Flows in enclosures have been studied for various applications. Transport in lower half heated upper half cooled enclosures becomes a focus of research recently due to its application on hydrothermal crystal growth. Natural convection flows in hydrothermal autoclaves are driven by the temperature differential on the enclosure walls; lower half hot and upper half cooled. Due to the difficulties associated with visualization of flow in cylindrical enclosures, flows were experimentally visualized in a rectangular model autoclave. In this study, flows and transport mechanisms in rectangular and cylindrical enclosures are studied numerically in the steady laminar flow regime. Flow structures and transport mechanisms are analyzed and compared. Parametric studies on aspect ratios are carried out for both enclosures. Results show that flows in rectangular and cylindrical enclosures have the same transports mechanism. Fluid in the wall layers in one half form streams that feed into the center of the other half. However, for two kinds of enclosures quantitative differences exist on the stream formation.

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