The Spectrum of Fluid Shear in a Mixing Vessel

Author:

James Y. Oldshue

The document explores the full spectrum of fluid shear rates and stresses present within a mixing vessel, emphasizing their influence on mixing efficiency and particle behavior. It distinguishes between shear rates caused by average fluid velocity and those from turbulent fluctuations, explaining how each affects different scales of mixing phenomena. Experimental data and practical examples highlight how impeller design, speed, and tank geometry impact shear distribution, scale-up performance, and process outcomes in various industrial mixing applications.

Key Learnings

• Fluid shear within a mixing vessel varies widely, influenced by both average flow velocity and turbulent velocity fluctuations.
• Understanding the difference between shear rate and shear stress is critical for predicting dispersion, coalescence, and particle deformation.
• Impeller design, geometry, and rotational speed significantly affect shear distribution and overall mixing performance.
• Shear phenomena occur at multiple scales—from bulk flow patterns to localized turbulence near impeller blades.
• The relationship between power number, Reynolds number, and energy dissipation provides a framework for quantifying mixing efficiency.
• Scale-up of mixing systems requires careful consideration of shear rate equivalence to maintain consistent product quality.
• Measurement tools such as hot-wire anemometers enable precise evaluation of turbulent shear components.
• Pseudoplastic and other non-Newtonian fluids exhibit unique shear behaviors that must be accounted for in industrial design.