Agitation flow patterns

Figure 7-9. Agitator flow patterns, (a) Axial or radial impellers without baffles produoe vortex, (b) Off-oenter looation reduoes the vortex, (o) Axial impeller with baffles, (d) Radial impeller with baffles. (Source Wales, S. M., Chemioal Prooess Equipment—Seleotion and Design, Butterworths Series in Chemical Engineering, 1988.)...

This chapter reviews the various types of impellers, die flow patterns generated by diese agitators, correlation of die dimensionless parameters (i.e., Reynolds number, Froude number, and Power number), scale-up of mixers, heat transfer coefficients of jacketed agitated vessels, and die time required for heating or cooling diese vessels. 

In fluid agitation, the direetion as well as the magnitude of the veloeity is eritieal. The direetions of the veloeity veetors throughout an agitated vessel are refeiTed to as the flow pattern. Sinee the veloeity distribution is eonstant in the viseous and turbulent ranges, the flow pattern in an agitated vessel is fixed. 

The overall nueleation rate in a erystallizer is determined by the interaetion of the seeondary nueleation eharaeteristies of the material being erystallized with the hydrodynamies of the erystal suspension. When erystallizing a given material, erystallizers of different size, agitation levels, flow patterns, ete. will... 

The propeller agitator with three blades rotates at relatively high speeds of 60-300 ips high efficient mixing is obtained. The generated flow pattern is axial flow since the fluid moves axially down to the centre and up the side of the tank. 

The flow patterns of agitated liquid have been studied extensively (Al, B11, F6, K5, M6, N2, R12, V5), usually by photographic methods. Apparently no work has been reported on bubble-flow patterns and relative velocities in agitated gas-liquid dispersions. Some simple pictures have been presented that only show the same details that may be seen with the unaided eye (Bll, F6, Y4). 

In Section I, a qualitative schematic description of the main connection between increased agitation intensity and increased total mass-transfer rate was given. It can readily be seen from this description that further research in gas and liquid flow patterns and in the area of relative bubble velocities in dispersions will contribute to the basic knowledge necessary for understand ing the real mechanisms occurring in these systems. 

The flow patterns for single phase, Newtonian and non-Newtonian liquids in tanks agitated by various types of impeller have been repotted in the literature.1 3 27 38 39) The experimental techniques which have been employed include the introduction of tracer liquids, neutrally buoyant particles or hydrogen bubbles, and measurement of local velocities by means of Pitot tubes, laser-doppler anemometers, and so on. The salient features of the flow patterns encountered with propellers and disc turbines are shown in Figures 7.9 and 7.10. 

Figure 7.12. Flow pattern with agitator offset from centre...

Carreav. P, J.. Patterson, I., and Yap, C. Y. Can J. Chem. Eng. 54 (1976) 135. Mixing of viscoelastic fluids with helical ribbon agitators 1. Mixing time and flow pattern. 

Figure 11. Flow patterns with an anchor agitator as in Figure 10 but at higher Reynolds numbers (above 10-20) where inertial effects become significant. Streamline schematic (12) shows stable trailing vortex.

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