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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