TURBULENT-FLOW IN A BAFFLED VESSEL STIRRED BY A 60-DEGREES PITCHED BLADE IMPELLER

The power characteristics of an impeller with six blades, each at 60 d egrees to the direction of rotation, in a fully baffled vessel have be en quantified as a function of Reynolds number and the mean and turbul ent-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 hig h Reynolds numbers. The bulk flow velocities, averaged over 360 degree s of impeller rotation, showed an axial jet with a ring vortex encompa ssing two-thirds of the vessel height, swirl velocity proportional to radius beneath the impeller as in solid body rotation and counter-rota ting vortices behind the baffles. The velocity measurements with respe ct to impeller rotation, averaged over 1.08 degrees of impeller rotati on, confirmed shedding of small vortices from the tip of the impeller blades and the turbulence around the impeller and in the discharge str eam was generally anisotropic with maximum values just below the impel ler in the discharging jet. The turbulence fluctuations were overestim ated by upto a factor of two when averaged over 360 degrees. Compariso n with a Rushton impeller of the same size shows the pumping efficienc y of the pitched blade impeller is some 2.5 times higher and that it d issipates less power within the impeller. Copyright (C) 1996 Elsevier Science Ltd