This paper considers a modified heat exchanger method (HEM) crystal growth system that combines the advantages of traditional HEM systems and Bridgman systems. New design consists of two heating units at the sidewalls, similar to the Bridgman design, and one cooling system at the bottom of the furnace, similar to the HEM design. A transient numerical model is developed to simulate optical crystal growth process in the proposed system. Melt flow and heat transfer as well as the solidification process inside the crucible has been simulated together with the heat conduction and radiation phenomena in the entire furnace. The simulation results show that the modified HEM system is particularly suitable for growing large single crystals that need accurate control of thermal environment. Benefited from the unidirectional solidification, the temperature gradient inside the crystal during the growth process is reduced, subsequently less undesirable defects and cracks. Meanwhile, the seeding process is also well controlled by a hemispherical convex interface at the bottom of the crucible due to the cooling effect of the heat exchanger. (c) 2007 Elsevier B.V. All rights reserved.