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In this paper, the Gidaspow, Wen and Yu, Brucato and modified Brucato Drag models are assessed. The implementation of Brucato drag model uses ltolmogorov length scale calculated for each cell, rather than using a value from the mean power dissipated in the system and applying it all over the domain as is commonly practiced.
4. Methodology and boundary conditions
4. 1. Vessel geome
In the current study, a flat bottomed cylindrical tanl‹ was simu- lated. The dimensions used are tanl‹ diameter, T = 0.2 m and tank height, II= T. The tanl‹ has four baffles mounted on the wall of width T/10. The shaft of the impeller (of diameter = 0.0J m) was concentric with the axis of the tanl‹. A six—bladed Rushton turbine was used as an impeller. The Rushton turbine has a diameter, D —— T/3. For each blade, the length - TJ4 2 and the height - T/15. The impeller off-bottom clearance was (C -- TJ3) measured from the level of the impeller disc. The fluid for the system was water
and the solids were small glass
particles of density 2550 kgJm' and diameter of 0.3 mm.
4.2. Numericol simulations
Fig. 4 shows half of the computational domain with baffles and stirrer. Owing to the rotationally periodic nature, half of the tank was simulated. Multiple reference frame (MRF) approach was used. A reference moving zone with dimensions r -0.06 in and 0.03995 < z « 0.09325 was created (where z is the axial distance from the bottom). The impeller rod outside this zone was consid- ered as a moving wall. The top of the tank was open, so it was de- fined as a wall of zero shear. The specularity coefficient is 0 for smooth walls and is 4 for rough walls. The walls of stirred tanl‹ were assumed to be smooth and a very small specularity coeffi- cient of 0.008 was given to all other walls. In the initial condition of the simulation, a uniform average concentration (0.04 v/v or
0.07 vJv) glass particles was taken in the tanl‹. The rotation speed of the impeller was 1000 rpm that was above the speed of just—sus— pension of glass particles in the liquid. For modelling the turbu- lence, a standard l‹-r mixture model was used. The model parameters were Cy: 0.09, C : 4.44, Cz: I.92, k•’ ! aH Tb 'd
In few cases the standard k-r dispersion model was also used with the turbulence Schmidt number, v, taken as equal to 0.8. The stea- dy state numerical solution of the system was obtained by using the commercial CFD solver ANSYS 12.1 FLUENT. In the present work, simple pressure—velocity coupling scheme was used along with the standard pressure discretization scheme. The grid inde- pendency of the geometry was checl‹ed by conducting single phase flow simulations on total number computational grid of 4 75460, 224280 and 428760 cells. The grid of 224280 predicted a correct
where, ft is constant with value of 8.76 x 10 ‘, dp is particle diam— eter and 7. is Kolmogorov length scale.
Ithopl‹ar et at. [3] performed DNS simulations for conditions closer to those in stirred tanks. Based on these simulations they ob— tained a modified version of Brucato di’ag that is more appropriate for stirred tanl‹s. This modified drag has a constant value of
8.76 ›‹ 10—'. Few more drag correlations that also tal‹e the depen—
Table 1
Details of cases simulated.