of fluid (VOF) method is adopted to track the free-surface movement. The simulation results elucidate that the oscillation wave number m and frequency of temperature fluctuation decrease with increasing Ar, while the amplitude of temperature fluctuation increases with increasing Ar. The deformation ratio ξ of the free-surface increases as Ar increases. The numerical results also reveal that the axial magnetic field causes a concentration of convection vortexes near the free-surface and effectively suppresses the flow in both the radial and axial directions. Moreover, the axial magnetic field effectively damps free-surface deformation, so that the deformation ratio ξ decreases as the Hartmann number Ha increases. The temperature displays a uniform and linear distribution along the free-surface and the axis of molten zone when Ba = 0.3 T.

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).

3-D Flow Measurement of Oscillatory Thermocapillary Convection in Liquid Bridge in MEIS

3-D Flow Measurement of Oscillatory Thermocapillary Convection in Liquid Bridge in MEIS

Hydrothermal instability of thermocapillary convection in large-Prandtl-number liquid bridges under microgravity

Linear stability analysis was performed to study the mechanism of transition of thermocapillary convection in liquid bridges with liquid volume ratios ranging from 0.4 to 1.2, aspect ratio of 0.75 and Prandtl number of 100. 2-D governing equations were solved to obtain the steady axi-symmetric basic flow and temperature distributions. 3-D perturbation equations were discretized at the collocation grid points using the Chebyshev-collocation method. Eigenvalues and eigenfunctions were obtained by using the Q–R method. The predicted critical Marangoni numbers and critical frequencies were compared with data from space experiments. The disturbance of the temperature distribution on the free surface causes the onset of oscillatory convection. It is shown that the origin of instability is related to the hydrothermal origin for convections in large-Prandtl-number liquid bridges.

Azimuthal Wave Number Control of Oscillatory Thermocapillary Convection in a Liquid Bridge by Local Heating

The thermocapillary convection in a half-zone liquid bridge of high Prandtl number fluid is widely known to exhibit a three-dimensional oscillatory flow after onset of oscillation. The oscillatory convection presents 'Standing' and 'Traveling' flows depending upon the temperature difference between the top and bottom rods. In the oscillatory state, the flow indicates a modal structure with azimuthal wave number depending upon the aspect ratio of liquid bridge and the intensity of the thermocapillarity expressed by the Marangoni number. The present study attempted to control the azimuthal wave number by heating the free surface locally with a prescribed frequency and intensity. The flow of the liquid bridge exhibited different modal structures depending upon the heating condition and a relation between the frequency and the modal structure was obtained.

Azimuthal wave number control of oscillatory thermocapillary convection in a liquid bridge

Thermocapillary convection in a half-zone liquid bridge of high Prandtl number fluid is widely known to exhibit a three-dimensional oscillatory flow after the onset of oscillation. The oscillatory flow presents ‘standing’ and ‘traveling’ flows depending upon the temperature difference between the top and bottom rods. In the oscillatory state, the flow shows a modal structure with an azimuthal wave number that depends on the aspect ratio of the liquid bridge and the intensity of the thermocapillarity expressed by the Marangoni number. The present study attempted to control the azimuthal wave number by heating the free surface locally with a prescribed frequency and intensity. The flow in the liquid bridge exhibited different modal structures depending on the heating conditions and a relationship between the frequency and the modal structure was obtained. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(7): 460–469, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20086

D152 液柱内マランゴニ対流に対する周囲気体の影響(一般セッション 対流)

D152 液柱内マランゴニ対流に対する周囲気体の影響(一般セッション 対流)

Thermocapillary convection in cylindrical liquid bridges and annuli

Thermocapillary convection in liquid bridges and open cylindrical annuli is investigated in two- and three-dimensional numerical studies. The nondeformable free surfaces are either flat or curved as determined by the fluid volume, V, and the Young–Laplace equation. Dynamic free-surface deformations are discussed only in the axisymmetric models. Convection is steady and axisymmetric at sufficiently low values of the Reynolds number, Re, with either nondeformable or deformable surfaces. For the parameter ranges considered, it is found that only steady convection is possible at any Re in strictly axisymmetric computations. Transition to oscillatory three-dimensional motions occurs as Re increases beyond a critical value dependent on the aspect ratio, the Prandtl number, Pr, and V. Good agreement with available experiments is achieved in all cases. To cite this article: B.-C. Sim, A. Zebib, C. R. Mecanique 332 (2004).

Linear-stability analysis of thermocapillary convection in liquid bridges with highly deformed free surface

The float-zone crystal-growth process is studied in the framework of the half-zone model. Linear-stability analysis is used to compare the onset of oscillations in liquid-bridge thermocapillary convection upon liquid-bridge volume with experimental data for high-Prandtl fluid. Well-known structure of neutral stability curve for high-Prandtl fluid, consisting of two branches separated by an “overstability” gap, has been clearly reproduced. Moreover, for low-volume branch there is a good quantitative correspondence between experimental data and linear-stability analysis. Travelling hydrothermal waves with unit azimuthal wave number correspond to critical perturbations for both branches. In addition, an influence of temperature-dependent viscosity on stability limits is determined.

Oscillatory Thermocapillary Convection in Liquid Bridge under Microgravity

The floating zone is a widely used technique in crystal growth, and also a promising containerless method to realize high-quality crystals of semiconductors under microgravity. Thermocapillary convection driven by unbalanced surface tension is important for the mass and heat transports, and therefore finally crystal quality in a floating zone. The halfzone liquid bridge is a typical model to study thermocapillary flow in the floating zone, and therefore is adopted in the present study. The oscillatory instability of thermocapillary convection is believed to be responsible for the presence of an undesirable striation of material growth with the floating zone method under microgravity, in which the buoyancy convection is greatly reduced. For this reason, a lot of literature has contributed to the studies of the instability on thermocapillary flow in a half-zone liquid bridge model. A recent review on thermocapillary convection in a model of crystal growth can be found in Ref. 1.

Change of flow patterns in thermocapillary convection in liquid bridge

A parametric investigation of the onset of chaotic flows in a non-isothermal liquid bridge is numerically carried out for a liquid with medium Prandtl number, Pr=4, and unit aspect ratio. Spatiotemporal patterns of thermocapillary flow are successively studied beginning from onset of instability up to the appearance of the non-periodic flow. Well tested numerical code is used for solving the three-dimensional, time-dependent Navier–Stokes equations in cylindrical coordinate system. The dependence of viscosity upon temperature is taken into account. The two-dimensional steady-state flow becomes oscillatory with azimuthal wave number m cr=2 resulting from a Hopf bifurcation at Re 0 cr=630. Between this critical Reynolds number and up to Re≈810 convective flow evolves being uniquely determined by the set of characteristic parameters and independent of initial conditions. A secondary flow appears with an independent frequency and independent wave number m=3 at Re 1 cr=810. Two solutions with m=2 and 3, each with a different pattern of symmetry, co-exist for Re> Re 1 cr. Different modes reveal different behaviors in the supercritical area. Moreover, the traveling wave belonging to the critical mode m cr=2 always remains periodic, but the mode excited later on, with azimuthal wave number m=3, exhibits chaotic features at Re≈4200.