Mixing

Key points

1. 01

   Turbulence is a spectrum of eddy sizes: the impeller feeds kinetic energy into the largest eddies, it cascades down to progressively smaller ones, and viscous dissipation into heat happens almost entirely at the smallest (Kolmogorov microscale) eddies.

2. 02

   Shear damage is worst near the impeller blades, where the smallest, most energetic eddies form — this is what fragments filamentous fungal hyphae and lyses wall-less animal cells.

3. 03

   Mixing time should be minimized — a slow mix lets local pH or concentration spikes near the addition point kill nearby cells.

4. 04

   Baffles break circulatory flow and vortex formation, converting it into turbulence and eddies; a standard reactor uses 4.

5. 05

   Power number ($N_p$) is constant only in turbulent flow — in laminar flow it depends on Reynolds number, so always check the flow regime first.

6. 06

   Flow number ($F_l$) scales with $D_i^3$ while power number scales with $D_i^5$ — the two formulas look alike but respond very differently to impeller size.

7. 07

   Radial (Rushton) impellers give high shear and high $O_2$ transfer; axial impellers (propeller, hydrofoil, pitched blade) give low-to-moderate shear and high pumping — never use Rushton for shear-sensitive cultures.

8. 08

   Viscosity ladder for impeller choice: pitched blade → anchor → helical ribbon, as fluids get thicker and more non-Newtonian; anchor is the only one built to scrape the tank wall.