Dynamics of flow pattern in a baffled mixing vessel with axial impeller
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media (T2-7P)
Keywords: mixing, stirred tank, macro instability, kinetic energy
The paper deals with an experimental study of flow in flat-bottomed cylindrical stirred tank of inner diameter of T=0,29 m filled with water to the height equal to the tank diameter (H=T). The vessel was equipped with four radial baffles and stirred with pitched blade impeller with six blades (45°, diameter D=T/3), pumping downwards. The experiments were localized on the lower part of the vessel, namely on the time and space pulsations of the primary loop, initiated by the impeller pumping action. The attention was focused on this area as this area is supposed to be a “birth place” of macro instabilities. The occurrence of macro instabilities has been studied and described by several authors last years, but the very origin of this phenomenon has not been satisfactorily explained yet. The flow was observed in a vertical plane coming through the vessel axis. Flow pattern was visualised by means of aluminium micro particles spread in water and illuminated by a vertical light knife 5 mm wide. The impeller speed was adjusted at 300 rpm, 400 rpm and 500 rpm and the impeller off-bottom clearances were 1/2T, 1/3T and 1/4T. The visualised flow was scanned by digital camera, records and shots obtained were of satisfactory quality. The defined exposure time allowed obtaining information on approximate local flow speed. The analysis of the records and shots resulted in following conclusions:
The primary loop reaches from 1/2 to 3/4 height of surface level. The flow field corresponding to the primary loop has an elliptical shape with a core. The flow in the elliptical annual area is intensive and streamlined, while the core is steady or chaotic; the flow in the remaining upper part is markedly steadier. The main diameter of the primary loop is not constant, it increases in time and after reaching a certain value, a part of loop separates and the loop disintegrates. The separated formation which is ejected upwards sometimes reaches the fluid level and then disintegrates, sometimes disintegrates under the level. This development is characterized by a certain periodicity and the period proved to be correlated to the macro instability occurrence. A dimensionless energy parameter, characterizing the periodic behaviour of the loop, was introduced. This parameter is proportional to the kinetic energy of the rotating annual area. Angular velocity of the annual area is derived from the loop surface speed. The magnitude of the surface speed is determined as the average flow speed in the impeller discharge stream characterized by the impeller pumping capacity. As a final result of experiments, statistically clear time behaviour of the energy parameter is obtained.
See the full pdf manuscript of the abstract.
Presented Monday 17, 13:30 to 15:00, in session Transport Phenomena in Porous/Granular Media (T2-7P).
