This paper presents three-dimensional (3D) large eddy simulation (LES) calculations of natural convection in laterally-heated cylindrical enclosures using ANSYS FLUENT. The problem considered here is at a Rayleigh number, (Ra) of 8.8 x 10(6), based on a characteristic length scale defined by the ratio of the reactor's volume to the lateral area. The numerical simulations are carried out for three different geometrical configurations: an empty reactor, a reactor with a rack and 32 seeds, and thirdly, a reactor with a rack and 80 seeds mounted inside. The objective of the current study is to understand the effects of the rack and seeds on the flow patterns and thermal distribution inside the reactor. In order to investigate these effects, a comprehensive analysis of velocity and temperature on the selected planes and points inside the domain were performed. Results showed that with an increase in seeds within the reactor, the flow slowed down more, the turbulence intensity reduced, and the extent of temperature inversion increased. These have important implications for the choice of ammonothermal method in terms of acidic or basic, and also for species transport and reactions, which eventually affect the deposition rates of GaN crystals in autoclaves. (C) 2017 Elsevier Masson SAS. All rights reserved.