Introducing patterns from nature to control dynamic systems
14 February 2020
Fluidised beds are ubiquitous amongst gas-solid operations in energy, environmental, chemical and pharmaceutical industries. Bubbling fluidised beds of granular matter look a bit like a boiling liquid.
They provide an efficient way to put solid and gas phases into contact, but mixing and the transport of heat and mass depend upon the characteristics of each system, which brings challenges to predict and scale up their hydrodynamics, from lab to industry.
Inspiration from self-assembly mechanisms in nature can come to the rescue. Nature offers us many examples where macroscopic granular patterns result from interparticle and particle-fluid forces in sediments.
For example, sand particles form patterns under the action of waves on the beach or wind in dunes. Similarly, stripes, squares and hexagonal surface waves can be initiated in shallow granular layers by mechanical vibration, but they collapse in large vibro-assisted beds due to the energy dissipated in inelastic collisions.
Researchers at the UCL Centre for Nature Inspired Engineering (CNIE) have demonstrated that the use of oscillatory flows creates better coupling between the motion of the solids and the perturbation.
In deep, quasi-2D fluidised beds, the use of an oscillation can propagate the surface waves created in shallow layers into vertical, macroscopically ordered bubble flows.
The oscillatory flow stabilises an inherently chaotic system, impeding the coalescence of bubbles by intermittent dense particle clusters that redirect the gas flow, and reduce the long-range circulation of solids. The result is a predictable flow, where bubbles organise in a triangular lattice of given wavelength and angle.
This structured flow provides an unprecedented level of control over bubble size and solid mixing, thus defining a new operation sitting between fixed and fluidised beds, which we call “dynamically structured fluidised beds”. Here, the flow structure can be externally controlled, which is highly relevant to industrial operations.