Active volume of mean circulation for stirred tanks agitated with axial impellers

Abstract

The total volume of a stirred tank is currently treated as the active volume. In this work, laser doppler velocimetry (LDV) is used to show that for stirred tanks agitated with axial impellers the active volume of mean circulation for a stirred tank is not the whole tank, but a height equivalent to 2 3 of the tank diameter ( T). The active volume was defined using the decay of the three-dimensional wall jet in front of the baffle. At the point where the dimensionless slope of the axial velocity approaches zero the jet stops being effective for gross circulation. The active volume in fully turbulent flow remains constant for three types of axial impellers (PBT, A-310, HE3) independent of speed, diameter or off-bottom clearance. The absolute value of the velocities, of course, is a function of impeller speed, size and off-bottom clearance. The direction of the impeller discharge stream affects the location of the active volume. The discharge from axial impellers (A-310 and HE3) is always directed at the bottom of the tank, but this is not the case for the PBT. For the PBT, a transition point exists, above which the discharge stream impinges on the tank wall. The factors affecting the maximum clearance or transition point are the angle of the impeller blade, and the direction of the r–θ component of the impeller discharge stream. If the impeller discharge stream reaches the bottom of the tank, the active volume is the bottom two-thirds of the tank. If, however, the jet impinges on the tank wall, the zone of least activity is distributed between the bottom and the top of the tank.