A novel approach to vapor phase deposition of gallium nitride is presented. Though the chemistry of the process is based on the well known NH3 and GaCl precursors, the reactor geometry was drastically changed with respect to the standard horizontal VPE configuration. The reactants are injected from the ceiling of the vertical reactor and kept separated until they mix at the surface of the horizontal rotating substrate, This paper discusses the fluid dynamic behavior of this reactor A computer simulation provides the flow patterns according to the different growth conditions.