•Heat transfer mechanisms affecting surface temperature of the rod are investigated.•Transition points of mixed convection and natural convection is GrRed2=100.•Operational and geometric factors for temperature homogeneity are discussed. The temperature homogeneity at polysilicon rods significantly affects the uniformity growth of the polysilicon rod and the product quality in reduction furnace. The heat transfer phenomena on the rod surface includes electric heating, radiation heat transfer, convection heat transfer, heat conduction and reaction heat. In this paper, a representative configuration (RC) of heat transfer model was selected from a “honeycomb” industrial furnace. The flow patterns with different outlets have been numerically investigated and the dependency of surface temperature distribution on the inlet velocity as well as the geometric parameters has been investigated. Gr/Red2 = 100 is set as the transition point determining the natural convection and mixed convection heat transfer. The results indicated that the axial temperature difference at the rod surface has close relationship with the complicated convection heat transfer. With rods growth, the main difference of local heat loss is caused by radiation rather than convection heat transfer.