HYDRODYNAMIC ASPECT OF HYDROTHERMAL SYNTHESIS OF QUARTZ BULK FLOW REGIMES

In crystal growth autoclaves, dissolution zone and cristallisation zon e are connected through a diaphragm. The flow regimes in such a config uration are quite complex, as they result from two buoyancy mechanisms , solutal and thermal, acting in opposite directions. In addition, the autoclave walls are thick and heat transfer in it strongly affect hea t transfer in the bulk (conjugate problem). Several model with increas ing difficulties have been numerically tested to calculate the bulk fl ow. For a thermal boundary condition with a flux imposed at the extern al wall, the temperature distribution along the internal wall is found to be almost linear. For ''thermosiphon'' configuration, the axisymme tric solution is completely different, but still the flow goes up alon g the internal wall inside the two zones. For three-dimensional soluti ons, the flow is going up along the axis in the crystallisation zone; showing that the axisymmetric solution is not realistic. In addition, the stability theory indicates that after a certain threshold, the sol ution would become anti-symmetric and thus will be three-dimensional t oo.