Computational fluid dynamics (CFD) was used to evaluate how impeller type and vertical arrangement influence mixing performance and hydrodynamic stress in a nominal 30,000 L stirred-tank bioreactor operated at a 20,000 L working volume. Eight configurations containing different combinations of pitched-blade and Smith turbine impellers were simulated in COMSOL Multiphysics (v6.2) using the standard k-epsilon turbulence model. Mixing time, flow patterns, shear stress, and energy dissipation rate (EDR) were compared, and mixing-time mesh sensitivity was assessed using three mesh resolutions. Configurations with more pitched-blade impellers generally produced stronger axial circulation, shorter mixing times, and higher volume-average shear stress and EDR. The all-pitched-blade configuration mixed fastest, whereas the all-Smith configuration mixed slowest. Local maximum shear stress and EDR depended more strongly on impeller arrangement, and not predicted by vessel-average behavior alone. Comparison with a 3,000 L small-scale vessel showed that matching the selected impeller configuration and power input per unit volume (P/V) did not provide hydrodynamic equivalence. These results highlight the importance of evaluating impeller type, placement, and vessel scale together when balancing mixing performance and hydrodynamic exposure…