The Optical Methods on Measuring Surface Deformation and Surface Wave in The Thermal Capillary Convection

An optical diagnostic system consisting of the Michelson interferometer with the image processor has been developed for the study of the kinetics of the thermal capillary convection. The capillary convection, surface deformation, surface wave and the velocity field in a rectangular cavity with different temperature's sidewalls have been investigated by optical interference method and PIV technique. In order to calculate the surface deformation from the interference fringe, Fourier transformation is used to grating analysis. The quantitative results of the surface deformation and surface wave have been calculated from the interference fringe pattern.

The convection during NaClO3 crystal growth observed by the phase shift interferometer

An optical diagnostic system consisting of the Mach-Zehnder interferometer with the phase shift device and an image processor has been developed for the study of the kinetics of the crystal growing process. The dissolution and crystallization process of NaClO3 crystal has been investigated. The concentration distributions around a growing and dissolving crystal have been obtained by using phase-shift of four-steps theory for the interpretation of the interferograms. The convection (a plume flow) has been visualized and analyzed in the process of the crystal growth. The experiment demonstrates that the buoyancy convection dominates the growth rate of the crystal growing face on the ground-based experiment.

Experimental Study on Benard-Marangoni Convection by PIV and TCL

An experimental investigation of Benard-Marangoni convection has been performed in double immiscible liquid layers of rectangular configuration. The two kinds of liquid are 10cst silicon oil and FC-70 respectively. The velocity fields in the vertical cross-section are obtained by PIV. Flow patterns and/or temperature distributions on the horizontal interface are displayed by using thermal color liquid crystal (TLC), and the velocity distributions on the interface were also obtained with the help of the serial particle image of TCL. The evolution processes of convection are observed in the differential thickness ratio of two liquid layers, and the convection styles are discussed.

Instability from steady and axisymmetric to steady and asymmetric floating half zone convection in a fat liquid bridge of larger Prandtl number

The linear instability analysis of the present paper shows that the thermocapillary convection in a half floating zone of larger Prandtl number has a steady instability mode w(i) = 0 and m = 1 for a fat liquid bridge V = 1.2 with small geometrical aspect ratio A = 0.6. This conclusion is different from the usual idea of hydrothermal instability, and implies that the instability of the system may excite a steady and axial asymmetric state before the onset of oscillation in the ease of large Prandtl number.

Influence of Forced Convection on Columnar Microstructure During Directional Solidification of Al-Ni Alloys

This paper presents a summary of cellular and dendritic morphologies resulting from the upward directional solidification of Al - Ni alloys in a cylindrical crucible. We analysed the coupling of solid-liquid interface morphology with natural and forced convection. The influence of natural convection was first analyzed as a function of growth parameters (solute concentration, growth rate and thermal gradient). In a second step, the influence of axial vibrations on solidification microstructure was investigated by varying vibration parameters (amplitude and frequency). Experimental results were compared to preliminary numerical simulations and a good agreement is found for natural convection. In this study, the critical role of the mushy zone in the interaction between fluid flow and solidification microstructure is pointed out.

An experimental study of the influence of weak convection on perikinetic coagulation

A remarkably increased coagulation rate for 2-mu m PS spheres was previously reported for a perikinetic coagulation experiment performed under microgravity conditions (1998, R. Folkersma, A. J. G. van Diemen, and H. N. Stein, J. Colloid Interface Sci. 206, 482); from this experiment, it was assumed that the leading factor slowing the coagulation process under normal gravitation was free convection due to gravity (1998, R. Folkersma, and H. N. Stein, J. Colloid Interface Sci. 206, 494). To test the influence of free convection as a single-effect factor on the coagulation process, a ground-based experiment was constructed. The coagulation rate of 2-mu m PS spheres dispersed in water was determined by measuring the turbidity of the dispersion solution while convection-driven flows in the solution were checked with a visual magnification system. We found that it was possible to cease free convection-driven particle flows on the ground, as long as the experiments were carefully operated. The strength of convection was controlled by changing the temperature gradient applied to the sample cell. By monitoring both the coagulation rate and convection-driven flows simultaneously, our experiments showed that weak free convection (maximum speed <150 mu m/s) actually has negligible effects on the coagulation rate.

