Catherine Xuereb

Laser Doppler Velocimetry in Agitated Vessels: Effect of Continuous Liquid Stream On Flow Patterns

Liquid velocities were measured by laser Doppler velocimetry for a standard Rushton turbine and an axial-flow Mixel TT agitator, with a stream of liquid, having a volumetric flowrate such that the residence time was approximately 1/10 of the mixing time, being fed continuously in the agitated vessel. Measurements taken in the feeding-tube plane showed a major disruption of the flow pattern, especially of axial velocities, due to the continuous liquid stream. The incoming liquid jet combined with the stream discharged by the impeller in a complex 3-D way; however, the disruption of the flow gradually attenuated, and measurements taken in the plane rotated 90° clockwise indicated a flow pattern, with characteristics like velocity magnitudes, turbulence intensity and energy dissipation, similar to the batch case (with no through flow).

Determination of 3-D Flow Fields in Agitated Vessels by Laser-Doppler Velocimetry: Use and Interpretation of RMS Velocities

Laser Doppler velocimetry measurements have been used to calculate the hydraulic and energetic efficiency and turbulence structure for three agitators, a Rushton turbine and two axial agitators: a Mixel TT and a Lightnin A310. The Mixel TT had the highest efficiencies, while in a viscous liquid (1% CMC solution) all three had comparably low efficiencies. Energy was dissipated mainly in the stream flowing out of the impeller region and in the main circulation loop. The turbulence length scale was estimated to be of the order of D/5 for the turbulent flow regime and of D/20 for the transitional flow regime. The mean and rms velocities were used to calculate the switching probability, used to simulate stochastically the flow in agitated vessels.