Marangoni convection flow in NaNO 3 -KNO 3 mixture under microgravity
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I.
INTRODUCTION
In the process of growing single crystals, the natural and Marangoni convection are produced in the melt due to the temperature distribution, and could have an important effect on the quality of crystals. Marangoni flow is also known to enhance significantly the mass and heat transfer rates at the interface and is investigated from the chemical engineering point of view. Therefore, from not only a theoretical but also a practical point of view, it is very important to understand the behavior of Marangoni flow. Many works[2–7,9,10] on the Marangoni flow have been carried out. The quantitative evaluation of Marangoni flow, however, has not yet been firmly established. Under normal gravity conditions, the temperature gradient produces not only the Marangoni convection but also natural convection inevitably. Therefore, it is not easy to evaluate the contribution of Marangoni convection flow to total flow. Recent easy accesses to the drop-shaft facilities make it possible to carry out the experiments under microgravity condition. Since the natural convection can be suppressed under microgravity condition, the role of Marangoni convection can be clearly evaluated. In the present study, the measurements of surface flow for the 50 mass pct NaNO3-50 mass pct KNO3 mixture driven by Marangoni convection under microgravity were carried out by using the drop-shaft type microgravity facility of the Japan Microgravity Center (JAMIC). The total path length of free fall is 490 m and the duration of free fall is about 10 seconds. During this free fall, a microgravity environment of less than 1023 G (G 5 9.8m/s2) is created continuously. A default experimental rack size is 0.870 3 0.850 3 0.886 (m). In this study, a one-quarter rack, of size 0.870 3 0.425 3 0.443 (m), was used. The Marangoni convection in the melt was analyzed by Y. MURAYAMA, formerly Graduate Student, Graduate School of Engineering, Hokkaido University, is Research Engineer with Honda Motors Co. Ltd., Sayama, 350-13, Japan. Y. SASAKI, Research Fellow, Y. KANEKI, Research Assistant, K. KASHIWAYA, Associate Professor, and K. ISHII, Professor, are with the Graduate School of Engineering, Hokkaido University, Sapporo, 060-8628, Japan. Manuscript submitted August 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
means of an order-of-magnitude evaluation.[6,8] This yields a quantitative evaluation of the relation between the Ma and Re numbers. II.
EXPERIMENTAL
The schematic of an experimental apparatus is shown in Figure 1. The experimental chamber is a stainless steel cylinder of 90 mm in diameter and 100 mm in length, and it has four holes (20-mm diameter) at each quarter position to insert the electrodes and noble metal thermocouples. A pair of counterpositioned windows is set along the gravitational direction, and another pair is perpendicular to it in order to evaluate the effect of gravity direction. Both ends of the cylinder were covered by silica glasses to observe the flow of molten salt. The two thermocouples were held just in the center
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