As a first toehold an axial symmetric thermal model of a lab-scale ammonothermal autoclave was developed in order to investigate different heater configurations and their influence on the temperature distribution inside the autoclave. This thermal model used in the calculations was validated by special in-situ temperature measurements directly under ammonothermal conditions. The results of the numerical 2D studies were presented elsewhere.

Based on the results of the global thermal 2D simulations of the autoclave, a transient local 3D model was developed for calculation of convective phenomena. The GaN feedstock is considered as a porous medium and was calculated by the Darcy-Brinkman-Forchheimer model. The residual volume is considered as a fluid layer and was solved by a large eddy simulation with a one equation turbulence model. The temperature and flow distribution for different baffle geometries, as well as for different temperature gradients, were investigated, since baffles are used to control the flow pattern in ammonothermal systems. CFD calculations of an inclined baffle construction revealed the formation of a jet stream through the baffle opening depending on the angle.

Generally, large fluctuations of the temperature as well as of the velocities occur, indicating that 3D considerations are absolutely necessary for an accurately evaluation of the heat and mass transport in ammonothermal systems. Further studies will include mineralizers and the mass transport during the crystal growth process.

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