A good understanding of the natural convective flow in hydrothermal autoclaves is essential for the control of the growth rate and the quality of the grown crystals. This paper presents a numerical simulation of turbulent natural convection in industry-size autoclaves with various aspect ratios. A simplified 2D axisymmetric model was developed. Numerical results were obtained for autoclaves with aspect ratios ranging from 2 to 20. Results show typical two counter-flow cells in both the bottom and upper chambers. Heat conduction through the baffle is negligible. Detailed analyses and discussions are presented to characterize the bulk flows in the upper chamber under various aspect ratios, and their effect on crystal growth. As the aspect ratio increases the gradients in flow velocity and temperature decrease making uniform growth possible in larger autoclaves.