We present a numerical study of vapor pressure controlled Czochralski growth of GaAs bulk crystals, using a global two-dimensional (2D) model of conjugated heat transfer and flows in the GaAs melt, argon gas, and boron oxide encapsulant. At first, the model is examined with respect to the prediction of melt/crystal interface shape, whose accuracy is closely related to a correct simulation of the heat transfer near the crystallization front. A qualitative agreement between the computed and measured interface geometries exhibits, nevertheless, some overestimation of the interface convexity, provided by the 2D model. Then, an analysis of three-dimensional unsteady melt convection has been performed to clarify whether the 2D model is sufficient to describe correctly convective heat transfer. It has been shown that in spite of strong spatial and time variations of the melt flow, the 2D approximation is capable of providing reliable information on averaged melt characteristics, but does not reproduce properly the convection intensity in the under-crystal region. Finally, the 2D model is applied to study the specificities of the melt turbulent convection and transport mechanisms for various growth parameters. (C) 2003 Elsevier Science B.V. All rights reserved.