Experimental study of thermocapillary flows in a thin liquid layer with heat fluxes imposed on the free surface
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INTRODUCTION
T H E space environment can be very beneficial for material processing as both the gravitational body forces and the buoyancy convection are greatly reduced, as are the sedimentation caused by such forces, and turbulences generated by such convection. Among the various applications in material processing, float-zone crystal growth offers not only a promising technique in the practical sense but also a challenging opportunity in understanding the thermocapillary flow phenomena. There are basically two configurations considered in the past studies. One is that with fixed end temperatures, the other, with imposed heat flux. Most of the studies have dealt with the former. For the imposed-heat-flux configuration, there still exist certain basic and important phenomena to be understood before further studies can be properly pursued. The surface temperature distribution of the thin liquid layer as a result of the coupling between the imposed heat flux and the thermocapillary convection in a twodimensional thin liquid layer is the one of particular interest to us. For a finite container, which is more realistic than the infinite layer, the end wall effects (insulating or conducting) on the flow structure are important. The present study is aimed at understanding the above-mentioned flow phenomena experimentally. Based on the analysis done by Lai and Chai, f~l a long tray with 50 cm in length and 9.5 cm in width filled with silicone oil with 0.5 cm to 1.0 cm in depth was used to simulate a two-dimensional, infinite thin layer. Heat fluxes were generated by a heating element extending above the center of the container in the width direction. Flow patterns induced by the imposed heat fluxes through the thermocapillary effect should be symmetric and perpendicular to the heating element. The silicone oil, the layer depth, and the distance between the heating element and the liquid surface CHUN-LIANG LAI, formerly with NASA Lewis Research Center, is with the National Taiwan University, Taipei, Taiwan. PAUL S. GREENBERG, Engineer, and AN-TI CHAI, Physicist, are with NASA Lewis Research Center, Cleveland, OH 44135. This paper is based on a presentation made in the symposium "Experimental Methods for Microgravity Materials Science Research" presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25-29, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee. METALLURGICALTRANSACTIONS A
were varied to cover a certain flow range with A 2 Ma between 102 and 104, where A = D / L , with D denoting the layer depth and L, the reference length scale in the flow direction, is the aspect ratio and Ma, the Marangoni number. The surface temperature distribution was then measured and compared with the theoretical predictions.ill In order to study the end wall effects, a short tray (10 and 12 cm in length) was employed. Copper was used for the conducting wall; plexiglass, for the insulating wall. For ease of comparison of the flow structures with and without
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