Particle-wave duality and coherent instability control in dense gas–solid flows

The collective effect of transport behaviors in a multibody system can either drastically enhance or deteriorate system performance depending on the nature of the internal interactions (i.e., constructive or deconstructive) and the structure established. For most powder processes, flow instability leads to poor performance. Control strategies have been attempted previously, but with limited success. The ability to drive such a system that is far from equilibrium into its “ordered” state by tuning the interactions can effectively reduce internal energy dissipation, which may lead to a technological breakthrough.By using a hybrid dynamics simulation and multiphase flow experiments, we will first elucidate two fundamental mechanisms underlying flow instabilities in a dense gas–solid flow: nonlinear drag and collisional dissipation. Then we clarify how gas-fluidized particles exhibit “particle-wave” duality (e.g., exhibit standing waves in a thin layer of granular bed that are driven by superimposed oscillating air, when the exciting frequencies of the oscillating air match the system's natural frequency). On this basis, we show experimentally that dense gas-fluidized granules can be synchronized into “ordered” structures by developing an adaptively exciting fluid wave. The introduction of an additional fluid wave enables the flow structures to be fine-tuned. Our method results in remarkably improved fluidization: highly expanded particulate beds with significantly suppressed gas bubble formation (for coarse particles) and channel formation (for ultra-fine powders), as has always prevailed in conventional dense gas–particle systems. By applying our methodology to several systems that are normally difficult to fluidize, we achieve unprecedented, well-controlled suspension of solids in gas flow.A scientific understanding of complex, dense gas–solid flows should enable the dispersion of solids in the gas flow to be controlled effectively. This work contributes to the basic science of dense gas–solid flows and would have impacts on powder technology, pharmaceutical manufacturing, and the optimum design of the third generation of fluidized bed reactors, such as the use of fast pyrolysis, to produce fuels from biomass and coal feedstock.