Fluid Mixing Principles and Shear Rate Effects

Author:

J.Y. Oldshue, Ph.D.

The article explores the principles of fluid mixing, focusing on how pumping capacity and fluid shear effects influence mixing performance and scale-up. It explains the importance of different shear rates in impeller-driven systems, their impact on processes such as emulsion polymerization, fermentation, and pseudoplastic fluid mixing, and how turbulence and velocity gradients contribute to particle dispersion and viscosity behavior. The author also discusses scale-up and scale-down challenges, emphasizing the role of impeller design, power consumption, and non-geometric adjustments in achieving consistent mixing results.

Key Learnings

• Fluid mixing performance depends on balancing pumping capacity and fluid shear effects.
• Six main types of shear rates exist in mixing systems, each influencing particle size, dispersion, and turbulence differently.
• Maximum impeller shear rate determines the ultimate shear impact on particles, while average shear rates govern overall system behavior.
• Scale-up often increases maximum shear rates and reduces average shear rates, making it difficult to replicate pilot-scale results directly.
• Non-geometric scale-up may be required when fluid shear effects must be maintained consistently across scales.
• Pumping capacity plays a critical role in blending and solid suspension processes, often favoring large, slow-moving impellers for efficiency.
• Viscosity in pseudoplastic fluids decreases with increasing shear rate, and measuring power consumption helps estimate effective shear conditions.
• Turbulence and velocity gradients drive energy dispersion and mixing intensity, impacting both macro- and micro-scale processes.
• Industrial applications like fermentation, polymerization, and emulsification require careful control of shear rates to balance performance and prevent damage.
• Power consumption and mixer design must be evaluated not only for process effectiveness but also for long-term economic efficiency.