This paper presents an experimental and numerical investigation on the natural convection in a rectangular enclosure with lower-half-heated, upper-half-cooled sidewalls. The flow structure contains thin wall-layers and large central vortical circulating zones. The mixing process and the fluid paths caused by the wall-layer head-on collisions and stream–stream interaction are fundamental to the development of the heat and fluid transport mechanisms. The understanding of the flow structures and their mutual interaction provides a guide leading to the optimization of the autoclave internal architecture and to practical suggestions for the design strategy of crystal growth vessels.