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.