Fermentation is a common method for producing bio-products such as alcohols and pharmaceutical compounds or alternative feedstocks. In the primary stage, presence of oxygen drives the metabolism and hence growth of the organic substance. Oxygen is supplied via air in stirred vessel type fermenters. While a certain configuration tested in a lab may result in satisfactory distribution of the dissolved oxygen (DO), large scale fermenters need careful design considerations.
Computational methods (CFD) have been reported in large amount of literature to investigate the flow patterns and consequently the gas hold up in such equipment. However, the air bubble injected vary in size throughout the reactors due to the choice of impellers, rpm, baffles, and other internals such as coils or dip tubes. The size of the air bubbles affects directly the gas hold up and the interfacial area. Therefore it is necessary to account for the varying bubble size distribution due to coalescence and breakup.
In this work, we present the use of a size distribution model to account for the bubble size distribution called S-gamma model in the commercial code STAR-CCM+. This model shows the differences in assuming constant bubble size but is not as computationally expensive as the quadrature (DQMOM/QMOM) type of models.
Features of the S-gamma model will be discussed and a case study presented where the two methods are compared. An outlook on future work and advances will also be given.
Lo, S. and Zang, D., Modelling of Break-up and Coalescence in Bubbly Two-Phase Flows, Journal of Computational Multiphase Flows, Vol 1(1), p 23, 2009
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