Aerated and unaerated power consumption and flow patterns in a 0.56 m diameter agitated vessel containing water with dual Rushton turbines have been studied. Under unaerated conditions with a liquid height-to-diameter ratio of 2, an impeller spacing of 2 to 3 times the impeller is required for each to draw an amount of power equal to a single impeller. For aerated conditions, if a similar spacing is used, equations for the flooding-loading transition and for power consumption for a single Rushton impeller can be extended relatively easily to dual systems. All results for this spacing are explained by reference to bulk flow patterns and gassed-filled cavity structures and the proportion of sparged gas flowing through the upper impeller is also estimated. Such a spacing is generally recommended since it maximizes the power draw and hence the potential for oxygen mass transfer. Data are presented for other spacings but the results do not fit in easily with single agitator studies because strong impeller-impeller flow pattern interactions occur.
A simulation of flow field and tracer homogenization was performed using the commercial CFD software FLUENT 6.1. The aim is to investigate the potential of CFD software to predict concentration distribution of added tracer in cylindrical vessels. The calculated results ± dimensionless velocity profiles, power and pumping numbers, dimensionless concentration curves, and mixing times ± were compared with experiments in stirred vessels. In Part I, the study was performed for vessels agitated by one or two impellers on a centric shaft. Two different impellers were used ± a 6-bladed 45 pitched blade turbine and a standard Rushton turbine. The standard k-e turbulence model and multiple reference frames method were used for the simulations. The influence of the grid type was also investigated; three types of grid ± a structured, unstructured and a special user-defined grid ± were studied.
Power for agitation has been measured under aerated and unaerated conditions in a 0.29 m vessel of Rushton dimensions at specific powers up to 18 W/kg. The lluids studied were water. Newtonian solutions up to 19 m Pas and non-Newtonian shear thinning fluids some of which also exhibited a yield stress and some of which were viscoelastic. For the unaerated case. the power number-Reynolds number plot is in good agreement with the literature. For the aerated case, the resull can conveniently be divided into three Reynolds number regions. At Re > -900. the data obtained for the solutions is not markedly different [0 that for water except that a higher impeller speed is required to achieve complete gas dispersion. For -10 < Re < -900. the power drawn is independent of gassing rate and greater levels of elasticity and the presence of a yield stress leads to the lowest power numbers. For Re < --10. the gassed and ungassed power numbers are equal.
The process of homogenization of liquids in a tall vessel equipped with a multiple impeller was studied. Up to four standard Rushton turbines and/or six pitched-blade turbines were used. The mixing time in the system was measured by the conductivity method. A continuous time history of the tracer concentration at several points in the system was recorded. The data were interpreted by means of an adapted cell model of the flow within the stirred vessel with several impellers based on the assumption of well mixed cells and intercellular flow of liquid. The liquid transfer flow rate between the cells, as a parameter of the model, was calculated from experimental data. A good agreement between the time dependence of concentration obtairied experimentally and that calculated from theory was obtained. A direct relationship between the flow numbers between cells and those of impellers was established.
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