In order to improve the accuracy and efficiency of global modeling of a directional solidification process for solar silicon, we developed a structured/unstructured combined mesh scheme. The multi-block structured mesh was used to discretize those subdomains with regular boundaries. The unstructured mesh was used to discretize those sub-domains wim highly irregular boundaries. Then it was applied in the global modeling of heat transfer for a directional solidification process to investigate the effect of argon flow rate on heat transfer and impurities transport (oxygen as an example) in the furnace. It was found that the argon flow has little effect on the global heat transfer compared with thermal radiation, while its effect on impurities transport is remarkable. The amount of oxygen carried away by the argon flow at the melt surface is closely correlated with the argon flow rate.