Experimental Study of Surface Deformation and Flow Pattern on Buoyant-Thermocapillary Convection

Free surface deformation is one of the most important physical phenomena in fluids with free surface. In the present paper, convection and surface deformation caused by thermocapillary effect in a rectangular cavity were investigated. In ground experiments, the convection was also affected by gravity. The cavity has a horizontal cross section of 52mm×42mm and the thikkness of the liquid layer is 4mm. Temperature difference between two sides of the liquid layer was increased gradually, and the flow in liquid layer will develop from steady to unstable convection. An optical diagnostic system consisting of a revised Michelson interferometer with image processor was developed to study fluid surface deformation in convection, and the displacements of free surface oscillation were determined. PIV technique was adopted to observe the evolution of flow pattern, and the velocity fields were obtained quantitatively. The present experiments demonstrate that surface deformation is quite distinct in buoyant-thermocapillary convection. in order to understand the mechanism of buoyant-thermocapillary convection, not only the hydrothermal wave instability but also the surface wave instability should be discussed.

Natural convection in metal foams with open cells

This paper presents a combined experimental and numerical study on natural convection in open-celled metal foams. The effective thermal conductivities of steel alloy (FeCrAlY) samples with different relative densities and cell sizes are measured with the guarded-hot-plate method. To examine the natural convection effect, the measurements are conducted under both vacuum and ambient conditions for a range of temperatures. The experimental results show that natural convection is very significant, accounting for up to 50% of the effective foam conductivity obtained at ambient pressure. This has been attributed to the high porosity (ε > 0.9) and inter-connected open cells of the metal foams studied. Morphological parameters characterizing open-celled FeCrAlY foams are subsequently identified and their cross-relationships are built. The non-equilibrium two-equation energy transfer model is employed, and selected calculations show that the non-equilibrium effect between the solid foam skeleton and air is significant. The study indicates that the combined parameter, i.e., the porous medium Rayleigh number, is no longer appropriate to correlate natural convection by itself when the Darcy number is sufficiently large as in the case of natural convection in open-celled metal foams. Good agreement between model predictions and experimental measurements is obtained. © 2005 Elsevier Ltd. All rights reserved.

Linear Stability Analysis of Convection in Two-Layer System with an Evaporating Vapor-Liquid Interface

Classical theories have successfully provided an explanation for convection in a liquid layer heated from below without evaporation. However, these theories are inadequate to account for the convective instabilities in an evaporating liquid layer, especially in the case when it is cooled from below. In the present paper, we study the onset of Marangoni convection in a liquid layer being overlain by a vapor layer.A new two-sided model is put forward instead of the one-sided model in previous studies. Marangoni-Bénard instabilities in evaporating liquid thin layers are investigated with a linear instability analysis. We define a new evaporation Biot number, which is different from that in previous studies and discuss the influences of reference evaporating velocity and evaporation Biot number on the vapor-liquid system. At the end, we explain why the instability occurs even when an evaporating liquid layer is cooled from below.

A study of the influence of free convection on coagulation process

It was assumed [1, 2] that gravity affects the coagulation process in two ways: free convection, which is hard to be avoided on the ground and sedimentation, which can be greatly reduced by the density-matching method. We present a ground-based experiment set-up to study the influence of convection on the perikinetic coagulation for aqueous polystyrene (PS) dispersions. The turbidity measurement was used to evaluate the relative coagulation rate and convection-driven flows in the solution were checked with a visual-magnification system. The pattern of flow field temperature profile in the sample cell is given. Our experiments show that there was no noticeable difference of coagulation rate observed no matter whether convection flows exist (with the flow speed up to 180 mu m/s) or not.

