Due to the experimental difficulties brought about by high pressure and temperature growth conditions, flow and heat transfer in industrial hydrothermal autoclaves for the growth of single quartz crystals have been studied mostly numerically. To date, most of the numerical models and associated results are not experimentally validated; only qualitative validation data from experiments done during actual crystal growth production is partly available. In this study, the authors used a model simulated reactor represented by an enclosure with two lower half sidewalls uniformly heated while the upper half sidewalls are uniformly cooled. Flow structures in the reactor are qualitatively visualized using full field flow tracking; particle image velocimetry (PIV) is used for quantitative velocity pattern evaluation. Based on the physical setup and experimentally determined boundary conditions, flow is numerically simulated and the corresponding model is validated through comparison to the experimental results. The ensuing parametric studies show the changing of flow patterns and velocity magnitudes for a variety of enclosure aspect ratios.