Laminar Flow in Static Mixers With Helical Elements

Authors:

André Bakker

Richard D. LaRoche

Elizabeth M. Marshall

This research paper investigates the performance of Kenics static mixers by using computational fluid dynamics to analyze flow patterns, pressure drop, and mixing efficiency under laminar conditions. The study demonstrates that mixing occurs through a continuous cycle of flow splitting at element junctions combined with a stretching and folding mechanism that effectively redistributes fluid from the pipe walls to the center.



Ultimately, the simulations validate that these helical elements provide excellent radial mixing and correlate closely with established industrial pressure drop data, confirming that computer modeling is a reliable tool for static mixer design.

Key Learnings

• Mixing Mechanism: Mixing is achieved through a combination of flow splitting and shearing at element junctions, followed by a stretching and folding mechanism within the helical elements.
• Radial Efficiency: The "inside-out" flipping of the concentration field every two elements makes the Kenics design an excellent tool for radial mixing, effectively moving material from the wall to the core.
• Flow Characteristics: In the laminar regime (𝓡𝓮 = 10), there is no visible fluid circulation in a Cartesian frame; instead, mixing relies on the relative motion created by the 180° twist of the blades.
• Pressure Drop Validation: The CFD simulations predicted pressure drops within 14% of standard Kenics design correlations, confirming that computer modeling is a reliable alternative to traditional experimental testing.
• Velocity Patterns: High-speed fluid cores are split at each element division, forming a "flower-like" pattern that ensures the fluid is continuously redistributed across the pipe diameter.
• Grid Density Requirements: While flow patterns and pressure drops are easily captured, high-resolution grids (over 350,000 cells) and advanced interpolation schemes like QUICK are necessary to minimize numerical diffusion and accurately predict the degree of chemical mixing.