Many piezo-electrical crystals such as quartz are grown hydrothermally in highly pressurized cylindrical autoclaves. In hydrothermal crystal growth of quartz, quartz pellets are put in the bottom of an autoclave where the temperature is higher and the quartz dissolves into an aqueous solution. The nutrient is then brought to the upper part of the autoclave by natural convection of the solution due to a temperature difference and deposited onto the seed crystals at a lower temperature. A good understanding of such a bulk flow is therefore essential for the control of the rate of crystal growth and the quality of the grown crystals. Due to a large dimension of the system and a large length to diameter ratio (10 to 30), complex flow patterns with strong turbulence are expected. This paper presents the first numerical analysis of turbulent natural convection in industry-size autoclaves with various aspect ratios. A simplified, two-dimensional axisymmetric model that does not consider the seed and the porous raw material bed has been developed. The Wilcox κ-ω turbulent model is employed to describe turbulent transport. Using a CFD package, CFX, numerical results have been obtained for cases with aspect ratios ranging from 2 to 30. Results show typical two counter-flow cells in both the bottom and upper parts of the autoclave. Strong turbulent interaction is observed in the region near the baffle opening where the bottom and upper two flow cells meet, although there is no cross flow in the average flow field. Detailed analyses and discussions are presented to characterize the bulk flow in the upper chamber under various aspect ratios, and their effect on crystal growth.