We carried out transient global simulations of heat transfer in an industrial heat exchanger method (HEM) furnace for sapphire crystal to investigate the helium flow effect on the melt convection, growth rate and melt-crystal (m-c) interface shape during the growth process. We found that an m-c interface reversal due to crystal internal radiation occurred in the growth process, which is easy to cause high thermal stress and the formation of grain boundary. Smaller helium flow rate has influence on reducing the growth rate at the time of interface reversal, which is conductive to improve the crystal quality. At the middle stage of growth process, the melt convection increases as helium flow rate increases, while the temperature difference between the m-c interface and melt free surface has no change due to the minor cooling helium gas effect on the melt. When the whole melt is solidified, helium flow rate has a significant effect on the axial temperature gradient at the bottom region of crystal, while in other regions of crystal, the impact is slight.

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