0924 -8831 /$ - see front matter 20a 2 The Society of Powder Technology ]apan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved. http:JJdx.doi.org/10.104 6/j.apt.204 2.03.007

446 D. VVad ilerkar ct at. Advanced Powder technology 23 (2012) 445-453

in a stirred tanl‹ system. Inertial, gravitational and drag forces were It is quite clear from the review that solids suspension and dis- included, but  Saffman, Magnus  and  stress forces were  kept  condi- tribution   is  highly  dependent  on  the  turbulence   and    interphase tional  and  their  effect were  studied. The effect  of these  forces was drag  in  the  tanl‹.  At  low  impeller  speeds,  turbulent   fluctuations found  to be  negligible  due to the  high  magnitude  of  drag, inertial are  less  and  hence  do  not  affect  the  predictions  much. However, and  gravitational force. at higher  impeller  speeds, the drag and turbulence  become  increas- Ochieng   and   Lewis   [7]  simulated   nicl‹e1   solids   loading of ingly  important. Moreover,  there  is  no  consensus  on the appropri-

4 —20  www with impeller speeds between 200 and 700 rpm using  ate drag for liquid—solid stirred tanl‹s. Therefore, in this study, the both steady and transient simulations and found out that transient impact of drag model on the flow distribution and the velocity simulations, although time consuming, are better for stirred tanl‹ fields is investigated. Different drag models are assessed to provide simulations. The initial flow field was generated using the multiple a clear understanding of the selection criterion of drag in a partic- reference  frame  (MRF)  appi oach  and  then  the  simulations  were         ular case.

carried out using SG. The Gidaspow model was used for the   drag

factor, which is a combination of the Wen and Yu model and the Ergun equation [10]. Wen and Yu drag is appropriate for dilute systems and Ergun is used for dense packing. For the study of just suspended of solids using solids at the bottom of the tan1‹ as an initial condition, it provided satisfactory results.

The suspension can also be modelled as a continuous phase using a viscosity law and the shear induced migration phenomenon generated by gradients in shear rates or concentration gradients can be captured at a macroscopic scale. For the prediction of shear-induced particle migration, the Shear Induced Migration Model (SIMM) was used. which states that, in a viscous concen— ti-ated suspension, small but non-Brownian particles migrate from regions of high shear rate to regions of low shear rate. and from re— gions of high concentrations to regions of low concentrations in addition to which settling by gravity is added. In the case of a mix- ing process, owing to the action of shear and inertia, the particles may segregate and demix, thereby generating concentration gradi— ents in the vessel. This shear-induced migration phenomenon can be simulated at the macroscopic scale, where the suspension is modelled  as one continuous  phase  through  a viscosity  law  [1 ].

3. CFD model

3. 1. Mo0eI equations