For a fully baffled tank stirred by a Rushton turbine (RT)

the flow pattern will change from double-to single-loop as the off bottom clearance (

C

) of the RT decreases from one third of the tank diameter. Such a flow pattern transition as well as its influence on the macro mixing efficiency was investigated

via

CFD simulation. The transient sliding mesh approach coupled with the standard

k-ε

turbulence model could correctly and efficiently reproduce the reported critical

C

range where the flow pattern changes. Simulation results indicated that such a critical

C

range varied hardly with the impeller rotation speed but decreased significantly with increasing impeller diameter. Small RTs are preferable to generating the single-loop flow pattern. A mechanism of the flow pattern transition was further proposed to explain these phenomena. The discharge stream from the RT deviates downwards from the horizontal direction for small

C

values; if it meets the tank wall first

the double-loop will form; if it hits the tank bottom first

the single-loop will form. With the flow pattern transition

t

he mixing time decreased by about 35% at the same power input (

P

)

indicating that the single-loop flow pattern was more efficient than the double-loop to enhance the macro mixing in the tank. A comparison was further made between the single-loop RT and pitched blade turbine (PBT

45°) from macro mixing perspective. The single-loop RT was found to be less efficient than the PBT and usually required 60% more time to achieve the same level of macro mixing at the same

P

.