Static Mixers

 

A static mixer is a device for mixing two fluid materials. It consists of a series of baffles, contained in a cylindrical or squared housing. As the two fluid streams move through the mixer, the non-moving elements continuously blend the materials.

The video below shows a model mixer which we use here for the sake of illustration. This mixer is very similar to industrial mixers, but has subtile design flaws that are predicted here by computer simulation. The mixer consists of a sequence of 6 elements which blend the fluids in successively crossed space directions. This relative crossing of the elements is illustrated in the video.

In a laminar flow, a processed material divides at the leading edge of each element and follows the channels created by the element shape. The channels are further divided at each succeeding element, resulting in an exponential increase in stratification. The number of striations produced is approximated by 2n, where n is the number of elements in the mixer, although the actual picture in a mixer is more complex and includes small, secondary striations.

Computer simulations in high resolution

The picture below is obtained by computer simulation and shows the mixing of fluids obtained for a laminar flow in four different cross-sections of the model mixer.

As it can be seen, small, secondary striations are present already in the first mixer unit, and the average size of the produced strations decreases exponentially in the downstream direction of the mixer. The animation below shows the details of the mixing process reproduced in this numerical experiment:

It is highly challenging to reproduce the physics of a static mixer in a computer simulation, because the size of the strations rapidly falls below the resolution of the mesh used to simulate the fluid. We solve this problem through a hybrid approach, using a classical Eulerian mesh to simulate the flow equations, and a mesh-less, particle based approach to reproduce the mixing process.

In this particular example, extended, unblended fluid portions remain even in the sixth mixer element. This is due to a flaw in the mixer design (size, spacing and inclination of the baffles) which can be spotted by computer simulation.

What we offer

  • Accurate computation of mixer features: pressure drop, mixing quality, and more.
  • Simulation of time-dependent and stationary flows in mixers.
  • High definition: more than 100 parallel striations are resolved.
  • The mixer is simulated as it is, without restrictive mathematical assumptions. It is for example not necessary that successive mixer elements are equal or similar.
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