Abstract
Bubbling fluidised reactors provide good mixing and transport rates for a broad
range of gas–solid processes in the industry. Their overall performance relies highly
on the macroscopic flow of bubbles, which is usually chaotic and unstable due to
the complex dissipative interparticle collisions and interfacial friction. Applying an
oscillatory gas flow, under certain conditions, can induce a recursive flow structure
where bubble positions become perfectly ordered following an array of triangle
tessellation. The phenomenon manifests great potential to benefit the reactor design,
control and rational scale-up.
The thesis combines both experiments and simulations to explore the dynamically structured flows in pulsed fluidised beds. Its reproducibility has been evaluated experimentally with a designed intensity index that quantifies the
degree-of-order introduced in the flow pattern. The operation regimes of structured bubbling flows were identified via the comparison across flow conditions and
particulate bed depths. Unlike a steady flow fluidised system, the bubbles in a
structured flow exhibit a narrower size and separation distribution, which is directly
correlated to the oscillation applied, and hereby can be controlled and manipulated
precisely.
In the same work, the dynamics of the structured flows has been investigated
using two different modelling strategies, two-fluid models (Euler–Euler approach)
and discrete element methods (Euler–Lagrange approach). The simulated flow
reveals the dynamic nature that the particulate phase periodically alternates between
the viscous and plastic regimes. When solids contract, the system creates temporally
locked regions of solids that were found to synchronise the nucleation of bubbles
and stabilise the flow structures. Besides, due to inherent assumptions of the
two-fluid model, the simulated system fails to find a structured flow. These results,
in turn, highlight the power of pattern formation as a robust “fingerprint” for gas–
solid multiphase model validation and development.
xi
Bubbling fluidised reactors provide good mixing and transport rates for a broad
range of gas–solid processes in the industry. Their overall performance relies highly
on the macroscopic flow of bubbles, which is usually chaotic and unstable due to
the complex dissipative interparticle collisions and interfacial friction. Applying an
oscillatory gas flow, under certain conditions, can induce a recursive flow structure
where bubble positions become perfectly ordered following an array of triangle
tessellation. The phenomenon manifests great potential to benefit the reactor design,
control and rational scale-up.
The thesis combines both experiments and simulations to explore the dynamically structured flows in pulsed fluidised beds. Its reproducibility has been evaluated experimentally with a designed intensity index that quantifies the
degree-of-order introduced in the flow pattern. The operation regimes of structured bubbling flows were identified via the comparison across flow conditions and
particulate bed depths. Unlike a steady flow fluidised system, the bubbles in a
structured flow exhibit a narrower size and separation distribution, which is directly
correlated to the oscillation applied, and hereby can be controlled and manipulated
precisely.
In the same work, the dynamics of the structured flows has been investigated
using two different modelling strategies, two-fluid models (Euler–Euler approach)
and discrete element methods (Euler–Lagrange approach). The simulated flow
reveals the dynamic nature that the particulate phase periodically alternates between
the viscous and plastic regimes. When solids contract, the system creates temporally
locked regions of solids that were found to synchronise the nucleation of bubbles
and stabilise the flow structures. Besides, due to inherent assumptions of the
two-fluid model, the simulated system fails to find a structured flow. These results,
in turn, highlight the power of pattern formation as a robust “fingerprint” for gas–
solid multiphase model validation and development.
xi