Reimagining static mixing

The Challenge

How far can radical thinking take us when considering existing design and assembly challenges? When the limitations of traditional manufacturing methods are removed, we are able to explore organic and free-form geometries that serve multiple functions.

In-line static mixers are a great example of complex assemblies used in several industries, ranging from polymer, plastic, pulp, food and energy, where low shear blending and mixing of raw materials or ingredients needs to be held at a constant temperature. Due to manufacturing constraints, there is poor integration between mixing and heat exchange. An example of a typical helical baffled mixer is shown below. Achieving a higher degree of mixing for a given length would be difficult without affecting pressure drop across the mixer.

Our Solution

Additive Manufacturing allows us to re-imagine these mixers to improve their performance in a number of key areas. Our design draws inspiration from braiding: Braiding is a method of interlacing three or more yarns in such a way that they cross one another and are laid together in diagonal formation, forming a narrow strip of flat or tubular fabric. A feature that stands out and could be particularly useful for mixing is that each strand zigzags forward through the overlapping mass of the others. Could this help us control how the volume of viscous and immiscible fluids efficiently get divided and recombined while maintaining low shear? Channel profiles can be tailored to remove areas of low or stagnant flow and make cleaning and maintenance easier.

As we evolve the design, simulation and computational fluid dynamics, enable us to gain insight into how these braided structures might divide and recombine viscous fluids during mixing. In our first design, it is evident that some degree of mixing occurs as the two fluids progress down the length of the mixer, but there are still two regions that remain unmixed. Making the braided section longer will improve its performance but given the geometric freedom computational design and additive manufacturing gives us, there is a better way to achieve this without expanding the design envelope!

We know that the braiding approach favours mixing, but we need to increase connectivity between the two regions. The original braided mixer was generated by simply adding a thickness to the threads and merging them all into a solid with largely open channels. In our final design, we demonstrate a field-driven computational design approach, in which a charge is assigned to each braid. Their resulting field, which can be seen below, was evaluated at a 3D array to produce an isosurface. This is the surface that corresponds to all the points in 3D space where the charge density is equal to a specified value. The resulting geometry essentially wraps around the original braid and offers more opportunity for mixing the two fluids.

Our simulations confirm the drastic change in fluid behaviour, offering a more effective folding of the two fluids.

With our primary fluidic domain designed to achieve a good level of mixing, we move on to addressing another common requirement in these applications: Heat transfer! A major benefit of the braided structure over conventional helical baffles positioned in a tube is the vast amount of surface area that is available for heat transfer. We have intertwined an additional fluidic domain to maximise contact with our primary domain and improve heat transfer efficiency. This design approach allows us to tailor heat transfer locally by altering channel dimensions and the resulting flow, delivering/removing heat to/from where it’s most needed.

Impact

This is a good example of how leaving past design approaches behind and working from first principles allow us to think outside the box and generate innovative design solutions that fully benefit from Additive Manufacturing.

It’s important to make a clear distinction at this point: these geometries have been created with purpose, as opposed to the common paradigm of filling a volume with generic pre-set geometries that are not designed with flow in mind. This is our definition of Design for Additive Manufacturing. What is yours?