When formulators talk about shear, the conversation often stops at rotational speed or tip velocity. Yet CFD analysis shows that shear is not a single value and not a uniform field. What matters for emulsification is how much of the material experiences high shear, how often, and under what pressure and flow constraints. Geometry, not speed alone, defines this reality.

The Micelvak CFD study was designed to answer a very specific question: which rotor tooth geometry maximises effective shear for droplet breakup while remaining mechanically viable at industrial flow rates. The results provide a rare, quantitative view of how inline mixer design directly influences emulsion formation pathways.

Why geometry matters more than nominal shear

Inline emulsification relies on controlled deformation of droplets as they pass repeatedly through high shear regions. However, droplets only break efficiently if a sufficient fraction of the flow is exposed to shear rates high enough to overcome interfacial restoring forces.

The CFD simulations make this explicit. Shear rate is not evenly distributed across the rotor domain. Instead, it is concentrated in discrete regions defined by tooth height, channel number and channel width. A design that produces very high local shear in a small volume may be less effective overall than one that produces slightly lower shear across a much larger fraction of the flow.

This distinction explains why geometry optimisation is central to inline emulsifier performance.

CFD methodology and operating conditions

The Micelvak study evaluated 27 tooth configurations, varying three parameters:

• Tooth height
• Number of channels
• Channel width

All simulations were run under identical operating conditions to ensure comparability:

• Rotor speed: 4500 rpm
• Peripheral speed: 50 m/s
• Flow rate: 10 m³/h
• Fluid: water

Shear rate was selected as the primary performance metric because of its direct relevance to droplet deformation, interfacial stress and emulsification efficiency.

In parallel, inlet pressure was monitored to assess mechanical feasibility and axial load risk.

Tooth height as the dominant shear driver

The colour maps from the CFD simulations show a clear and consistent trend. Lower tooth height generates higher shear rates, with a significantly larger proportion of the rotor domain operating in the high shear regime.

Configurations with 6 mm tooth height exhibit substantially more red and orange regions in the shear rate maps compared to 10 mm and 15 mm designs. This is not a marginal effect. It fundamentally changes how much of the product experiences emulsifying conditions during each pass.

This finding aligns with emulsification theory. Shorter teeth reduce the distance over which velocity gradients develop, intensifying shear while maintaining flow continuity.

Why channel number and width require statistical analysis

In contrast, visual inspection alone was insufficient to distinguish the effects of channel number and channel width. Colour maps showed no obvious qualitative difference between many configurations.

To resolve this, the study moved beyond visualisation and analysed shear rate distributions using histograms, focusing on the 6 mm tooth height group where performance was already superior.

Shear rate values between 20,000 s⁻¹ and 200,000 s⁻¹ were binned, and the volume fraction of the rotor domain experiencing each shear range was calculated.

This step is critical. Emulsification depends on how much material experiences effective shear, not just the peak value.

Shear rate distribution reveals the real performance trend

The histogram analysis shows a clear pattern:

• Configurations with higher channel number and wider channels exhibit a larger volume fraction exposed to shear rates above 100,000 s⁻¹.
• Among the eight 6 mm configurations, HSM6, HSM7 and HSM8 stand out as the most effective in populating the high shear regime.

This confirms that channel geometry does matter, but its effect is statistical rather than obvious from instantaneous maps. Increasing the number and width of channels does not necessarily increase peak shear, but it increases the probability that any given fluid element experiences high shear during its residence time.

This directly links geometry to shear exposure uniformity, a concept introduced earlier in our blog about droplet size distribution. https://blog.olallaconsulting.com/processing-o/w-emulsions-why-distribution-matters-more-than-size

Weighted average shear rate as an objective metric

To identify the optimal design, the study calculated the weighted average shear rate, combining shear intensity with the corresponding volume fraction.

This approach avoids the trap of chasing extreme values that affect only a small fraction of the flow.

The results confirm a consistent trend:

• Weighted average shear rate increases with channel number and channel width
• The HSM6 configuration delivers the highest weighted average shear rate among all viable designs

HSM6 geometry:

• Tooth height: 6 mm
• Number of channels: 30
• Channel width: 8 mm

This configuration represents the best balance between shear intensity and shear coverage.

Pressure constraints and mechanical viability

High shear is only useful if it can be delivered safely and reliably. Excessive inlet pressure translates directly into axial loads on bearings and the rotor assembly.

The CFD pressure analysis establishes a practical constraint:

• Maximum acceptable inlet pressure at 10 m³/h: 7.5 bar

The HSM6 configuration reaches approximately 7.6 bar at maximum flow. This places it just above the conservative mechanical limit.

The study therefore recommends operating HSM6 at 8–9 m³/h, where inlet pressure remains safely below the threshold while maintaining high shear performance.

This is a crucial insight. It shows that geometry optimisation must consider process windows, not just peak capability.

What HSM6 changes for O/W emulsification

From an emulsion science perspective, the HSM6 geometry delivers three critical advantages:

• Higher probability of droplet breakup
  A larger fraction of the flow experiences shear sufficient to overcome interfacial tension.
• More uniform droplet size distributions
  Repeated exposure to controlled shear reduces the population of under processed droplets.
• Lower dependence on extreme operating conditions
  Effective emulsification is achieved without pushing speed or pressure to mechanical limits.

This aligns directly with the principles discussed in the blogs in this series. Stability begins at the interface, but processing geometry determines whether that interface is stressed constructively or destructively.

Why this matters for scale up

Because the shear field in HSM6 is geometry driven rather than speed driven, scale up becomes more predictable. Performance is maintained by preserving flow rate and geometry ratios, not by increasing energy input blindly.

This is precisely why multi-stage inline systems can replace more energy intensive technologies for many oil in water emulsions.

Conclusion

The Micelvak CFD study demonstrates that emulsification performance is not defined by speed alone. Tooth geometry controls shear distribution, residence time exposure and mechanical feasibility. By combining shear rate mapping, statistical distribution analysis and pressure constraints, the study identifies HSM6 as the most efficient and practical configuration for inline emulsification.

This geometry does not simply increase shear. It puts shear where it matters, for the largest possible fraction of the product, within a safe operating window.

This CFD work will be central to my CHEMUK presentation, where I will connect interfacial science, droplet formation dynamics and inline mixer geometry in a single, practical framework.
Join me at CHEMUK, 20 May, Stage 4 at 15:45. 

A very special thank you to Vak Kimsa for their outstanding case study and for showcasing their impressive capabilities in pilot plant trials, inline emulsification geometry development and advanced processing science. Their expertise continues to elevate how the industry approaches shear optimisation and scale up.

Learn more at Olalla Consulting

Explore more of Vak Kimsa’s engineering leadership and processing solutions in the full case study. For support with inline emulsification strategy, process optimisation and scale up decisions, visit:
https://www.olallaconsulting.com