This research describes a three dimensional (3-D) computational fluid dynamics (CFD) simulation coupled with population balance equations (PBE) to study hydrodynamics and biomass production in a laboratory-scale stirred-tank bioreactor. The gas-liquid-solid flow was modelled using a Eulerian multiphase and k-epsilon turbulence (RNG) model. The energy dissipation rates, gas holdup, flow patterns, Sauter mean bubble diameter, and volumetric mass transfer coefficient were investigated for three different types of impeller using a multiple reference frame (MRF) model within the whole multiphase bioreactor. The effects of aeration rate and impeller speed on gas holdup and volumetric mass transfer coefficient were investigated owing to oxygen limitation in high cell density cultivation (HCDC). As high viscosity puts a limit on the efficiency of the bioreactor, the influences of viscosity on Sauter mean diameter, gas holdup, and volumetric mass transfer coefficient were also assessed. To determine growth kinetics as well as gas holdup, a set of experiments was performed. The numerical results of gas holdup and k(L)a were compared with the experimental data. Obtained results suggest that the Scaba impeller results in higher values of volumetric mass transfer coefficient, and subsequently higher biomass concentrations. One of the greatest problems in HCDC is feed accumulation in particular places. As depletion of substrate occurs near the impeller, the best spot for feeding purposes is in the vicinity of the impeller. Current research gives insight into the determination of the optimal operating conditions of HCDC in stirred bioreactors.