The numerical simulation of an ammonothermal process for growing GaN bulk single crystals has been performed by using the SC/Tetra computational fluid dynamics software. The autoclave is assumed to be axisymmetric. Heat transfer by natural convection is discussed in the case of flat and funnel-shaped baffles. Simulation results show that the optimum baffle angle is approximately 20 degrees. This result is identical to that obtained in our previous study on the hydrothermal ZnO crystal growth process. (C) 2009 Elsevier Ltd. All rights reserved.