Four different outlet hub geometries of a large scale axial fan were designed and investigated. CFD methods were used to compare the fan characteristics and analyze the flow field structures in the blade tip and blade root. The results show that variation of outlet hub geometry has little effect on the fan's tip flow structure; there is a large separated flow in the blade root region which occupies 20% of blade height when the fan isn't provided with outlet hub

causing serious loss and decrease of efficiency; with the same blade tip clearance

the fan's efficiency increases when the outlet hub diffuser angle decreases; a matching straight or convergent hub could make the blade root separated vortex move backwards

reduce the separated area to 5% of blade height and increase at least 2 percentage points of the fan efficiency.