During the growth process of single bulk crystals from melt, the defect density is strongly affected by the shape of the melt/crystal interface. The shape of the interface is governed by the construction of the growth equipment including the heating system and the convection in the melt. In this paper the flow in a GaAs melt and the boron oxide encapsulant in an equipment used for vapour pressure controlled Czochralski growth has been calculated. 2D-axisymmetric calculations have been performed by using the commercial general purpose program FIDAP(TM). A simple model has been developed to describe the phase change problem in the weak form.