Flow and Heat Transfer in an Upper Half Cooled Lower Half Heated Enclosure With Horizontal Temperature Deviations

Abstract

The nature and patterns of solution flow in hydrothermal autoclaves are critical to the quality, growth uniformity, and growth rates of synthetic single crystals. Small horizontal temperature deviations, which exist in industrial practice, were found to be critical in establishing flow patterns. However, the mechanism that determines how temperature deviations affect flow pattern is not well understood. In this study, an experimental system is set-up to study the flow in a model reactor (an enclosure). Temperature in the enclosure is visualized using liquid crystals. With the experimental results, a numerical model is validated and then used to simulate flows in enclosures that are subjected to similar thermal condition as industrial autoclaves. Flow patterns are obtained with various temperature deviations, for various aspect ratios and various Rayleigh (Ra) number between 4.05E8 to 3.24E9. Flows studied are unsteady in nature. Without temperature deviations, the overall flow pattern is anti-symmetric. With a temperature deviation, the wall layers are un-balanced. The impingement of streams on the wall layers does not affect the wall layer flow at low Ra numbers. At high Ra number, wall layers are broken by the impinging streams. The dominant heat transfer mechanism in the enclosure changes significantly as the aspect ratio of the enclosure changes. In enclosures of high aspect ratios that heat transfer resistance is mainly at the fluid exchange between the two halves, temperature deviations significantly affect heat transfer by stabilizing the direction of the streams.