The Hydrothermal Wave Of Large-Prandtl-Number Fluid In A Shallow Cavity

The hydrothermal wave was investigated numerically for large-Prandtl-number fluid (Pr = 105.6) in a shallow cavity with different heated sidewalls. The traveling wave appears and propagates in the direction opposite to the surface flow (upstream) in the case of zero gravity when the applied temperature difference grows and over the critical value. The phase relationships of the disturbed velocity, temperature and pressure demonstrate that the traveling wave is driven by the disturbed temperature, which is named hydrothermal wave. The hydrothermal wave is so weak that the oscillatory flow field and temperature distribution can hardly be observed in the liquid layer. The exciting mechanism of hydrothermal wave is analyzed and discussed in the present paper.

Influence Of Rayleigh Effect Combined With Marangoni Effect On The Onset Of Convection In A Liquid Layer Overlying A Porous Layer

The coupling mechanism of Rayleigh effect and Marangoni effect in a liquid-porous system is investigated using a linear stability analysis. The eigenvalue problem is solved by means of a Chebyshev tau method. Results indicate that there are three coupling modes between the Rayleigh effect and the Marangoni effect for different depth ratios. (C) 2008 Elsevier Ltd. All rights reserved.

Influence Of Free Surface Curvature Of A Liquid Layer On The Critical Marangoni Convection

The Pearson instability was suggested to discuss the onset of Marangoni convection in a liquid layer of large Prandtl number under an applied temperature difference perpendicular to the free surface in the microgravity environment. In this case, the temperature distribution on the curved free surface is nonuniform, and the thermocapillary convection is induced and coupled with the Marangoni convection. In the present paper the effect of volume ratio of the liquid layer on the critical Marangoni convection and the corresponding spatial variation of the convection structure in zero-gravity condition were numerically investigated by two-dimensional model. (C) 2008 Elsevier Ltd. All rights reserved.

Thermocapillary Flows In Liquid Bridges Of Molten Tin With Small Aspect Ratios

Numerical simulations were conducted to study thermocapillary flows in short half-zone liquid bridges of molten tin with Prandtl number Pr = 0.009, under ramped temperature difference. The spatio-temporal structures in the thermocapillary flows in short half-zone liquid bridges with aspect ratios As = 0.6, 0.8, and 1.0 were investigated. The first critical Marangoni numbers were compared with those predicted by linear stability analyses (LSA). The second critical Marangoni numbers for As = 0.6 and 0.8 were found to be larger than that for As = 1.0. The time evolutions of the thermocapillary flows exhibited unusual features such as a change in the azimuthal wave number during the three-dimensional stationary (non-oscillating) flow regime, a change in the oscillation mode during the three-dimensional oscillatory flow regime, and the decreasing and then increasing of amplitudes in a single oscillation mode. The effects of the ramping rate of the temperature difference on the flow modes and critical conditions were studied as well. In this paper, the experimental observability of the critical conditions was also discussed. (C) 2008 Elsevier Inc. All rights reserved.

A Linear Stability Analysis Of Large-Prandtl-Number Thermocapillary Liquid Bridges

A linear stability analysis is applied to determine the onset of oscillatory thermocapillary convection in cylindrical liquid bridges of large Prandtl numbers (4 <= Pr <= 50). We focus on the relationships between the critical Reynolds number Re-c, the azimuthal wave number m, the aspect ratio F and the Prandtl number Pr. A detailed Re-c-Pr stability diagram is given for liquid bridges with various Gamma. In the region of Pr > 1, which has been less studied previously and where Re, has been usually believed to decrease with the increase of Pr, we found Re-c exhibits an early increase for liquid bridges with Gamma around one. From the computed surface temperature gradient, it is concluded that the boundary layers developed at both solid ends of liquid bridges strengthen the stability of basic axisymmetric thermocapillary convection at large Prandtl number, and that the stability property of the basic flow is determined by the "effective" part of liquid bridge. (c) 2008 Published by Elsevier Ltd on behalf of COSPAR.