Single- and multi-hole baffles—a heat transfer and fluid flow control for hydrothermal growth
Introduction
Hydrothermal growth, the industry method of preference for producing high quality single crystals for resonators and surface acoustic wave devices [1], [2], [3], is usually carried out in a closed, cylindrical steel container called an autoclave. Autoclaves are separated into a lower and an upper chamber by a baffle with a small opening and filled with aqueous solution (Fig. 1). The raw material, small pieces of quartz crystals packed into the lower chamber, is dissolved. High quality seed crystal plates are hung in the upper chamber and quartz is deposited at a very low rate from the solution. Two independently controlled heaters maintain the temperatures of the two chambers, a higher temperature in the lower chamber and a lower temperature in the upper chamber. The transport of nutrient from the dissolving chamber to the growing chamber is through the natural convection flow induced by the temperature differential, [4], [5].
Natural convection of solution has strong effects on the growth morphology and quality of the grown crystals [6], [7]. Therefore, extensive studies have been carried out to investigate the fluid flow and heat transfer in industry-size hydrothermal autoclaves [8], [9]. A recent numerical study by the present authors found that the flow and thus the temperature fields in the autoclave are significantly different between cases with and without a baffle [9]. It is generally believed that the baffle design has a strong influence on the crystal growth but no study (experimental and/or numerical) of these effects has been reported. This paper presents a systematic study of the effects of the baffle designs on the fluid flow and temperature fields in an industry-size autoclave.
Section snippets
Physical model and mathematical formulation
A simplified physical model for the fluid flow and heat transfer in an industry-size autoclave is shown in Fig. 2. Only the aqueous solution inside the autoclave is considered. The geometry of the model is from an actual industry autoclave with diameter , aspect ratio . Isothermal boundary conditions are employed for the walls of both the lower and upper chambers with temperatures and , respectively. Correspondingly the flow driving temperature differential is
Single-hole baffles
Since a baffle is used to limit the flow and to separate the two chambers into the two temperature zones, baffles with a hole-opening larger than 25% by area are not recommended in practice. On the other hand, for baffle hole area openings smaller than 2% the thickness of the baffle will come into play. When the diameter of the baffle hole is comparable to the thickness of the baffle, fluid and heat exchange rates are significantly reduced. The hydrothermal process cannot be maintained as a
Conclusions
This paper examines the fluid flow and heat transfer in industry-size autoclaves with various baffles for hydrothermal crystal growth. Single-hole baffles with the hole area opening ranging from 2 to 25% and multi-hole baffles with a fixed 15% hole opening and various numbers of holes and hole arrangements have been investigated. Five kinds of multi-hole baffles, including one 4-hole baffle, three 8-hole baffles, and one 16-hole baffle, are proposed and studied. It is found that with a
Acknowledgments
This research is partially founded by National Science Foundation CRCD Project, Grant #9980325. The authors also give thanks to Sawyer Research Products, Inc. and College of Engineering in the University of Akron.
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