Modeling of fluid flow and heat transfer in a hydrothermal crystal growth system: use of fluid-superposed porous layer theory

Hydrothermal synthesis, which uses aqueous solvents under high pressure and relatively low temperature, is an important technique for difficult to gro w crystalline materials. It is a replica of crystal growth under geological conditions. A hydrothermal growth system usually consists of finely divide d particles of the nutrient, predetermined volume of a solvent and a suitab ly oriented crystal seed (Fig. 1) under very high pressures, generally seve ral thousand bar. The nutrient dissolves at a higher temperature in the low er region, moves to the upper region due to buoyancy-induced convective flo ws, and deposits on the seed due to lower solubility if the seed region is maintained at a lower temperature. The system can be modeled as a composite fluid and porous layer using the Darcy-Brinkman-Forchheimer flow model in the porous bed. Since the growth process is very slow, the process is consi dered quasi-steady and the effect of dissolution and growth is neglected. T his first study on transport phenomena in a hydrothermal system therefore f ocuses on the flow and temperature fields without the presence of the seed and,mass transfer A three-dimensional algorithm is used to simulate the flo w and heat transfer in a typical autoclave system. An axisymmetric flow pat tern at low Grashof numbers becomes three-dimensional at high Grashof numbe rs. A reduction in the porous bed height for fired healed and cooled region s can result in oscillatory flows. These results, for the first time, depic t the possible flow patterns in a hydrothermal system, that can have far re aching consequences on the growth process and crystal quality.