The current understanding of the transport phenomena involved in the operation of industrial fermenters is not sufficient. This is reflected by the limitations seen in their design and operation. A better insight in the local processes taking place (hydrodynamics, gas dispersion, mixing, microbial kinetics) is required to be able to make a step change in the design of those reactors. At the scale of industrially relevant fermenters, experimental methods become quickly limited when detailed information is needed. It was the aim of this research to provide a framework where such information could be gained by means of Computational Fluid Dynamics (CFD) simulations with a manageable computational burden so that it could readily be used by the industrial practitioners. The main focus of this thesis is on the hydrodynamics of bubbly flows in stirred reactors, although, scalar mixing and substrate uptake kinetics studies were also conducted. Because the literature on the standard configured lab/pilot scale single-impeller reactors is vast and the experimental data is abundant, we chose to start with such a system first and, with the learnings gained, moved ultimately to realistic industrial scale multi-impeller fermenters. We also limited ourselves to Rushton type radial pumping disk turbine systems, again on the basis of available data for validation, and also due to time limitations.@en