In this study, we present a comprehensive characterization of the power number (Np) across a wide range of Reynolds numbers and baffling configurations in a Millipore Sigma 3-liter single-use bioreactor. Using a torque cell method, we recover the commonly referenced power number of ~0.3 at high Reynolds numbers under fully baffled conditions. However, by modifying the baffling configuration, we observe that the power number can drop by up to 50%. Conversely, at lower Reynolds numbers, the power number increases by up to an order of magnitude.
To validate and provide mechanistic insight into these trends, we performed CFD simulations, which demonstrate how vortex formation and transitions between turbulent, transitional, and laminar flow regimes drive the observed changes in Np. The CFD results show that variations in the underlying flow field are consistent with the experimental measurements, underscoring that the widely cited value of Np ≈ 0.3 should be treated as a general guideline rather than a universal constant.
This work provides a more nuanced understanding of power number behavior that can inform improved process design and scale-up under varying operating and geometry conditions.
