Turbulent flow in a baffled vessel stirred by a 60° pitched blade impeller

Abstract

The power characteristics of an impeller with six blades, each at 60° to the direction of rotation, in a fully baffled vessel have been quantified as a function of Reynolds number and the mean and turbulent-flow characteristics measured for Reynolds and power numbers of 48,000 and 2.25, respectively. The power number decreased rapidly by 12% at a Reynolds number of 1200 and was constant at 2.25 ± 0.05 at high Reynolds numbers. The bulk flow velocities, averaged over 360° of impeller rotation, showed an axial jet with a ring vortex encompassing two-thirds of the vessel height, swirl velocity proportional to radius beneath the impeller as in solid body rotation and counter-rotating vortices behind the baffles. The velocity measurements with respect to impeller rotation, averaged over 1.08° of impeller rotation, confirmed shedding of small vortices from the tip of the impeller blades and the turbulence around the impeller and in the discharge stream was generally anisotropic with maximum values just below the impeller in the discharging jet. The turbulence fluctuations were overestimated by upto a factor of two when averaged over 360°. Comparison with a Rushton impeller of the same size shows the pumping efficiency of the pitched blade impeller is some 2.5 times higher and that it dissipates less power within the impeller.