Impact Statement
Macroscopic flow structure is essential to the performance of gas–solid bubbling
fluidised beds, but the complex hydrodynamics make it challenging to control and
predict, even in a small simplified reactor. The motion and evolution of bubbles,
being the major driving factors, motivate solid circulation and mixing. Due to
nonlinear particle–fluid interactions occurring at multiple spatiotemporal scales,
bubble rising is often irregular, and solids circulate in complex size-dependent
patterns inside a bubbling fluidised bed. Thereby, such a coupled solid–gas flow
easily degenerates into a chaotic structure, where interphase contact time and
mass/heat transfer become difficult to predict. Without proper control over these
critical design parameters, operations and scale-up of a gas–solid bubbling fluidised
bed remain largely empirical.
One may overcome some disadvantages of chaotic dynamics by “structuring”
the flow using additional degrees of freedom. A conventional practice to control
hydrodynamics is via modifications to the geometry through internals (i.e. baffles
and heat exchange tubes). Such objects intrude the domain and force certain flow
fields, but at a cost of additional energy dissipation, particle attrition and substantial
pressure drop. These improvements are strongly tied not only to the specific particle
type and operation conditions but also to the system geometry and scale. As a
result, it is still challenging to migrate the same flow behaviour from
laboratory-scale elementary studies to a commercial size reactor.
The thesis demonstrates experimentally the use of oscillation in a gas flow to
suppress the inherent instability and impose a regular flow pattern that results from
dissipative contacts via drag and friction. The emerged flow structure is scalable
and able to tightly tailor the hydrodynamics of gas–solid coupled flows (i.e. size
distribution, residence time and spatial deposition of bubbles) in a controlled
manner. The numerical results reveal its underlying dynamics and features to
establish a series of modulated short-range solid mixing in uniform contacts with
the gas phase. These unique features are highly desired, as it can not only bypass
certain engineering challenges occurring in conventional operations (i.e. flow
maldistribution, non-uniform contact and geometry dependence) but also decouple
conflicting design objectives (i.e. solid mixing and gas–solid contact).
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Macroscopic flow structure is essential to the performance of gas–solid bubbling
fluidised beds, but the complex hydrodynamics make it challenging to control and
predict, even in a small simplified reactor. The motion and evolution of bubbles,
being the major driving factors, motivate solid circulation and mixing. Due to
nonlinear particle–fluid interactions occurring at multiple spatiotemporal scales,
bubble rising is often irregular, and solids circulate in complex size-dependent
patterns inside a bubbling fluidised bed. Thereby, such a coupled solid–gas flow
easily degenerates into a chaotic structure, where interphase contact time and
mass/heat transfer become difficult to predict. Without proper control over these
critical design parameters, operations and scale-up of a gas–solid bubbling fluidised
bed remain largely empirical.
One may overcome some disadvantages of chaotic dynamics by “structuring”
the flow using additional degrees of freedom. A conventional practice to control
hydrodynamics is via modifications to the geometry through internals (i.e. baffles
and heat exchange tubes). Such objects intrude the domain and force certain flow
fields, but at a cost of additional energy dissipation, particle attrition and substantial
pressure drop. These improvements are strongly tied not only to the specific particle
type and operation conditions but also to the system geometry and scale. As a
result, it is still challenging to migrate the same flow behaviour from
laboratory-scale elementary studies to a commercial size reactor.
The thesis demonstrates experimentally the use of oscillation in a gas flow to
suppress the inherent instability and impose a regular flow pattern that results from
dissipative contacts via drag and friction. The emerged flow structure is scalable
and able to tightly tailor the hydrodynamics of gas–solid coupled flows (i.e. size
distribution, residence time and spatial deposition of bubbles) in a controlled
manner. The numerical results reveal its underlying dynamics and features to
establish a series of modulated short-range solid mixing in uniform contacts with
the gas phase. These unique features are highly desired, as it can not only bypass
certain engineering challenges occurring in conventional operations (i.e. flow
maldistribution, non-uniform contact and geometry dependence) but also decouple
conflicting design objectives (i.e. solid mixing and gas–solid contact).
xv